diff --git a/compiler/test/stdlib/immutablemap.test.gr b/compiler/test/stdlib/immutablemap.test.gr
new file mode 100644
index 0000000000..41783affc3
--- /dev/null
+++ b/compiler/test/stdlib/immutablemap.test.gr
@@ -0,0 +1,331 @@
+import ImmutableMap from "immutablemap"
+import List from "list"
+import Array from "array"
+
+// Data types used in multiple tests
+enum Resource {
+  Grain,
+  Sheep,
+  Brick,
+  Wood,
+}
+record ResourceData {
+  name: String,
+  emoji: String,
+}
+
+let strKeys = ImmutableMap.fromList([("🌾", 1), ("🐑", 2), ("🧱", 3)])
+let numKeys = ImmutableMap.fromList([(1, "🌾"), (2, "🐑"), (3, "🧱")])
+let varKeys = ImmutableMap.fromList(
+  [(Grain, "🌾"), (Sheep, "🐑"), (Brick, "🧱")]
+)
+let recordKeys = ImmutableMap.fromList(
+  [
+    ({ name: "Grain", emoji: "🌾" }, 1),
+    ({ name: "Sheep", emoji: "🐑" }, 2),
+    ({ name: "Brick", emoji: "🧱" }, 3),
+  ]
+)
+
+// ImmutableMap.isEmpty()
+
+let mut e = ImmutableMap.empty
+
+assert ImmutableMap.isEmpty(e)
+e = ImmutableMap.remove("🌾", e)
+assert ImmutableMap.isEmpty(e)
+let newE = ImmutableMap.set("🌾", "🌾", e)
+assert !ImmutableMap.isEmpty(newE)
+assert ImmutableMap.isEmpty(e)
+
+// ImmutableMap.size()
+
+let m = strKeys
+
+assert ImmutableMap.size(m) == 3
+
+// ImmutableMap.contains()
+
+assert ImmutableMap.contains("🌾", m)
+assert ImmutableMap.contains("🐑", m)
+assert ImmutableMap.contains("🧱", m)
+assert !ImmutableMap.contains("🌳", m)
+
+// ImmutableMap.set() & ImmutableMap.get()
+
+// With Number keys
+let nums = numKeys
+
+assert ImmutableMap.get(1, nums) == Some("🌾")
+assert ImmutableMap.get(2, nums) == Some("🐑")
+assert ImmutableMap.get(3, nums) == Some("🧱")
+assert ImmutableMap.get(4, nums) == None
+
+// With String keys
+let mut strs = strKeys
+
+assert ImmutableMap.get("🌾", strs) == Some(1)
+assert ImmutableMap.get("🐑", strs) == Some(2)
+assert ImmutableMap.get("🧱", strs) == Some(3)
+assert ImmutableMap.get("🌳", strs) == None
+
+// With variant keys
+let vars = varKeys
+
+assert ImmutableMap.get(Grain, vars) == Some("🌾")
+assert ImmutableMap.get(Sheep, vars) == Some("🐑")
+assert ImmutableMap.get(Brick, vars) == Some("🧱")
+assert ImmutableMap.get(Wood, vars) == None
+
+// With record keys
+let recs = recordKeys
+
+assert ImmutableMap.get({ name: "Grain", emoji: "🌾" }, recs) == Some(1)
+assert ImmutableMap.get({ name: "Sheep", emoji: "🐑" }, recs) == Some(2)
+assert ImmutableMap.get({ name: "Brick", emoji: "🧱" }, recs) == Some(3)
+assert ImmutableMap.get({ name: "Wood", emoji: "🌳" }, recs) == None
+
+// Overwriting data
+let mut o = ImmutableMap.empty
+
+o = ImmutableMap.set(1, "🐑", o)
+o = ImmutableMap.set(1, "🌾", o)
+
+assert ImmutableMap.get(1, o) == Some("🌾")
+
+// ImmutableMap.remove()
+
+let mut r = strKeys
+
+assert ImmutableMap.size(r) == 3
+
+r = ImmutableMap.remove("🐑", r)
+
+assert ImmutableMap.size(r) == 2
+assert ImmutableMap.get("🐑", r) == None
+
+r = ImmutableMap.remove("🌳", r)
+
+assert ImmutableMap.size(r) == 2
+
+r = ImmutableMap.remove("🌾", r)
+assert ImmutableMap.get("🌾", r) == None
+
+r = ImmutableMap.remove("🧱", r)
+assert ImmutableMap.get("🧱", r) == None
+
+assert ImmutableMap.isEmpty(r)
+
+// ImmutableMap.forEach()
+
+let fe = varKeys
+
+let mut called = 0
+
+ImmutableMap.forEach((key, value) => {
+  called += 1
+  match (key) {
+    Grain => assert value == "🌾",
+    Sheep => assert value == "🐑",
+    Brick => assert value == "🧱",
+    _ => fail "ImmutableMap.forEach() should not contain this value.",
+  }
+}, fe)
+
+assert called == 3
+
+// ImmutableMap.reduce()
+
+let mut r = ImmutableMap.empty
+
+r = ImmutableMap.set(Grain, 1, r)
+r = ImmutableMap.set(Sheep, 2, r)
+r = ImmutableMap.set(Brick, 3, r)
+
+let mut called = 0
+
+let result = ImmutableMap.reduce((acc, key, value) => {
+  called += 1
+  match (key) {
+    Grain => assert value == 1,
+    Sheep => assert value == 2,
+    Brick => assert value == 3,
+    _ => fail "ImmutableMap.reduce() should not contain this value.",
+  }
+  acc + value
+}, 0, r)
+
+assert called == 3
+assert result == 6
+
+// ImmutableMap.keys() & ImmutableMap.values();
+
+let kvs = varKeys
+
+let keys = ImmutableMap.keys(kvs)
+
+assert List.contains(Grain, keys)
+assert List.contains(Sheep, keys)
+assert List.contains(Brick, keys)
+assert !List.contains(Wood, keys)
+
+let vals = ImmutableMap.values(kvs)
+
+assert List.contains("🌾", vals)
+assert List.contains("🐑", vals)
+assert List.contains("🧱", vals)
+assert !List.contains("🌳", vals)
+
+// ImmutableMap.toList()
+
+let tl = varKeys
+
+let lis = ImmutableMap.toList(tl)
+
+// No order is guaranteed
+assert List.contains((Grain, "🌾"), lis)
+assert List.contains((Sheep, "🐑"), lis)
+assert List.contains((Brick, "🧱"), lis)
+assert !List.contains((Wood, "🌳"), lis)
+
+// ImmutableMap.fromList()
+
+let fl = ImmutableMap.fromList(
+  [(Grain, "🌾"), (Sheep, "🐑"), (Brick, "🧱")]
+)
+
+assert ImmutableMap.contains(Grain, fl)
+assert ImmutableMap.contains(Sheep, fl)
+assert ImmutableMap.contains(Brick, fl)
+assert !ImmutableMap.contains(Wood, fl)
+
+// ImmutableMap.toArray()
+
+let ta = varKeys
+
+let arr = ImmutableMap.toArray(ta)
+
+// No order is guaranteed
+assert Array.contains((Grain, "🌾"), arr)
+assert Array.contains((Sheep, "🐑"), arr)
+assert Array.contains((Brick, "🧱"), arr)
+assert !Array.contains((Wood, "🌳"), arr)
+
+// ImmutableMap.fromArray()
+
+let fa = ImmutableMap.fromArray(
+  [> (Grain, "🌾"), (Sheep, "🐑"), (Brick, "🧱")]
+)
+
+assert ImmutableMap.contains(Grain, fa)
+assert ImmutableMap.contains(Sheep, fa)
+assert ImmutableMap.contains(Brick, fa)
+assert !ImmutableMap.contains(Wood, fa)
+
+// ImmutableMap.filter()
+
+let makeFilterTestImmutableMap = () =>
+  ImmutableMap.fromList([(Grain, "g"), (Sheep, "s"), (Brick, "b")])
+
+let mut filterTestImmutableMap = makeFilterTestImmutableMap()
+
+filterTestImmutableMap = ImmutableMap.filter((key, value) =>
+  key == Sheep, filterTestImmutableMap)
+
+assert !ImmutableMap.contains(Grain, filterTestImmutableMap)
+assert ImmutableMap.contains(Sheep, filterTestImmutableMap)
+assert !ImmutableMap.contains(Brick, filterTestImmutableMap)
+
+let mut filterTestImmutableMap = makeFilterTestImmutableMap()
+
+filterTestImmutableMap = ImmutableMap.filter((key, value) =>
+  value == "b" || value == "s", filterTestImmutableMap)
+
+assert !ImmutableMap.contains(Grain, filterTestImmutableMap)
+assert ImmutableMap.contains(Sheep, filterTestImmutableMap)
+assert ImmutableMap.contains(Brick, filterTestImmutableMap)
+
+let mut filterTestImmutableMap = makeFilterTestImmutableMap()
+
+filterTestImmutableMap = ImmutableMap.filter((key, value) =>
+  value == "invalid", filterTestImmutableMap)
+
+assert ImmutableMap.size(filterTestImmutableMap) == 0
+
+let mut filterTestImmutableMap = makeFilterTestImmutableMap()
+
+filterTestImmutableMap = ImmutableMap.filter((key, value) =>
+  true, filterTestImmutableMap)
+
+assert ImmutableMap.size(filterTestImmutableMap) == 3
+
+// ImmutableMap.reject()
+
+let mut rejectTestImmutableMap = makeFilterTestImmutableMap()
+
+rejectTestImmutableMap = ImmutableMap.reject((key, value) =>
+  key == Sheep, rejectTestImmutableMap)
+
+assert ImmutableMap.contains(Grain, rejectTestImmutableMap)
+assert !ImmutableMap.contains(Sheep, rejectTestImmutableMap)
+assert ImmutableMap.contains(Brick, rejectTestImmutableMap)
+
+let mut rejectTestImmutableMap = makeFilterTestImmutableMap()
+
+rejectTestImmutableMap = ImmutableMap.reject((key, value) =>
+  value == "b" || value == "s", rejectTestImmutableMap)
+
+assert ImmutableMap.contains(Grain, rejectTestImmutableMap)
+assert !ImmutableMap.contains(Sheep, rejectTestImmutableMap)
+assert !ImmutableMap.contains(Brick, rejectTestImmutableMap)
+
+let mut rejectTestImmutableMap = makeFilterTestImmutableMap()
+
+rejectTestImmutableMap = ImmutableMap.reject((key, value) =>
+  true, rejectTestImmutableMap)
+
+assert ImmutableMap.size(rejectTestImmutableMap) == 0
+
+let mut rejectTestImmutableMap = makeFilterTestImmutableMap()
+
+rejectTestImmutableMap = ImmutableMap.reject((key, value) =>
+  false, rejectTestImmutableMap)
+
+assert ImmutableMap.size(rejectTestImmutableMap) == 3
+
+// ImmutableMap.update()
+
+let mut toUpdate = ImmutableMap.fromList([("a", 1), ("b", 2), ("c", 3)])
+
+toUpdate = ImmutableMap.update(
+  "b",
+  old => {
+    assert old == Some(2)
+    Some(4)
+  },
+  toUpdate
+)
+
+assert ImmutableMap.get("b", toUpdate) == Some(4)
+
+toUpdate = ImmutableMap.update(
+  "d",
+  old => {
+    assert old == None
+    Some(10)
+  },
+  toUpdate
+)
+
+assert ImmutableMap.get("d", toUpdate) == Some(10)
+
+toUpdate = ImmutableMap.update(
+  "c",
+  old => {
+    assert old == Some(3)
+    None
+  },
+  toUpdate
+)
+
+assert ImmutableMap.contains("c", toUpdate) == false
diff --git a/compiler/test/stdlib/immutableset.test.gr b/compiler/test/stdlib/immutableset.test.gr
new file mode 100644
index 0000000000..d31074f160
--- /dev/null
+++ b/compiler/test/stdlib/immutableset.test.gr
@@ -0,0 +1,197 @@
+import ImmutableSet from "immutableset"
+import List from "list"
+import Array from "array"
+
+// Data types used in multiple tests
+enum Resource {
+  Grain,
+  Sheep,
+  Brick,
+  Wood,
+}
+record ResourceData {
+  name: String,
+  emoji: String,
+}
+
+// ImmutableSet.isEmpty
+
+let mut e = ImmutableSet.empty
+
+assert ImmutableSet.isEmpty(e)
+e = ImmutableSet.remove("🌾", e)
+assert ImmutableSet.isEmpty(e)
+let newE = ImmutableSet.add("🌾", e)
+assert !ImmutableSet.isEmpty(newE)
+assert ImmutableSet.isEmpty(e)
+
+// ImmutableSet.size
+
+let s = ImmutableSet.fromList(["🌾", "🐑", "🧱"])
+
+assert ImmutableSet.size(s) == 3
+
+// ImmutableSet.contains
+
+let h = ImmutableSet.fromList(["🌾", "🐑", "🧱"])
+
+assert ImmutableSet.contains("🌾", h)
+assert ImmutableSet.contains("🐑", h)
+assert ImmutableSet.contains("🧱", h)
+assert !ImmutableSet.contains("🌳", h)
+
+// ImmutableSet.add
+
+let mut vars = ImmutableSet.empty
+
+vars = ImmutableSet.add(Grain, vars)
+vars = ImmutableSet.add(Sheep, vars)
+vars = ImmutableSet.add(Grain, vars)
+
+assert ImmutableSet.size(vars) == 2
+
+let mut recs = ImmutableSet.empty
+
+recs = ImmutableSet.add({ name: "Grain", emoji: "🌾" }, recs)
+recs = ImmutableSet.add({ name: "Sheep", emoji: "🐑" }, recs)
+recs = ImmutableSet.add({ name: "Brick", emoji: "🧱" }, recs)
+recs = ImmutableSet.add({ name: "Grain", emoji: "🌾" }, recs)
+recs = ImmutableSet.add({ name: "Sheep", emoji: "🐑" }, recs)
+recs = ImmutableSet.add({ name: "Brick", emoji: "🧱" }, recs)
+
+assert ImmutableSet.size(recs) == 3
+
+// ImmutableSet.remove
+
+let mut r = ImmutableSet.fromList(["🌾", "🐑", "🧱"])
+
+assert ImmutableSet.size(r) == 3
+
+r = ImmutableSet.remove("🌾", r)
+
+assert ImmutableSet.size(r) == 2
+assert !ImmutableSet.contains("🌾", r)
+
+r = ImmutableSet.remove("🐑", r)
+r = ImmutableSet.remove("🧱", r)
+
+assert ImmutableSet.isEmpty(r)
+
+// ImmutableSet.filter
+
+let makeTestSet = () => ImmutableSet.fromList([Grain, Sheep, Brick])
+
+let mut filterTestSet = makeTestSet()
+
+ImmutableSet.filter(key => fail "Shouldn't be called", ImmutableSet.empty)
+filterTestSet = ImmutableSet.filter(key => key == Sheep, filterTestSet)
+
+assert !ImmutableSet.contains(Grain, filterTestSet)
+assert ImmutableSet.contains(Sheep, filterTestSet)
+assert !ImmutableSet.contains(Brick, filterTestSet)
+
+// ImmutableSet.reject
+
+let mut rejectTestSet = makeTestSet()
+
+ImmutableSet.reject(key => fail "Shouldn't be called", ImmutableSet.empty)
+rejectTestSet = ImmutableSet.reject(key => key == Sheep, rejectTestSet)
+
+assert ImmutableSet.contains(Grain, rejectTestSet)
+assert !ImmutableSet.contains(Sheep, rejectTestSet)
+assert ImmutableSet.contains(Brick, rejectTestSet)
+
+// ImmutableSet.reduce
+
+let reduceTestSet = ImmutableSet.fromList([1, 3, 2, 5, 4])
+
+let result = ImmutableSet.reduce((acc, key) => fail "Shouldn't be called",
+0,
+ImmutableSet.empty
+)
+
+assert result == 0
+
+let mut called = 0
+
+let result = ImmutableSet.reduce((acc, key) => {
+  called += 1
+  [key, ...acc]
+}, [], reduceTestSet)
+
+assert called == 5
+assert result == [5, 4, 3, 2, 1]
+
+// ImmutableSet.forEach
+
+let forEachTestSet = makeTestSet()
+
+ImmutableSet.forEach(key => fail "Shouldn't be called", ImmutableSet.empty)
+
+let mut called = 0
+
+ImmutableSet.forEach(key => {
+  called += 1
+  match (key) {
+    Grain => void,
+    Sheep => void,
+    Brick => void,
+    _ => fail "ImmutableSet.forEach() should not contain this value.",
+  }
+}, forEachTestSet)
+
+assert called == 3
+
+// ImmutableSet.diff
+
+let d = ImmutableSet.fromList([0, 1, 2, 3, 4, 5, 6])
+let e = ImmutableSet.fromList([4, 2, 1, 3, 0, -2, -1, -3])
+
+let diffSet = ImmutableSet.diff(d, e)
+
+assert ImmutableSet.size(diffSet) == 5
+assert [-3, -2, -1, 5, 6] == ImmutableSet.toList(diffSet)
+
+// ImmutableSet.union
+
+let unionSet = ImmutableSet.union(d, e)
+
+assert ImmutableSet.size(unionSet) == 10
+assert [-3, -2, -1, 0, 1, 2, 3, 4, 5, 6] == ImmutableSet.toList(unionSet)
+
+// ImmutableSet.intersect
+
+let intersectSet = ImmutableSet.intersect(d, e)
+
+assert ImmutableSet.size(intersectSet) == 5
+assert [0, 1, 2, 3, 4] == ImmutableSet.toList(intersectSet)
+
+// ImmutableSet.fromList
+
+let k = ImmutableSet.fromList([1, 1, 1])
+
+assert ImmutableSet.size(k) == 1
+assert ImmutableSet.contains(1, k)
+
+// ImmutableSet.toList
+
+let o = ImmutableSet.fromList([0, 2, 1, 3, 0, 4, 2])
+
+let l = ImmutableSet.toList(o)
+
+assert l == [0, 1, 2, 3, 4]
+
+// ImmutableSet.fromArray
+
+let q = ImmutableSet.fromArray([> 0, 0, 0])
+
+assert ImmutableSet.size(q) == 1
+assert ImmutableSet.contains(0, q)
+
+// ImmutableSet.toArray
+
+let p = ImmutableSet.fromArray([> 0, 1, 2, 3, 4, 3, 2, 1, 0])
+
+let r = ImmutableSet.toArray(p)
+
+assert r == [> 0, 1, 2, 3, 4]
diff --git a/compiler/test/suites/stdlib.re b/compiler/test/suites/stdlib.re
index 86efe51522..0dd5f260bd 100644
--- a/compiler/test/suites/stdlib.re
+++ b/compiler/test/suites/stdlib.re
@@ -100,6 +100,7 @@ describe("stdlib", ({test, testSkip}) => {
   assertStdlib("int64.test");
   assertStdlib("list.test");
   assertStdlib("map.test");
+  assertStdlib("immutablemap.test");
   assertStdlib("marshal.test");
   assertStdlib("number.test");
   assertStdlib("option.test");
@@ -108,6 +109,7 @@ describe("stdlib", ({test, testSkip}) => {
   assertStdlib("range.test");
   assertStdlib("result.test");
   assertStdlib("set.test");
+  assertStdlib("immutableset.test");
   assertStdlib("regex.test");
   assertStdlib("stack.test");
   assertStdlib("priorityqueue.test");
diff --git a/stdlib/immutablemap.gr b/stdlib/immutablemap.gr
new file mode 100644
index 0000000000..9dcb6e72c6
--- /dev/null
+++ b/stdlib/immutablemap.gr
@@ -0,0 +1,493 @@
+/**
+ * @module ImmutableMap: An ImmutableMap holds key-value pairs. Any value may be used as a key or value. Operations on an ImmutableMap do not mutate the map's internal state.
+ * @example import ImmutableMap from "immutablemap"
+ *
+ * @since v0.5.4
+ */
+
+import List from "list"
+import Array from "array"
+import Option from "option"
+
+// implementation based on the paper "Implementing Sets Efficiently in a
+// Functional Language" by Stephen Adams
+record Node<k, v> {
+  key: k,
+  val: v,
+  size: Number,
+  left: ImmutableMap<k, v>,
+  right: ImmutableMap<k, v>,
+},
+/**
+ * @section Types: Type declarations included in the ImmutableMap module.
+ */
+enum ImmutableMap<k, v> {
+  Empty,
+  Tree(Node<k, v>),
+}
+
+/**
+ * @section Values: Functions and constants for working with ImmutableMaps.
+ */
+
+// semi-arbitrary value chosen for algorithm for determining when to balance
+// trees; no tree can have a left subtree containing this number of times
+// more elements than its right subtree or vice versa
+let weight = 4
+
+/**
+ * An empty map
+ *
+ * @since v0.5.4
+ */
+export let empty = Empty
+
+// returns the key-value pair of the minimum key in a tree
+let rec min = node => {
+  match (node) {
+    Tree({ key, val, left: Empty, _ }) => (key, val),
+    Tree({ left, _ }) => min(left),
+    Empty => fail "Impossible: min of empty element in ImmutableMap",
+  }
+}
+
+/**
+ * Provides the count of key-value pairs stored within the map.
+ *
+ * @param map: The map to inspect
+ * @returns The count of key-value pairs in the map
+ *
+ * @since v0.5.4
+ */
+export let size = map => {
+  match (map) {
+    Empty => 0,
+    Tree({ size, _ }) => size,
+  }
+}
+
+/**
+ * Determines if the map contains no key-value pairs.
+ *
+ * @param map: The map to inspect
+ * @returns `true` if the given map is empty or `false` otherwise
+ *
+ * @since v0.5.4
+ */
+export let isEmpty = map => {
+  match (map) {
+    Empty => true,
+    Tree(_) => false,
+  }
+}
+
+let unwrapTree = node => {
+  match (node) {
+    Empty => fail "Impossible: ImmutableMap unwrapTree got an empty tree node",
+    Tree(tree) => tree,
+  }
+}
+
+// helper function for creating a tree node with correct size from
+// two balanced trees
+let makeNode = (key, val, left, right) => {
+  Tree({ key, val, size: 1 + size(left) + size(right), left, right })
+}
+
+// note: see Figure 1 of paper referenced above for visual illustration of
+// the rotations below
+
+// node rotation moving the left subtree of the right node to the left side
+let singleL = (key, val, left, right) => {
+  let { key: rKey, val: rVal, left: rl, right: rr, _ } = unwrapTree(right)
+  makeNode(rKey, rVal, makeNode(key, val, left, rl), rr)
+}
+
+// node rotation moving left child of right tree to the root
+let doubleL = (key, val, left, right) => {
+  let { key: rKey, val: rVal, left: rl, right: rr, _ } = unwrapTree(right)
+  let { key: rlKey, val: rlVal, left: rll, right: rlr, _ } = unwrapTree(rl)
+  makeNode(
+    rlKey,
+    rlVal,
+    makeNode(key, val, left, rll),
+    makeNode(rKey, rVal, rlr, rr)
+  )
+}
+
+// node rotation moving the right subtree of the left node to the right side
+let singleR = (key, val, left, right) => {
+  let { key: lKey, val: lVal, left: ll, right: lr, _ } = unwrapTree(left)
+  makeNode(lKey, lVal, ll, makeNode(key, val, lr, right))
+}
+
+// node rotation moving right child of left tree to the root
+let doubleR = (key, val, left, right) => {
+  let { key: lKey, val: lVal, left: ll, right: lr, _ } = unwrapTree(left)
+  let { key: lrKey, val: lrVal, left: lrl, right: lrr, _ } = unwrapTree(lr)
+  makeNode(
+    lrKey,
+    lrVal,
+    makeNode(lKey, lVal, ll, lrl),
+    makeNode(key, val, lrr, right)
+  )
+}
+
+// creates a new node after either the left or right trees have just had an
+// element inserted or removed from them, maintaining balance in the tree
+let balancedNode = (key, val, left, right) => {
+  let makeNodeFn = if (size(left) + size(right) < 2) {
+    makeNode
+  } else if (size(right) > weight * size(left)) {
+    // if the right tree is too much larger than the left then move part of
+    // the right tree to the left side
+    let { left: rl, right: rr, _ } = unwrapTree(right)
+    if (size(rl) < size(rr)) singleL else doubleL
+  } else if (size(left) > weight * size(right)) {
+    // if the left tree is too much larger than the right then move part of
+    // the left tree to the right side
+    let { left: ll, right: lr, _ } = unwrapTree(left)
+    if (size(lr) < size(ll)) singleR else doubleR
+  } else {
+    // if neither tree is too much larger than the other then simply create
+    // a new node
+    makeNode
+  }
+
+  makeNodeFn(key, val, left, right)
+}
+
+/**
+ * Produces a new map containing a new key-value pair. If the key already exists in the map, the value is replaced.
+ *
+ * @param key: The unique key in the map
+ * @param value: The value to store
+ * @param map: The base map
+ * @returns A new map containing the new key-value pair
+ *
+ * @since v0.5.4
+ */
+export let rec set = (key, val, map) => {
+  match (map) {
+    Empty => Tree({ key, val, size: 1, left: Empty, right: Empty }),
+    Tree({ key: nodeKey, val: nodeVal, left, right, _ }) => {
+      match (compare(key, nodeKey)) {
+        cmp when cmp < 0 =>
+          balancedNode(nodeKey, nodeVal, set(key, val, left), right),
+        cmp when cmp > 0 =>
+          balancedNode(nodeKey, nodeVal, left, set(key, val, right)),
+        _ => makeNode(key, val, left, right),
+      }
+    },
+  }
+}
+
+/**
+ * Retrieves the value for the given key.
+ *
+ * @param key: The key to access
+ * @param map: The map to access
+ * @returns `Some(value)` if the key exists in the map or `None` otherwise
+ *
+ * @since v0.5.4
+ */
+export let rec get = (key, map) => {
+  match (map) {
+    Empty => None,
+    Tree({ key: nodeKey, val: nodeVal, left, right, _ }) => {
+      match (compare(key, nodeKey)) {
+        cmp when cmp < 0 => get(key, left),
+        cmp when cmp > 0 => get(key, right),
+        _ => Some(nodeVal),
+      }
+    },
+  }
+}
+
+/**
+ * Determines if the map contains the given key. In such a case, it will always contain a value for the given key.
+ *
+ * @param key: The key to search for
+ * @param map: The map to search
+ * @returns `true` if the map contains the given key or `false` otherwise
+ *
+ * @since v0.5.4
+ */
+export let rec contains = (key, map) => {
+  Option.isSome(get(key, map))
+}
+
+// removes the minimum element from a tree
+let rec removeMin = node => {
+  match (node) {
+    Tree({ left: Empty, right, _ }) => right,
+    Tree({ key, val, left, right, _ }) =>
+      balancedNode(key, val, removeMin(left), right),
+    _ => fail "Impossible: ImmutableMap removeMin on empty node",
+  }
+}
+
+// helper function for removing a node by creating a new node containing the
+// removed node's left and right subtrees
+let removeInner = (left, right) => {
+  match ((left, right)) {
+    (Empty, node) | (node, Empty) => node,
+    (left, right) => {
+      let (minKey, minVal) = min(right)
+      balancedNode(minKey, minVal, left, removeMin(right))
+    },
+  }
+}
+
+/**
+ * Produces a new map without the key-value pair corresponding to the given
+ * key. If the key doesn't exist in the map, the map will be returned unmodified.
+ *
+ * @param key: The key to exclude
+ * @param map: The map to exclude from
+ * @returns A new map without the given key
+ *
+ * @since v0.5.4
+ */
+export let rec remove = (key, map) => {
+  match (map) {
+    Empty => Empty,
+    Tree({ key: nodeKey, val: nodeVal, left, right, _ }) => {
+      match (compare(key, nodeKey)) {
+        cmp when cmp < 0 =>
+          balancedNode(nodeKey, nodeVal, remove(key, left), right),
+        cmp when cmp > 0 =>
+          balancedNode(nodeKey, nodeVal, left, remove(key, right)),
+        _ => removeInner(left, right),
+      }
+    },
+  }
+}
+
+/**
+ * Produces a new map by calling an updater function that receives the
+ * previously stored value as an `Option` and returns the new value to be
+ * stored as an `Option`. If the key didn't exist previously, the value
+ * will be `None`. If `None` is returned from the updater function, the
+ * key-value pair is excluded.
+ *
+ * @param key: The unique key in the map
+ * @param fn: The updater function
+ * @param map: The base map
+ * @returns A new map with the value at the given key modified according to the function's output
+ *
+ * @since v0.5.4
+ */
+export let update = (key, fn, map) => {
+  let val = get(key, map)
+  match (fn(val)) {
+    Some(next) => set(key, next, map),
+    None => remove(key, map),
+  }
+}
+
+/**
+ * Iterates the map, calling an iterator function with each key and value.
+ *
+ * @param fn: The iterator function to call with each key and value
+ * @param map: The map to iterate
+ *
+ * @since v0.5.4
+ */
+export let forEach = (fn, map) => {
+  let rec forEachInner = node => {
+    match (node) {
+      Empty => void,
+      Tree({ key, val, left, right, _ }) => {
+        forEachInner(left)
+        fn(key, val): Void
+        forEachInner(right)
+      },
+    }
+  }
+  forEachInner(map)
+}
+
+/**
+ * Combines all key-value pairs of a map using a reducer function.
+ *
+ * @param fn: The reducer function to call on each key and value, where the value returned will be the next accumulator value
+ * @param init: The initial value to use for the accumulator on the first iteration
+ * @param map: The map to iterate
+ * @returns The final accumulator returned from `fn`
+ *
+ * @since v0.5.4
+ */
+export let reduce = (fn, init, map) => {
+  let rec reduceInner = (acc, node) => {
+    match (node) {
+      Empty => acc,
+      Tree({ key, val, left, right, _ }) => {
+        let newAcc = fn(reduceInner(acc, left), key, val)
+        reduceInner(newAcc, right)
+      },
+    }
+  }
+  reduceInner(init, map)
+}
+
+// joins two trees with a value, preserving the BST property of left children
+// being less the node and right children being greater than the node
+let rec concat3 = (key, val, left, right) => {
+  match ((left, right)) {
+    (Empty, node) | (node, Empty) => set(key, val, node),
+    (Tree(left) as leftOpt, Tree(right) as rightOpt) => {
+      if (weight * left.size < right.size) {
+        balancedNode(
+          right.key,
+          right.val,
+          concat3(key, val, leftOpt, right.left),
+          right.right
+        )
+      } else if (weight * right.size < left.size) {
+        balancedNode(
+          left.key,
+          left.val,
+          left.left,
+          concat3(key, val, left.right, rightOpt)
+        )
+      } else {
+        makeNode(key, val, leftOpt, rightOpt)
+      }
+    },
+  }
+}
+
+// concatenates two trees of arbitrary size
+let concat = (node1, node2) => {
+  match (node2) {
+    Empty => node1,
+    _ => {
+      let (minKey, minVal) = min(node2)
+      concat3(minKey, minVal, node1, removeMin(node2))
+    },
+  }
+}
+
+let reduceRight = (fn, init, map) => {
+  let rec reduceInner = (acc, node) => {
+    match (node) {
+      Empty => acc,
+      Tree({ key, val, left, right, _ }) => {
+        let newAcc = fn(reduceInner(acc, right), key, val)
+        reduceInner(newAcc, left)
+      },
+    }
+  }
+  reduceInner(init, map)
+}
+
+/**
+ * Enumerates all keys in the given map.
+ *
+ * @param map: The map to enumerate
+ * @returns A list containing all keys from the given map
+ *
+ * @since v0.5.4
+ */
+export let keys = map => {
+  reduceRight((list, key, _) => [key, ...list], [], map)
+}
+
+/**
+ * Enumerates all values in the given map.
+ *
+ * @param map: The map to enumerate
+ * @returns A list containing all values from the given map
+ *
+ * @since v0.5.4
+ */
+export let values = map => {
+  reduceRight((list, _, value) => [value, ...list], [], map)
+}
+
+/**
+ * Produces a new map excluding the key-value pairs where a predicate function returns `false`.
+ *
+ * @param fn: The predicate function to indicate which key-value pairs to exclude from the map, where returning `false` indicates the key-value pair should be excluded
+ * @param map: The map to iterate
+ * @returns A new map excluding the key-value pairs not fulfilling the predicate
+ *
+ * @since v0.5.4
+ */
+export let filter = (fn, map) => {
+  let rec filterInner = node => {
+    match (node) {
+      Empty => Empty,
+      Tree({ key, val, left, right, _ }) => {
+        if (fn(key, val)) {
+          concat3(key, val, filterInner(left), filterInner(right))
+        } else {
+          concat(filterInner(left), filterInner(right))
+        }
+      },
+    }
+  }
+  filterInner(map)
+}
+
+/**
+ * Produces a new map excluding the key-value pairs where a predicate function returns `true`.
+ *
+ * @param fn: The predicate function to indicate which key-value pairs to exclude from the map, where returning `true` indicates the key-value pair should be excluded
+ * @param map: The map to iterate
+ * @returns A new map excluding the key-value pairs fulfilling the predicate
+ *
+ * @since v0.5.4
+ */
+export let reject = (fn, map) => {
+  filter((key, val) => !fn(key, val), map)
+}
+
+/**
+ * Creates a map from a list.
+ *
+ * @param list: The list to convert
+ * @returns A map containing all key-value pairs from the list
+ *
+ * @since v0.5.4
+ */
+export let fromList = list => {
+  List.reduce((map, (key, val)) => set(key, val, map), empty, list)
+}
+
+/**
+ * Enumerates all key-value pairs in the given map.
+ *
+ * @param map: The map to enumerate
+ * @returns A list containing all key-value pairs from the given map
+ *
+ * @since v0.5.4
+ */
+export let toList = map => {
+  reduceRight((list, key, val) => [(key, val), ...list], [], map)
+}
+
+/**
+ * Creates a map from an array.
+ *
+ * @param array: The array to convert
+ * @returns A map containing all key-value pairs from the array
+ *
+ * @since v0.5.4
+ */
+export let fromArray = array => {
+  Array.reduce((map, (key, val)) => set(key, val, map), empty, array)
+}
+
+/**
+ * Converts a map into an array of its key-value pairs.
+ *
+ * @param map: The map to convert
+ * @returns An array containing all key-value pairs from the given map
+ *
+ * @since v0.5.4
+ */
+export let toArray = map => {
+  Array.fromList(toList(map))
+}
diff --git a/stdlib/immutablemap.md b/stdlib/immutablemap.md
new file mode 100644
index 0000000000..6260cd1f6c
--- /dev/null
+++ b/stdlib/immutablemap.md
@@ -0,0 +1,479 @@
+---
+title: ImmutableMap
+---
+
+An ImmutableMap holds key-value pairs. Any value may be used as a key or value. Operations on an ImmutableMap do not mutate the map's internal state.
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+import ImmutableMap from "immutablemap"
+```
+
+## Types
+
+Type declarations included in the ImmutableMap module.
+
+### ImmutableMap.**ImmutableMap**
+
+```grain
+type ImmutableMap<k, v>
+```
+
+## Values
+
+Functions and constants for working with ImmutableMaps.
+
+### ImmutableMap.**empty**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+empty : ImmutableMap<a, b>
+```
+
+An empty map
+
+### ImmutableMap.**size**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+size : ImmutableMap<a, b> -> Number
+```
+
+Provides the count of key-value pairs stored within the map.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`map`|`ImmutableMap<a, b>`|The map to inspect|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Number`|The count of key-value pairs in the map|
+
+### ImmutableMap.**isEmpty**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+isEmpty : ImmutableMap<a, b> -> Bool
+```
+
+Determines if the map contains no key-value pairs.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`map`|`ImmutableMap<a, b>`|The map to inspect|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Bool`|`true` if the given map is empty or `false` otherwise|
+
+### ImmutableMap.**set**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+set : (a, b, ImmutableMap<a, b>) -> ImmutableMap<a, b>
+```
+
+Produces a new map containing a new key-value pair. If the key already exists in the map, the value is replaced.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`key`|`a`|The unique key in the map|
+|`value`|`b`|The value to store|
+|`map`|`ImmutableMap<a, b>`|The base map|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableMap<a, b>`|A new map containing the new key-value pair|
+
+### ImmutableMap.**get**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+get : (a, ImmutableMap<a, b>) -> Option<b>
+```
+
+Retrieves the value for the given key.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`key`|`a`|The key to access|
+|`map`|`ImmutableMap<a, b>`|The map to access|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Option<b>`|`Some(value)` if the key exists in the map or `None` otherwise|
+
+### ImmutableMap.**contains**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+contains : (a, ImmutableMap<a, b>) -> Bool
+```
+
+Determines if the map contains the given key. In such a case, it will always contain a value for the given key.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`key`|`a`|The key to search for|
+|`map`|`ImmutableMap<a, b>`|The map to search|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Bool`|`true` if the map contains the given key or `false` otherwise|
+
+### ImmutableMap.**remove**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+remove : (a, ImmutableMap<a, b>) -> ImmutableMap<a, b>
+```
+
+Produces a new map without the key-value pair corresponding to the given
+key. If the key doesn't exist in the map, the map will be returned unmodified.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`key`|`a`|The key to exclude|
+|`map`|`ImmutableMap<a, b>`|The map to exclude from|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableMap<a, b>`|A new map without the given key|
+
+### ImmutableMap.**update**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+update :
+  (a, (Option<b> -> Option<b>), ImmutableMap<a, b>) -> ImmutableMap<a, b>
+```
+
+Produces a new map by calling an updater function that receives the
+previously stored value as an `Option` and returns the new value to be
+stored as an `Option`. If the key didn't exist previously, the value
+will be `None`. If `None` is returned from the updater function, the
+key-value pair is excluded.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`key`|`a`|The unique key in the map|
+|`fn`|`Option<b> -> Option<b>`|The updater function|
+|`map`|`ImmutableMap<a, b>`|The base map|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableMap<a, b>`|A new map with the value at the given key modified according to the function's output|
+
+### ImmutableMap.**forEach**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+forEach : (((a, b) -> Void), ImmutableMap<a, b>) -> Void
+```
+
+Iterates the map, calling an iterator function with each key and value.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`(a, b) -> Void`|The iterator function to call with each key and value|
+|`map`|`ImmutableMap<a, b>`|The map to iterate|
+
+### ImmutableMap.**reduce**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+reduce : (((a, b, c) -> a), a, ImmutableMap<b, c>) -> a
+```
+
+Combines all key-value pairs of a map using a reducer function.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`(a, b, c) -> a`|The reducer function to call on each key and value, where the value returned will be the next accumulator value|
+|`init`|`a`|The initial value to use for the accumulator on the first iteration|
+|`map`|`ImmutableMap<b, c>`|The map to iterate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`a`|The final accumulator returned from `fn`|
+
+### ImmutableMap.**keys**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+keys : ImmutableMap<a, b> -> List<a>
+```
+
+Enumerates all keys in the given map.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`map`|`ImmutableMap<a, b>`|The map to enumerate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`List<a>`|A list containing all keys from the given map|
+
+### ImmutableMap.**values**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+values : ImmutableMap<a, b> -> List<b>
+```
+
+Enumerates all values in the given map.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`map`|`ImmutableMap<a, b>`|The map to enumerate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`List<b>`|A list containing all values from the given map|
+
+### ImmutableMap.**filter**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+filter : (((a, b) -> Bool), ImmutableMap<a, b>) -> ImmutableMap<a, b>
+```
+
+Produces a new map excluding the key-value pairs where a predicate function returns `false`.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`(a, b) -> Bool`|The predicate function to indicate which key-value pairs to exclude from the map, where returning `false` indicates the key-value pair should be excluded|
+|`map`|`ImmutableMap<a, b>`|The map to iterate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableMap<a, b>`|A new map excluding the key-value pairs not fulfilling the predicate|
+
+### ImmutableMap.**reject**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+reject : (((a, b) -> Bool), ImmutableMap<a, b>) -> ImmutableMap<a, b>
+```
+
+Produces a new map excluding the key-value pairs where a predicate function returns `true`.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`(a, b) -> Bool`|The predicate function to indicate which key-value pairs to exclude from the map, where returning `true` indicates the key-value pair should be excluded|
+|`map`|`ImmutableMap<a, b>`|The map to iterate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableMap<a, b>`|A new map excluding the key-value pairs fulfilling the predicate|
+
+### ImmutableMap.**fromList**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+fromList : List<(a, b)> -> ImmutableMap<a, b>
+```
+
+Creates a map from a list.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`list`|`List<(a, b)>`|The list to convert|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableMap<a, b>`|A map containing all key-value pairs from the list|
+
+### ImmutableMap.**toList**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+toList : ImmutableMap<a, b> -> List<(a, b)>
+```
+
+Enumerates all key-value pairs in the given map.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`map`|`ImmutableMap<a, b>`|The map to enumerate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`List<(a, b)>`|A list containing all key-value pairs from the given map|
+
+### ImmutableMap.**fromArray**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+fromArray : Array<(a, b)> -> ImmutableMap<a, b>
+```
+
+Creates a map from an array.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`array`|`Array<(a, b)>`|The array to convert|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableMap<a, b>`|A map containing all key-value pairs from the array|
+
+### ImmutableMap.**toArray**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+toArray : ImmutableMap<a, b> -> Array<(a, b)>
+```
+
+Converts a map into an array of its key-value pairs.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`map`|`ImmutableMap<a, b>`|The map to convert|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Array<(a, b)>`|An array containing all key-value pairs from the given map|
+
diff --git a/stdlib/immutableset.gr b/stdlib/immutableset.gr
new file mode 100644
index 0000000000..b7ff63d412
--- /dev/null
+++ b/stdlib/immutableset.gr
@@ -0,0 +1,498 @@
+/**
+ * @module ImmutableSet: An ImmutableSet is a collection of unique values. Operations on an ImmutableSet do not mutate the set's internal state.
+ * @example import ImmutableSet from "immutableset"
+ * 
+ * @since v0.5.4
+ */
+
+import List from "list"
+import Array from "array"
+
+// implementation based on the paper "Implementing Sets Efficiently in a
+// Functional Language" by Stephen Adams
+
+record Node<a> {
+  key: a,
+  size: Number,
+  left: ImmutableSet<a>,
+  right: ImmutableSet<a>,
+},
+/**
+ * @section Types: Type declarations included in the ImmutableSet module.
+ */
+enum ImmutableSet<a> {
+  Empty,
+  Tree(Node<a>),
+}
+
+/**
+ * @section Values: Functions and constants for working with ImmutableSets.
+ */
+
+// semi-arbitrary value chosen for algorithm for determining when to balance
+// trees; no tree can have a left subtree containing this number of times
+// more elements than its right subtree or vice versa
+let weight = 4
+
+/**
+ * An empty set
+ * 
+ * @since v0.5.4
+ */
+export let empty = Empty
+
+// returns the minimum value in a tree
+let rec min = node => {
+  match (node) {
+    Tree({ key, left: Empty, _ }) => key,
+    Tree({ left, _ }) => min(left),
+    Empty => fail "Impossible: min of empty element in ImmutableSet",
+  }
+}
+
+/**
+ * Provides the count of values within the set.
+ *
+ * @param set: The set to inspect
+ * @returns The count of elements in the set
+ * 
+ * @since v0.5.4
+ */
+export let size = set => {
+  match (set) {
+    Empty => 0,
+    Tree({ size, _ }) => size,
+  }
+}
+
+/**
+ * Determines if the set contains no elements.
+ *
+ * @param set: The set to inspect
+ * @returns `true` if the given set is empty or `false` otherwise
+ * 
+ * @since v0.5.4
+ */
+export let isEmpty = set => {
+  match (set) {
+    Empty => true,
+    Tree(_) => false,
+  }
+}
+
+let unwrapTree = node => {
+  match (node) {
+    Empty => fail "Impossible: ImmutableSet unwrapTree got an empty tree node",
+    Tree(tree) => tree,
+  }
+}
+
+// helper function for creating a tree node with correct size from
+// two balanced trees
+let makeNode = (key, left, right) => {
+  Tree({ key, size: 1 + size(left) + size(right), left, right })
+}
+
+// note: see Figure 1 of paper referenced above for visual illustration of
+// the rotations below
+
+// node rotation moving the left subtree of the right node to the left side
+let singleL = (key, left, right) => {
+  let { key: rKey, left: rl, right: rr, _ } = unwrapTree(right)
+  makeNode(rKey, makeNode(key, left, rl), rr)
+}
+
+// node rotation moving left child of right tree to the root
+let doubleL = (key, left, right) => {
+  let { key: rKey, left: rl, right: rr, _ } = unwrapTree(right)
+  let { key: rlKey, left: rll, right: rlr, _ } = unwrapTree(rl)
+  makeNode(rlKey, makeNode(key, left, rll), makeNode(rKey, rlr, rr))
+}
+
+// node rotation moving the right subtree of the left node to the right side
+let singleR = (key, left, right) => {
+  let { key: lKey, left: ll, right: lr, _ } = unwrapTree(left)
+  makeNode(lKey, ll, makeNode(key, lr, right))
+}
+
+// node rotation moving right child of left tree to the root
+let doubleR = (key, left, right) => {
+  let { key: lKey, left: ll, right: lr, _ } = unwrapTree(left)
+  let { key: lrKey, left: lrl, right: lrr, _ } = unwrapTree(lr)
+  makeNode(lrKey, makeNode(lKey, ll, lrl), makeNode(key, lrr, right))
+}
+
+// creates a new node after either the left or right trees have just had an
+// element inserted or removed from them, maintaining balance in the tree
+let balancedNode = (key, left, right) => {
+  let makeNodeFn = if (size(left) + size(right) < 2) {
+    makeNode
+  } else if (size(right) > weight * size(left)) {
+    // if the right tree is too much larger than the left then move part of
+    // the right tree to the left side
+    let { left: rl, right: rr, _ } = unwrapTree(right)
+    if (size(rl) < size(rr)) singleL else doubleL
+  } else if (size(left) > weight * size(right)) {
+    // if the left tree is too much larger than the right then move part of
+    // the left tree to the right side
+    let { left: ll, right: lr, _ } = unwrapTree(left)
+    if (size(lr) < size(ll)) singleR else doubleR
+  } else {
+    // if neither tree is too much larger than the other then simply create
+    // a new node
+    makeNode
+  }
+
+  makeNodeFn(key, left, right)
+}
+
+/**
+ * Produces a new set by inserting the given value into the set. If the value
+ * already exists, the new set will have the same elements as the input set.
+ *
+ * @param key: The value to add
+ * @param set: The base set
+ * @returns A new set containing the new element
+ * 
+ * @since v0.5.4
+ */
+export let rec add = (key, set) => {
+  match (set) {
+    Empty => Tree({ key, size: 1, left: Empty, right: Empty }),
+    Tree({ key: nodeKey, left, right, _ }) => {
+      match (compare(key, nodeKey)) {
+        cmp when cmp < 0 => balancedNode(nodeKey, add(key, left), right),
+        cmp when cmp > 0 => balancedNode(nodeKey, left, add(key, right)),
+        _ => makeNode(key, left, right),
+      }
+    },
+  }
+}
+
+/**
+ * Determines if the set contains the given value.
+ *
+ * @param key: The value to search for
+ * @param set: The set to search
+ * @returns `true` if the set contains the given value or `false` otherwise
+ * 
+ * @since v0.5.4
+ */
+export let rec contains = (key, set) => {
+  match (set) {
+    Empty => false,
+    Tree({ key: nodeKey, left, right, _ }) => {
+      match (compare(key, nodeKey)) {
+        cmp when cmp < 0 => contains(key, left),
+        cmp when cmp > 0 => contains(key, right),
+        _ => true,
+      }
+    },
+  }
+}
+
+// removes the minimum element from a tree
+let rec removeMin = node => {
+  match (node) {
+    Tree({ left: Empty, right, _ }) => right,
+    Tree({ key, left, right, _ }) => balancedNode(key, removeMin(left), right),
+    _ => fail "Impossible: ImmutableSet removeMin on empty node",
+  }
+}
+
+// helper function for removing a node by creating a new node containing the
+// removed node's left and right subtrees
+let removeInner = (left, right) => {
+  match ((left, right)) {
+    (Empty, node) | (node, Empty) => node,
+    (left, right) => {
+      balancedNode(min(right), left, removeMin(right))
+    },
+  }
+}
+
+/**
+ * Produces a new set without the given element. If the value doesn't exist in
+ * the set, the set will be returned unmodified.
+ *
+ * @param key: The value to exclude
+ * @param set: The set to exclude from
+ * @returns A new set without the excluded element
+ * 
+ * @since v0.5.4
+ */
+export let rec remove = (key, set) => {
+  match (set) {
+    Empty => Empty,
+    Tree({ key: nodeKey, left, right, _ }) => {
+      match (compare(key, nodeKey)) {
+        cmp when cmp < 0 => balancedNode(nodeKey, remove(key, left), right),
+        cmp when cmp > 0 => balancedNode(nodeKey, left, remove(key, right)),
+        _ => removeInner(left, right),
+      }
+    },
+  }
+}
+
+/**
+ * Iterates the set, calling an iterator function on each element.
+ *
+ * @param fn: The iterator function to call with each element
+ * @param set: The set to iterate
+ * 
+ * @since v0.5.4
+ */
+export let forEach = (fn, set) => {
+  let rec forEachInner = node => {
+    match (node) {
+      Empty => void,
+      Tree({ key, left, right, _ }) => {
+        forEachInner(left)
+        fn(key): Void
+        forEachInner(right)
+      },
+    }
+  }
+  forEachInner(set)
+}
+
+/**
+ * Combines all elements of a set using a reducer function.
+ *
+ * @param fn: The reducer function to call on each element, where the value returned will be the next accumulator value
+ * @param init: The initial value to use for the accumulator on the first iteration
+ * @param set: The set to iterate
+ * @returns The final accumulator returned from `fn`
+ * 
+ * @since v0.5.4
+ */
+export let reduce = (fn, init, set) => {
+  let rec reduceInner = (acc, node) => {
+    match (node) {
+      Empty => acc,
+      Tree({ key, left, right, _ }) => {
+        let newAcc = fn(reduceInner(acc, left), key)
+        reduceInner(newAcc, right)
+      },
+    }
+  }
+  reduceInner(init, set)
+}
+
+// joins two trees with a value, preserving the BST property of left children
+// being less the node and right children being greater than the node
+let rec concat3 = (key, left, right) => {
+  match ((left, right)) {
+    (Empty, node) | (node, Empty) => add(key, node),
+    (Tree(left) as leftOpt, Tree(right) as rightOpt) => {
+      if (weight * left.size < right.size) {
+        balancedNode(right.key, concat3(key, leftOpt, right.left), right.right)
+      } else if (weight * right.size < left.size) {
+        balancedNode(left.key, left.left, concat3(key, left.right, rightOpt))
+      } else {
+        makeNode(key, leftOpt, rightOpt)
+      }
+    },
+  }
+}
+
+// returns a tree containing all of the nodes in the input tree whose values
+// are less than the given value
+let rec splitLt = (splitKey, node) => {
+  match (node) {
+    Empty => Empty,
+    Tree({ key, left, right, _ }) => {
+      match (compare(key, splitKey)) {
+        // we want this node, join it to the output
+        cmp when cmp < 0 => concat3(key, left, splitLt(splitKey, right)),
+        cmp when cmp > 0 => splitLt(splitKey, left),
+        _ => left,
+      }
+    },
+  }
+}
+
+// returns a tree containing all of the nodes in the input tree whose values
+// are greater than the given value
+let rec splitGt = (splitKey, node) => {
+  match (node) {
+    Empty => Empty,
+    Tree({ key, left, right, _ }) => {
+      match (compare(key, splitKey)) {
+        // we want this node, join it to the output
+        cmp when cmp > 0 => concat3(key, splitGt(splitKey, left), right),
+        cmp when cmp < 0 => splitGt(splitKey, right),
+        _ => right,
+      }
+    },
+  }
+}
+
+// concatenates two trees of arbitrary size
+let concat = (node1, node2) => {
+  match (node2) {
+    Empty => node1,
+    _ => concat3(min(node2), node1, removeMin(node2)),
+  }
+}
+
+/**
+ * Produces a new set without the elements from the input set where a predicate function returns `false`.
+ *
+ * @param fn: The predicate function to indicate which elements to exclude from the set, where returning `false` indicates the value should be excluded
+ * @param set: The set to iterate
+ * @returns A new set excluding the elements not fulfilling the predicate
+ * 
+ * @since v0.5.4
+ */
+export let filter = (fn, set) => {
+  let rec filterInner = node => {
+    match (node) {
+      Empty => Empty,
+      Tree({ key, left, right, _ }) => {
+        if (fn(key)) {
+          concat3(key, filterInner(left), filterInner(right))
+        } else {
+          concat(filterInner(left), filterInner(right))
+        }
+      },
+    }
+  }
+  filterInner(set)
+}
+
+/**
+ * Produces a new set without the elements from the input set where a predicate function returns `true`.
+ *
+ * @param fn: The predicate function to indicate which elements to exclude from the set, where returning `true` indicates the value should be excluded
+ * @param set: The set to iterate
+ * @returns A new set excluding the elements fulfilling the predicate
+ * 
+ * @since v0.5.4
+ */
+export let reject = (fn, set) => {
+  filter(key => !fn(key), set)
+}
+
+/**
+ * Combines two sets into a single set containing all elements from both sets.
+ *
+ * @param set1: The first set to combine
+ * @param set2: The second set to combine
+ * @returns A set containing all elements of both sets
+ * 
+ * @since v0.5.4
+ */
+export let rec union = (set1, set2) => {
+  match ((set1, set2)) {
+    (Empty, node) | (node, Empty) => node,
+    (node1, Tree(node2)) => {
+      let l = splitLt(node2.key, node1)
+      let r = splitGt(node2.key, node1)
+      concat3(node2.key, union(l, node2.left), union(r, node2.right))
+    },
+  }
+}
+
+/**
+ * Combines two sets into a single set containing only the elements not shared between both sets.
+ *
+ * @param set1: The first set to combine
+ * @param set2: The second set to combine
+ * @returns A set containing only unshared elements from both sets
+ * 
+ * @since v0.5.4
+ */
+export let diff = (set1, set2) => {
+  let rec diffInner = (node1, node2) => {
+    match ((node1, node2)) {
+      (Empty, node) | (node, Empty) => node,
+      (node1, Tree(node2)) => {
+        let l = splitLt(node2.key, node1)
+        let r = splitGt(node2.key, node1)
+        concat(diffInner(l, node2.left), diffInner(r, node2.right))
+      },
+    }
+  }
+  union(diffInner(set1, set2), diffInner(set2, set1))
+}
+
+/**
+ * Combines two sets into a single set containing only the elements shared between both sets.
+ *
+ * @param set1: The first set to combine
+ * @param set2: The second set to combine
+ * @returns A set containing only shared elements from both sets
+ * 
+ * @since v0.5.4
+ */
+export let rec intersect = (set1, set2) => {
+  match ((set1, set2)) {
+    (Empty, _) | (_, Empty) => Empty,
+    (node1, Tree(node2)) => {
+      let l = splitLt(node2.key, node1)
+      let r = splitGt(node2.key, node1)
+      if (contains(node2.key, node1)) {
+        concat3(node2.key, intersect(l, node2.left), intersect(r, node2.right))
+      } else {
+        concat(intersect(l, node2.left), intersect(r, node2.right))
+      }
+    },
+  }
+}
+
+/**
+ * Creates a set from a list.
+ *
+ * @param list: The list to convert
+ * @returns A set containing all list values
+ * 
+ * @since v0.5.4
+ */
+export let fromList = list => {
+  List.reduce((set, key) => add(key, set), empty, list)
+}
+
+/**
+ * Converts a set into a list of its elements.
+ *
+ * @param set: The set to convert
+ * @returns A list containing all set values
+ * 
+ * @since v0.5.4
+ */
+export let toList = set => {
+  let rec toListInner = (acc, node) => {
+    match (node) {
+      Empty => acc,
+      Tree({ key, left, right, _ }) => {
+        toListInner([key, ...toListInner(acc, right)], left)
+      },
+    }
+  }
+  toListInner([], set)
+}
+
+/**
+ * Creates a set from an array.
+ *
+ * @param array: The array to convert
+ * @returns A set containing all array values
+ * 
+ * @since v0.5.4
+ */
+export let fromArray = array => {
+  Array.reduce((set, key) => add(key, set), empty, array)
+}
+
+/**
+ * Converts a set into an array of its elements.
+ *
+ * @param set: The set to convert
+ * @returns An array containing all set values
+ * 
+ * @since v0.5.4
+ */
+export let toArray = set => {
+  Array.fromList(toList(set))
+}
diff --git a/stdlib/immutableset.md b/stdlib/immutableset.md
new file mode 100644
index 0000000000..0e56706088
--- /dev/null
+++ b/stdlib/immutableset.md
@@ -0,0 +1,449 @@
+---
+title: ImmutableSet
+---
+
+An ImmutableSet is a collection of unique values. Operations on an ImmutableSet do not mutate the set's internal state.
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+import ImmutableSet from "immutableset"
+```
+
+## Types
+
+Type declarations included in the ImmutableSet module.
+
+### ImmutableSet.**ImmutableSet**
+
+```grain
+type ImmutableSet<a>
+```
+
+## Values
+
+Functions and constants for working with ImmutableSets.
+
+### ImmutableSet.**empty**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+empty : ImmutableSet<a>
+```
+
+An empty set
+
+### ImmutableSet.**size**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+size : ImmutableSet<a> -> Number
+```
+
+Provides the count of values within the set.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`set`|`ImmutableSet<a>`|The set to inspect|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Number`|The count of elements in the set|
+
+### ImmutableSet.**isEmpty**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+isEmpty : ImmutableSet<a> -> Bool
+```
+
+Determines if the set contains no elements.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`set`|`ImmutableSet<a>`|The set to inspect|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Bool`|`true` if the given set is empty or `false` otherwise|
+
+### ImmutableSet.**add**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+add : (a, ImmutableSet<a>) -> ImmutableSet<a>
+```
+
+Produces a new set by inserting the given value into the set. If the value
+already exists, the new set will have the same elements as the input set.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`key`|`a`|The value to add|
+|`set`|`ImmutableSet<a>`|The base set|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableSet<a>`|A new set containing the new element|
+
+### ImmutableSet.**contains**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+contains : (a, ImmutableSet<a>) -> Bool
+```
+
+Determines if the set contains the given value.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`key`|`a`|The value to search for|
+|`set`|`ImmutableSet<a>`|The set to search|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Bool`|`true` if the set contains the given value or `false` otherwise|
+
+### ImmutableSet.**remove**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+remove : (a, ImmutableSet<a>) -> ImmutableSet<a>
+```
+
+Produces a new set without the given element. If the value doesn't exist in
+the set, the set will be returned unmodified.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`key`|`a`|The value to exclude|
+|`set`|`ImmutableSet<a>`|The set to exclude from|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableSet<a>`|A new set without the excluded element|
+
+### ImmutableSet.**forEach**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+forEach : ((a -> Void), ImmutableSet<a>) -> Void
+```
+
+Iterates the set, calling an iterator function on each element.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`a -> Void`|The iterator function to call with each element|
+|`set`|`ImmutableSet<a>`|The set to iterate|
+
+### ImmutableSet.**reduce**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+reduce : (((a, b) -> a), a, ImmutableSet<b>) -> a
+```
+
+Combines all elements of a set using a reducer function.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`(a, b) -> a`|The reducer function to call on each element, where the value returned will be the next accumulator value|
+|`init`|`a`|The initial value to use for the accumulator on the first iteration|
+|`set`|`ImmutableSet<b>`|The set to iterate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`a`|The final accumulator returned from `fn`|
+
+### ImmutableSet.**filter**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+filter : ((a -> Bool), ImmutableSet<a>) -> ImmutableSet<a>
+```
+
+Produces a new set without the elements from the input set where a predicate function returns `false`.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`a -> Bool`|The predicate function to indicate which elements to exclude from the set, where returning `false` indicates the value should be excluded|
+|`set`|`ImmutableSet<a>`|The set to iterate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableSet<a>`|A new set excluding the elements not fulfilling the predicate|
+
+### ImmutableSet.**reject**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+reject : ((a -> Bool), ImmutableSet<a>) -> ImmutableSet<a>
+```
+
+Produces a new set without the elements from the input set where a predicate function returns `true`.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`a -> Bool`|The predicate function to indicate which elements to exclude from the set, where returning `true` indicates the value should be excluded|
+|`set`|`ImmutableSet<a>`|The set to iterate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableSet<a>`|A new set excluding the elements fulfilling the predicate|
+
+### ImmutableSet.**union**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+union : (ImmutableSet<a>, ImmutableSet<a>) -> ImmutableSet<a>
+```
+
+Combines two sets into a single set containing all elements from both sets.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`set1`|`ImmutableSet<a>`|The first set to combine|
+|`set2`|`ImmutableSet<a>`|The second set to combine|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableSet<a>`|A set containing all elements of both sets|
+
+### ImmutableSet.**diff**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+diff : (ImmutableSet<a>, ImmutableSet<a>) -> ImmutableSet<a>
+```
+
+Combines two sets into a single set containing only the elements not shared between both sets.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`set1`|`ImmutableSet<a>`|The first set to combine|
+|`set2`|`ImmutableSet<a>`|The second set to combine|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableSet<a>`|A set containing only unshared elements from both sets|
+
+### ImmutableSet.**intersect**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+intersect : (ImmutableSet<a>, ImmutableSet<a>) -> ImmutableSet<a>
+```
+
+Combines two sets into a single set containing only the elements shared between both sets.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`set1`|`ImmutableSet<a>`|The first set to combine|
+|`set2`|`ImmutableSet<a>`|The second set to combine|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableSet<a>`|A set containing only shared elements from both sets|
+
+### ImmutableSet.**fromList**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+fromList : List<a> -> ImmutableSet<a>
+```
+
+Creates a set from a list.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`list`|`List<a>`|The list to convert|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableSet<a>`|A set containing all list values|
+
+### ImmutableSet.**toList**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+toList : ImmutableSet<a> -> List<a>
+```
+
+Converts a set into a list of its elements.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`set`|`ImmutableSet<a>`|The set to convert|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`List<a>`|A list containing all set values|
+
+### ImmutableSet.**fromArray**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+fromArray : Array<a> -> ImmutableSet<a>
+```
+
+Creates a set from an array.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`array`|`Array<a>`|The array to convert|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`ImmutableSet<a>`|A set containing all array values|
+
+### ImmutableSet.**toArray**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+toArray : ImmutableSet<a> -> Array<a>
+```
+
+Converts a set into an array of its elements.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`set`|`ImmutableSet<a>`|The set to convert|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Array<a>`|An array containing all set values|
+
diff --git a/stdlib/set.gr b/stdlib/set.gr
index 9551b57807..142e760934 100644
--- a/stdlib/set.gr
+++ b/stdlib/set.gr
@@ -2,7 +2,7 @@
  * @module Set: A Set is an unordered collection of unique values. Operations on a Set mutate the internal state, so it never needs to be re-assigned.
  * @example import Set from "set"
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 import List from "list"
 import Array from "array"
@@ -33,7 +33,7 @@ record Set<k> {
  * @param size: The initial storage size of the set
  * @returns An empty set with the given initial storage size
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let makeSized = size => {
   let buckets = Array.make(size, None)
@@ -44,7 +44,7 @@ export let makeSized = size => {
  *
  * @returns An empty set
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let make = () => {
   makeSized(16)
@@ -121,7 +121,7 @@ let rec nodeInBucket = (key, node) => {
  * @param key: The value to add
  * @param set: The set to update
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let add = (key, set) => {
   let buckets = set.buckets
@@ -154,7 +154,7 @@ export let add = (key, set) => {
  * @param set: The set to search
  * @returns `true` if the set contains the given value or `false` otherwise
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let contains = (key, set) => {
   let buckets = set.buckets
@@ -186,7 +186,7 @@ let rec removeInBucket = (key, node) => {
  * @param key: The value to remove
  * @param set: The set to update
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let remove = (key, set) => {
   let buckets = set.buckets
@@ -214,7 +214,7 @@ export let remove = (key, set) => {
  * @param set: The set to inspect
  * @returns The count of elements in the set
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let size = set => {
   set.size
@@ -226,7 +226,7 @@ export let size = set => {
  * @param set: The set to inspect
  * @returns `true` if the given set is empty or `false` otherwise
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let isEmpty = set => {
   size(set) == 0
@@ -237,7 +237,7 @@ export let isEmpty = set => {
  *
  * @param set: The set to reset
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let clear = set => {
   set.size = 0
@@ -263,7 +263,7 @@ let rec forEachBucket = (fn, node) => {
  * @param fn: The iterator function to call with each element
  * @param set: The set to iterate
  * 
- * @since 0.3.0
+ * @since v0.3.0
  * @history v0.5.0: Ensured the iterator function return type is always `Void`
  */
 export let forEach = (fn, set) => {
@@ -288,7 +288,7 @@ let rec reduceEachBucket = (fn, node, acc) => {
  * @param set: The set to iterate
  * @returns The final accumulator returned from `fn`
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let reduce = (fn, init, set) => {
   let buckets = set.buckets
@@ -305,10 +305,10 @@ export let reduce = (fn, init, set) => {
  * @param fn: The predicate function to indicate which elements to remove from the set, where returning `false` indicates the value should be removed
  * @param set: The set to iterate
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
-export let filter = (predicate, set) => {
-  let keysToRemove = reduce((list, key) => if (!predicate(key)) {
+export let filter = (fn, set) => {
+  let keysToRemove = reduce((list, key) => if (!fn(key)) {
     [key, ...list]
   } else {
     list
@@ -324,10 +324,10 @@ export let filter = (predicate, set) => {
  * @param fn: The predicate function to indicate which elements to remove from the set, where returning `true` indicates the value should be removed
  * @param set: The set to iterate
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
-export let reject = (predicate, set) => {
-  filter(key => !predicate(key), set)
+export let reject = (fn, set) => {
+  filter(key => !fn(key), set)
 }
 
 /**
@@ -336,7 +336,7 @@ export let reject = (predicate, set) => {
  * @param set: The set to convert
  * @returns A list containing all set values
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let toList = set => {
   reduce((list, key) => [key, ...list], [], set)
@@ -348,7 +348,7 @@ export let toList = set => {
  * @param list: The list to convert
  * @returns A set containing all list values
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let fromList = list => {
   let set = make()
@@ -364,7 +364,7 @@ export let fromList = list => {
  * @param set: The set to convert
  * @returns An array containing all set values
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let toArray = set => {
   Array.fromList(toList(set))
@@ -376,7 +376,7 @@ export let toArray = set => {
  * @param array: The array to convert
  * @returns A set containing all array values
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let fromArray = array => {
   let set = make()
@@ -393,7 +393,7 @@ export let fromArray = array => {
  * @param set2: The second set to combine
  * @returns A set containing all elements of both sets
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let union = (set1, set2) => {
   let set = make()
@@ -413,7 +413,7 @@ export let union = (set1, set2) => {
  * @param set2: The second set to combine
  * @returns A set containing only unshared elements from both sets
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let diff = (set1, set2) => {
   let set = make()
@@ -437,7 +437,7 @@ export let diff = (set1, set2) => {
  * @param set2: The second set to combine
  * @returns A set containing only shared elements from both sets
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let intersect = (set1, set2) => {
   let set = make()
@@ -461,7 +461,7 @@ export let intersect = (set1, set2) => {
  * @param set: The set to inspect
  * @returns The internal state of the set
  * 
- * @since 0.3.0
+ * @since v0.3.0
  */
 export let getInternalStats = set => {
   (set.size, Array.length(set.buckets))