move bounds checking logic to purescript

Author: themattchan

src/Data/String.js

@@ -131,22 +131,10 @@ exports.drop = function (n) {
   };
 };
 
-exports._slice = function (just) {
-  return function (nothing) {
-    return function (b) {
-      return function (e) {
-        return function (s) {
-          var b1 = b < 0 ? s.length + b : b;
-          var e1 = e < 0 ? s.length + e : e;
-          if (  b1 < 0 || b1 >= s.length
-             || e1 < 0 || e1 >= s.length
-             || b1 > e1
-             )
-            return nothing;
-          else
-            return just(s.slice(b,e));
-        };
-      };
+exports._slice = function (b) {
+  return function (e) {
+    return function (s) {
+      return s.slice(b,e);
     };
   };
 };

src/Data/String.purs

@@ -186,12 +186,19 @@ dropWhile p s = drop (count p s) s
 -- | slice (-4) 3  "purescript" == Nothing
 -- | ```
 slice :: Int -> Int -> String -> Maybe String
-slice = _slice Just Nothing
-
-foreign import _slice
-  :: (forall a. a -> Maybe a)
-  -> (forall a. Maybe a)
-  -> Int -> Int -> String -> Maybe String
+slice b e s = if b' < 0 || b' >= l ||
+                 e' < 0 || e' >= l ||
+                 b' > e'
+              then Nothing
+              else Just (_slice b e s)
+  where
+    l = length s
+    norm x | x < 0 = l + x
+           | otherwise = x
+    b' = norm b
+    e' = norm e
+
+foreign import _slice :: Int -> Int -> String -> String
 
 -- | If the string starts with the given prefix, return the portion of the
 -- | string left after removing it, as a Just value. Otherwise, return Nothing.