ST.unsafeThaw (purescript#123)

addresses purescript#118

Author: matthewleon

src/Data/Array/ST.purs

@@ -14,8 +14,10 @@ module Data.Array.ST
   , modifySTArray
   , pushAllSTArray
   , spliceSTArray
-  , freeze, thaw
+  , freeze
+  , thaw
   , unsafeFreeze
+  , unsafeThaw
   , toAssocArray
   ) where
 
@@ -66,6 +68,11 @@ withArray f xs = do
 unsafeFreeze :: forall a r h. STArray h a -> Eff (st :: ST h | r) (Array a)
 unsafeFreeze = pure <<< (unsafeCoerce :: STArray h a -> Array a)
 
+-- | O(1) Convert an immutable array to a mutable array, without copying. The input
+-- | array must not be used afterward.
+unsafeThaw :: forall a r h. Array a -> Eff (st :: ST h | r) (STArray h a)
+unsafeThaw = pure <<< (unsafeCoerce :: Array a -> STArray h a)
+
 -- | Create an empty mutable array.
 foreign import emptySTArray :: forall a h r. Eff (st :: ST h | r) (STArray h a)
 

test/Test/Data/Array/ST.purs

@@ -4,7 +4,7 @@ import Prelude
 import Control.Monad.Eff (Eff)
 import Control.Monad.Eff.Console (log, CONSOLE)
 import Control.Monad.ST (ST, pureST)
-import Data.Array.ST (STArray, emptySTArray, freeze, peekSTArray, pokeSTArray, pushAllSTArray, pushSTArray, spliceSTArray, thaw, toAssocArray, unsafeFreeze)
+import Data.Array.ST (STArray, emptySTArray, freeze, peekSTArray, pokeSTArray, pushAllSTArray, pushSTArray, spliceSTArray, thaw, toAssocArray, unsafeThaw, unsafeFreeze)
 import Data.Foldable (all)
 import Data.Maybe (Maybe(..), isNothing)
 import Test.Assert (assert, ASSERT)
@@ -29,6 +29,10 @@ testArrayST = do
     arr <- thaw [1, 2, 3]
     freeze arr) == [1, 2, 3]
 
+  log "unsafeThaw should produce an STArray from a standard array"
+
+  assert $ run (unsafeThaw [1, 2, 3]) == [1, 2, 3]
+
   log "pushSTArray should append a value to the end of the array"
 
   assert $ run (do