Haskell Parsec - error messages are less helpful while using custom tokens

Question

I\'m working on seperating lexing and parsing stages of a parser. After some tests, I realized error messages are less helpful when I\'m using some tokens other than Parsec\'s C

Answer 1

A beginning of solution can be to define your choice function in the Parser, use a specific unexpected function to override unexpected error and finally use the operator to override the expecting message:

mychoice [] = mzero
mychoice (x:[]) = (tok x <|> myUnexpected)  show x 
mychoice (x:xs) = ((tok x <|> mychoice xs) <|> myUnexpected)   show (x:xs)

myUnexpected =  do 
             input <- getInput 
             unexpected $ (id $ first input )
           where 
            first [] = "eof"
            first (x:xs) = show $ fst x

and call your parser like that :

ghci> Parser.parseTest (mychoice [Ide "ok", Ide "nop"]) "asdf  "
parse error at (line 1, column 1):
unexpected Ide "asdf"
expecting [Ide "ok",Ide "nop"]