Add runSTArray for efficiently constructing arrays using ST.

Haskell has this already. Also add a test for withArray.

Author: Dretch

src/Data/Array/ST.purs

@@ -5,6 +5,7 @@
 module Data.Array.ST
   ( STArray(..)
   , Assoc
+  , runSTArray
   , withArray
   , empty
   , peek
@@ -25,7 +26,7 @@ module Data.Array.ST
 
 import Prelude
 
-import Control.Monad.ST (ST, kind Region)
+import Control.Monad.ST (ST, kind Region, run)
 import Data.Maybe (Maybe(..))
 import Unsafe.Coerce (unsafeCoerce)
 
@@ -41,6 +42,13 @@ foreign import data STArray :: Region -> Type -> Type
 -- | An element and its index.
 type Assoc a = { value :: a, index :: Int }
 
+-- | A safe way to create and work with a mutable array before returning an
+-- | immutable array for later perusal. This function avoids copying the array
+-- | before returning it - it uses unsafeFreeze internally, but this wrapper is
+-- | a safe interface to that function.
+runSTArray :: forall a. (forall h. ST h (STArray h a)) -> Array a
+runSTArray st = run (st >>= unsafeFreeze)
+
 -- | Perform an effect requiring a mutable array on a copy of an immutable array,
 -- | safely returning the result as an immutable array.
 withArray

test/Test/Data/Array/ST.purs

@@ -4,7 +4,7 @@ import Prelude
 
 import Control.Monad.ST (ST)
 import Control.Monad.ST as ST
-import Data.Array.ST (STArray)
+import Data.Array.ST (STArray, withArray)
 import Data.Array.ST as STA
 import Data.Foldable (all)
 import Data.Maybe (Maybe(..), isNothing)
@@ -18,6 +18,20 @@ run act = ST.run (act >>= STA.unsafeFreeze)
 testArrayST :: Effect Unit
 testArrayST = do
 
+  log "runSTArray should produce an immutable array by running a constructor operation"
+
+  assert $ STA.runSTArray (do
+    arr <- STA.empty
+    void $ STA.push 1 arr
+    void $ STA.push 2 arr
+    pure arr) == [1, 2]
+
+  log "withArray should run an operation on a copy of an array"
+
+  let original = [1, 2, 3]
+  assert $ ST.run (withArray (STA.push 42) original) == [1, 2, 3, 42]
+  assert $ original == [1, 2, 3]
+
   log "empty should produce an empty array"
 
   assert $ run STA.empty == nil