improve Unsafe

README.md

@@ -434,7 +434,7 @@ charCodeAt :: Number -> String -> Number
 
 Returns the numeric Unicode value of the character at the given index.
 
-**Unsafe:** returns `NaN` if the index is out of bounds.
+**Unsafe:** throws runtime exception if the index is out of bounds.
 
 #### `charAt`
 
@@ -444,7 +444,17 @@ charAt :: Number -> String -> Char
 
 Returns the character at the given index.
 
-**Unsafe:** returns an illegal value if the index is out of bounds.
+**Unsafe:** throws runtime exception if the index is out of bounds.
+
+#### `char`
+
+``` purescript
+char :: String -> Char
+```
+
+Converts a string of length `1` to a character.
+
+**Unsafe:** throws runtime exception if length is not `1`.
 
 
 

src/Data/String/Unsafe.purs

@@ -1,31 +1,51 @@
 -- | Unsafe string and character functions.
 module Data.String.Unsafe
-  ( charAt
+  ( char
+  , charAt
   , charCodeAt
   ) where
 
   import Data.Char
 
   -- | Returns the numeric Unicode value of the character at the given index.
   -- |
-  -- | **Unsafe:** returns `NaN` if the index is out of bounds.
+  -- | **Unsafe:** throws runtime exception if the index is out of bounds.
   foreign import charCodeAt
     """
     function charCodeAt(i) {
       return function(s) {
+        if (s.length <= i) {
+          throw new Error("Data.String.Unsafe.charCodeAt: Invalid index.");
+        };
         return s.charCodeAt(i);
       };
     }
     """ :: Number -> String -> Number
 
   -- | Returns the character at the given index.
   -- |
-  -- | **Unsafe:** returns an illegal value if the index is out of bounds.
+  -- | **Unsafe:** throws runtime exception if the index is out of bounds.
   foreign import charAt
     """
     function charAt(i) {
       return function(s) {
+        if (s.length <= i) {
+          throw new Error("Data.String.Unsafe.charAt: Invalid index.");
+        };
         return s.charAt(i);
       };
     }
     """ :: Number -> String -> Char
+
+  -- | Converts a string of length `1` to a character.
+  -- |
+  -- | **Unsafe:** throws runtime exception if length is not `1`.
+  foreign import char
+    """
+    function $$char(s) {
+      if (s.length != 1) {
+        throw new Error("Data.String.Unsafe.char: Expected string of length 1.");
+      };
+      return s.charAt(0);
+    }
+    """ :: String -> Char