追い討ちReal World HaskellのP.245 10.4 暗黙の状態

じゃってことで
runParse関数の型を
runParse :: ParseState -> Either String (a, ParseState)
から
runParse :: Parse a -> ParseState -> Either String (a, ParseState)
に変えて:loadするとどうなるか？

Prelude> :load Parse.hs
[1 of 1] Compiling Main             ( Parse.hs, interpreted )

Parse.hs:18:26:
    Couldn't match expected type `ParseState
                                  -> Either String (a, ParseState)'
           against inferred type `Either a1 (a, Parse a)'
    In the expression: Right (a, s)
    In the first argument of `Parse', namely `(\ s -> Right (a, s))'
    In the expression: Parse (\ s -> Right (a, s))

Parse.hs:21:47:
    Couldn't match expected type `Parse a'
           against inferred type `ParseState'
    In the second argument of `runParse', namely
        `(ParseState initState 0)'
    In the expression: runParse parser (ParseState initState 0)
    In the expression:
        case runParse parser (ParseState initState 0) of {
          Left err -> Left err
          Right (result, _) -> Right result }
Failed, modules loaded: none.

どないやねん、マジで！

まったくの謎・・・