Add toMap and fromMap to Data.Set.

bower.json

@@ -26,8 +26,7 @@
     "purescript-st": "master",
     "purescript-tailrec": "master",
     "purescript-tuples": "master",
-    "purescript-unfoldable": "master",
-    "purescript-unsafe-coerce": "master"
+    "purescript-unfoldable": "master"
   },
   "devDependencies": {
     "purescript-quickcheck": "master",

src/Data/Map.purs

@@ -6,8 +6,9 @@ module Data.Map
 import Prelude
 
 import Data.Map.Internal (Map, alter, catMaybes, checkValid, delete, empty, filter, filterKeys, filterWithKey, findMax, findMin, foldSubmap, fromFoldable, fromFoldableWith, fromFoldableWithIndex, insert, insertWith, isEmpty, isSubmap, lookup, lookupGE, lookupGT, lookupLE, lookupLT, member, pop, showTree, singleton, size, submap, toUnfoldable, toUnfoldableUnordered, union, unionWith, unions, intersection, intersectionWith, difference, update, values, mapMaybeWithKey, mapMaybe)
-import Data.Set (Set)
-import Unsafe.Coerce (unsafeCoerce)
+import Data.Set (Set, fromMap)
 
+-- | The set of keys of the given map.
+-- | See also `Data.Set.fromMap`.
 keys :: forall k v. Map k v -> Set k
-keys = (unsafeCoerce :: Map k Unit -> Set k) <<< void
+keys = fromMap <<< void

src/Data/Set.purs

@@ -27,6 +27,8 @@ module Data.Set
   , filter
   , mapMaybe
   , catMaybes
+  , toMap
+  , fromMap
   ) where
 
 import Prelude hiding (map)
@@ -195,3 +197,12 @@ mapMaybe f = foldr (\a acc -> maybe acc (\b -> insert b acc) (f a)) empty
 -- | Filter a set of optional values, discarding values that contain `Nothing`
 catMaybes :: forall a. Ord a => Set (Maybe a) -> Set a
 catMaybes = mapMaybe identity
+
+-- | A set is a map with no value attached to each key.
+toMap :: forall a. Set a -> M.Map a Unit
+toMap (Set s) = s
+
+-- | A map with no value attached to each key is a set.
+-- | See also `Data.Map.keys`.
+fromMap :: forall a. M.Map a Unit -> Set a
+fromMap = Set