stdlib: Add set

README.md

@@ -234,6 +234,11 @@ The type system looks as follows:
   -- `get i x` gets the i'th thing out of x. I should be (evaluate to) a
   -- number or a string, with x evaluating to array or object, respectively.
   get   : JSON → JSON → JSON
+
+  -- `set i x` coll sets the i'th thing in coll to x, where i
+  -- should be a number or a string, and coll should evaluate to
+  -- an array or object.
+  set   : JSON → JSON → JSON → JSON
   ```
 
 ## REPL

src/eval.rs

@@ -50,6 +50,12 @@ impl Display for EvalError {
   }
 }
 
+macro_rules! eval_err {
+  (WrongIndex, $ix:expr, $x:expr $(,)?) => {
+    EvalError::WrongIndex($ix.to_string(), $x.reify()?)
+  };
+}
+
 fn num_vars(names: &[String], var: &str) -> isize {
   names.iter().filter(|&v| v == var).count() as isize
 }
@@ -77,6 +83,8 @@ impl Sem {
     Sem::App(Box::new(self.clone()), Box::new(s2.clone()))
   }
 
+  fn str(s: &str) -> Self { Self::SConst(Const::String(s.to_string())) }
+
   fn is_truthy(&self) -> bool {
     !matches!(
       self,
@@ -144,12 +152,30 @@ impl Sem {
       // Get
       (App(box SBuiltin(Get), box SConst(Const::Num(i))), Arr(xs)) => xs
         .get(**i as usize)
-        .ok_or(EvalError::WrongIndex(i.to_string(), x.reify()?))
+        .ok_or(eval_err!(WrongIndex, i, x))
         .cloned(),
       (App(box SBuiltin(Get), box SConst(Const::String(s))), Obj(ob)) => ob
-        .get(&SConst(Const::String(s.to_string())))
-        .ok_or(EvalError::WrongIndex(s.to_string(), x.reify()?))
+        .get(&Sem::str(s))
+        .ok_or(eval_err!(WrongIndex, s, x))
         .cloned(),
+      // Set
+      (App(box App(box SBuiltin(Set), box SConst(Const::Num(i))), box to), Arr(xs)) => {
+        let mut ys = xs.clone();
+        *ys
+          .get_mut(**i as usize)
+          .ok_or(eval_err!(WrongIndex, i, x))? = to.clone();
+        Ok(Arr(ys))
+      },
+      (
+        App(box App(box SBuiltin(Set), box SConst(Const::String(s))), box to),
+        Obj(ob),
+      ) => {
+        let mut res = ob.clone();
+        *res
+          .get_mut(&Sem::str(s))
+          .ok_or(eval_err!(WrongIndex, s, x))? = to.clone();
+        Ok(Obj(res))
+      },
       // Map
       (App(box SBuiltin(Map), closure), Arr(xs)) => {
         Ok(Arr(xs.iter().flat_map(|x| closure.apply(x)).collect()))

src/eval/stdlib.rs

@@ -12,6 +12,7 @@ pub enum Builtin {
   Id,
   BConst,
   Get,
+  Set,
   Map,
   Filter,
   Foldl,
@@ -147,6 +148,23 @@ pub static STDLIB: LazyLock<BTreeMap<Builtin, StdFun>> = LazyLock::new(|| {
         .fancy("x")
         .plain("should evaluate to an array or object.")
     ),
+    mk_fun!(
+      Builtin::Set,
+      Type::arr(JSON, Type::arr(JSON, Type::arr(JSON, JSON))),
+      Blocks::new()
+        .fancy("set i x coll")
+        .plain("sets the")
+        .fancy("i'th")
+        .plain("thing in")
+        .fancy("coll")
+        .plain("to")
+        .fancy("x,")
+        .plain("where")
+        .fancy("i")
+        .plain("should be a number or a string, and")
+        .fancy("coll")
+        .plain("should evaluate to an array or object.")
+    ),
     mk_fun!(
       Builtin::Map, // (a → b) → [a] → [b]
       Type::forall(

src/eval/stdlib/pretty.rs

@@ -8,6 +8,7 @@ impl Builtin {
       Builtin::Id => "id",
       Builtin::BConst => "const",
       Builtin::Get => "get",
+      Builtin::Set => "set",
       Builtin::Map => "map",
       Builtin::Filter => "filter",
       Builtin::Foldl => "foldl",

src/eval/test.rs

@@ -2,7 +2,7 @@
 mod evaluator {
   use std::assert_matches::assert_matches;
 
-  use crate::{eval::stdlib::{STDLIB_CTX, STDLIB_TYPES}, expr::{arr, λ, num, obj, parser::parse, var, Expr}, TCExpr};
+  use crate::{eval::stdlib::{STDLIB_CTX, STDLIB_TYPES}, expr::{arr, expr_str, num, obj, parser::parse, var, λ, Expr}, TCExpr};
 
   macro_rules! eval_eq {
     ($left:expr, $right:expr $(,)?) => {
@@ -73,6 +73,8 @@ mod evaluator {
     eval_eq!("map (- 1) [1, 2, 3, 4]", arr(&[num(0), num(1), num(2), num(3)]));
     eval_eq!("map (1 +) [1, 2, 3, 4]", arr(&[num(2), num(3), num(4), num(5)]));
     eval_eq!("(filter (get \"age\" | (>= 42)) | map .age | foldl (+) 0) [{name: \"A\", age: 43},{name:\"B\"},{name:\"C\", age:42}]", num(85));
+    eval_eq!("set 1 2 [ 1, 10, 3, 4 ]", arr(&[num(1), num(2), num(3), num(4)]));
+    eval_eq!("(λx → set 1 x.name [ 1, 10, 3, 4 ]) { name: \"bob\" }", arr(&[num(1), expr_str("bob"), num(3), num(4)]));
   }
 
 }