Merge pull request #100 from purescript/span-tco

Restore compiler-optimized TCO for span

Author: garyb

src/Data/Array.purs

@@ -113,7 +113,7 @@ import Prelude
 import Control.Alt ((<|>))
 import Control.Alternative (class Alternative)
 import Control.Lazy (class Lazy, defer)
-import Control.Monad.Rec.Class (class MonadRec, Step(..), tailRecM2, tailRec)
+import Control.Monad.Rec.Class (class MonadRec, Step(..), tailRecM2)
 import Control.Monad.ST (pureST)
 import Data.Array.ST (unsafeFreeze, emptySTArray, pushSTArray)
 import Data.Array.ST.Iterator (iterator, iterate, pushWhile)
@@ -521,13 +521,13 @@ span p arr =
       { init: arr, rest: [] }
   where
   breakIndex = go 0
-  go = tailRec \i ->
+  go i =
     -- This looks like a good opportunity to use the Monad Maybe instance,
     -- but it's important to write out an explicit case expression here in
     -- order to ensure that TCO is triggered.
     case index arr i of
-      Just x -> if p x then Loop (i + 1) else Done (Just i)
-      Nothing -> Done Nothing
+      Just x -> if p x then go (i + 1) else Just i
+      Nothing -> Nothing
 
 -- | Group equal, consecutive elements of an array into arrays.
 -- |