What is wrong with my Clean program which compiler give the error "cannot unify demanded type with offered type"

Question

I'm trying to write my PDF parser and I got stick for this type checking failure and it bothered me for days.

Here's my app.icl

module app

import StdEnv

Start = "Hello, world!"


:: ParseResult a
    = ParseOk a [Char]
    | ParseFail [Char]

match :: [Char] Real -> ([Char] -> (ParseResult Real))
match x y = matchYes x y
matchYes :: [Char] Real [Char] -> ParseResult Real
matchYes [] t _          = ParseOk t
matchYes [_:_] _ []      = ParseFail []
matchYes [x:xs] t [y:ys] = if (x == y) (matchYes xs t ys) (ParseFail [])

the output is

Type error [app.icl,14,matchYes]: near (case ... ) : cannot unify demanded type with offered type:
 [Char] -> ParseResult Real
 ParseResult Real

I run nitrile build and it fails. I really don't get it. As far as I know, matchYes always fit the type [Char] Real [Char] -> ParseResult Real

Answer 1

ParseOk a [Char]

ParseOk takes two arguments: one of type a (Real in this case) and one of type [Char].

matchYes [] t _          = ParseOk t

Here you're partially applying ParseOk to a single argument of type Real. So what you're left with is a function of type [Char] -> ParseResult Real when the expected type would have been just ParseResult Real.