Merge pull request #26 from purescript/global-region

Add global region and MonadST

Author: garyb

bower.json

@@ -19,7 +19,8 @@
   "dependencies": {
     "purescript-prelude": "^4.0.0",
     "purescript-tailrec": "^4.0.0",
-    "purescript-partial": "^2.0.0"
+    "purescript-partial": "^2.0.0",
+    "purescript-unsafe-coerce": "^4.0.0"
   },
   "devDependencies": {
     "purescript-console": "^4.0.0"

src/Control/Monad/ST/Class.purs

@@ -0,0 +1,17 @@
+module Control.Monad.ST.Class where
+
+import Prelude
+
+import Control.Monad.ST (ST)
+import Control.Monad.ST.Global (Global)
+import Control.Monad.ST.Global as Global
+import Effect (Effect)
+
+class MonadST s m | m -> s where
+  liftST :: ST s ~> m
+
+instance monadSTEffect :: MonadST Global Effect where
+  liftST = Global.toEffect
+
+instance monadSTST :: MonadST s (ST s) where
+  liftST = identity

src/Control/Monad/ST/Global.purs

@@ -0,0 +1,18 @@
+module Control.Monad.ST.Global
+  ( Global
+  , toEffect
+  ) where
+
+import Prelude
+
+import Control.Monad.ST (ST, kind Region)
+import Effect (Effect)
+import Unsafe.Coerce (unsafeCoerce)
+
+-- | This region allows `ST` computations to be converted into `Effect`
+-- | computations so they can be run in a global context.
+foreign import data Global :: Region
+
+-- | Converts an `ST` computation into an `Effect` computation.
+toEffect :: ST Global ~> Effect
+toEffect = unsafeCoerce