check bounds etc

Author: themattchan

src/Data/String.js

@@ -136,8 +136,12 @@ exports._slice = function (just) {
     return function (b) {
       return function (e) {
         return function (s) {
-          var res = s.slice(b, e);
-          return res.length === 0 ? nothing : just(res);
+          var b1 = b < 0 ? s.length + b : b;
+          var e1 = e < 0 ? s.length + e : e;
+          if ( b1 < 0 || e1 >= s.length || b1 > e1 )
+            return nothing;
+          else
+            return just(s.slice(b,e));
         };
       };
     };

src/Data/String.purs

@@ -179,10 +179,11 @@ dropWhile p s = drop (count p s) s
 -- | 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 "ecr"
--- | slice -4 -1 "purescript" == Just "rip"
--- | slice -4 3  "purescript" == Nothing
+-- | slice 3 6   "purescript" == Just "esc"
+-- | slice (-4) (-1) "purescript" == Just "rip"
+-- | slice (-4) 3  "purescript" == Nothing
 -- | ```
 slice :: Int -> Int -> String -> Maybe String
 slice = _slice Just 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