add slice

src/Data/String.js

@@ -131,6 +131,14 @@ exports.drop = function (n) {
   };
 };
 
+exports._slice = function (b) {
+  return function (e) {
+    return function (s) {
+      return s.slice(b,e);
+    };
+  };
+};
+
 exports.count = function (p) {
   return function (s) {
     var i = 0;

src/Data/String.purs

@@ -26,6 +26,7 @@ module Data.String
   , drop
   , dropRight
   , dropWhile
+  , slice
   , stripPrefix
   , stripSuffix
   , count
@@ -172,6 +173,33 @@ takeWhile p s = take (count p s) s
 dropWhile :: (Char -> Boolean) -> String -> String
 dropWhile p s = drop (count p s) s
 
+-- | Returns the substring at indices `[begin, end)`.
+-- | If either index is negative, it is normalised to `length s - index`,
+-- | where `s` is the input string. `Nothing` is returned if either
+-- | index is out of bounds or if `begin > end` after normalisation.
+-- |
+-- | ```purescript
+-- | slice 0 0   "purescript" == Just ""
+-- | slice 0 1   "purescript" == Just "p"
+-- | slice 3 6   "purescript" == Just "esc"
+-- | slice (-4) (-1) "purescript" == Just "rip"
+-- | slice (-4) 3  "purescript" == Nothing
+-- | ```
+slice :: 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.
 -- |

test/Test/Data/String.purs

@@ -206,3 +206,10 @@ testString = do
   assert $ joinWith "" [] == ""
   assert $ joinWith "" ["a", "b"] == "ab"
   assert $ joinWith "--" ["a", "b", "c"] == "a--b--c"
+
+  log "slice"
+  assert $ slice 0 0   "purescript" == Just ""
+  assert $ slice 0 1   "purescript" == Just "p"
+  assert $ slice 3 6   "purescript" == Just "esc"
+  assert $ slice (-4) (-1) "purescript" == Just "rip"
+  assert $ slice (-4) 3  "purescript" == Nothing