"Galchin, Vasili"
ok guys .. what is this "phantom type" concept? Is it a type theory thing or just Haskell type concept?
Here's another example. Say you want to use bytestrings with different encodings. You obviously don't want to concatenate a string representing Latin characters with a string in Cyrillic. One way to do this, is to define phantom types for the encodings, and a bytestring type that takes additional type parameter data KOI8 data ISO8859_1 : data Bytestring enc = MkBS ... Operations like concat work on same-typed bytestrings: concat :: Bytestring e -> Bytestring e -> Bytestring e The parameter (enc) isn't used on the right hand side, so all Bytestrings will have the same representation, but Bytestring KOI8 and Bytestring ISO8859_1 will have different types, so although the runtime won't know the difference, trying to 'concat' them will give you a type error at compile time. -k -- If I haven't seen further, it is by standing in the footprints of giants