Add stripPrefix

docs/Data/String.md

@@ -78,6 +78,17 @@ dropWhile :: (Char -> Boolean) -> String -> String
 
 Returns the suffix remaining after `takeWhile`.
 
+#### `stripPrefix`
+
+``` purescript
+stripPrefix :: String -> String -> Maybe 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.
+* `stripPrefix "http:" "http://purescript.org" == Just "//purescript.org"
+* `stripPrefix "http:" "https://purescript.org" == Nothing
+
 #### `fromCharArray`
 
 ``` purescript

src/Data/String.purs

@@ -23,6 +23,7 @@ module Data.String
   , takeWhile
   , drop
   , dropWhile
+  , stripPrefix
   , split
   , toCharArray
   , toLower
@@ -94,6 +95,16 @@ takeWhile p s = take (count p s) s
 dropWhile :: (Char -> Boolean) -> String -> String
 dropWhile p s = drop (count p s) s
 
+-- | 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.
+-- | * `stripPrefix "http:" "http://purescript.org" == Just "//purescript.org"
+-- | * `stripPrefix "http:" "https://purescript.org" == Nothing
+stripPrefix :: String -> String -> Maybe String
+stripPrefix prefix str =
+  case indexOf prefix str of
+    Just 0 -> Just $ drop (length prefix) str
+    _      -> Nothing
+
 -- | Converts an array of characters into a string.
 foreign import fromCharArray :: Array Char -> String