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

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 Char tokens.
Here are some examples of Parsec's error messages while using Char tokens:
ghci> P.parseTest (string "asdf" >> spaces >> string "ok") "asdf wrong"
parse error at (line 1, column 7):
unexpected "w"
expecting space or "ok"
ghci> P.parseTest (choice [string "ok", string "nop"]) "wrong"
parse error at (line 1, column 1):
unexpected "w"
expecting "ok" or "nop"
So, string parser shows what string is expected when found an unexpected string, and choice parser shows what are alternatives.
But when I use same combinators with my tokens:
ghci> Parser.parseTest ((tok $ Ide "asdf") >> (tok $ Ide "ok")) "asdf "
parse error at "test" (line 1, column 1):
unexpected end of input
In this case, it doesn't print what was expected.
ghci> Parser.parseTest (choice [tok $ Ide "ok", tok $ Ide "nop"]) "asdf "
parse error at (line 1, column 1):
unexpected (Ide "asdf","test" (line 1, column 1))
And when I use choice, it doesn't print alternatives.
I expect this behavior to be related with combinator functions, and not with tokens, but seems like I'm wrong. How can I fix this?
Here's the full lexer + parser code:
Lexer:
module Lexer
( Token(..)
, TokenPos(..)
, tokenize
) where
import Text.ParserCombinators.Parsec hiding (token, tokens)
import Control.Applicative ((<*), (*>), (<$>), (<*>))
data Token = Ide String
| Number String
| Bool String
| LBrack
| RBrack
| LBrace
| RBrace
| Keyword String
deriving (Show, Eq)
type TokenPos = (Token, SourcePos)
ide :: Parser TokenPos
ide = do
pos <- getPosition
fc <- oneOf firstChar
r <- optionMaybe (many $ oneOf rest)
spaces
return $ flip (,) pos $ case r of
Nothing -> Ide [fc]
Just s -> Ide $ [fc] ++ s
where firstChar = ['A'..'Z'] ++ ['a'..'z'] ++ "_"
rest = firstChar ++ ['0'..'9']
parsePos p = (,) <$> p <*> getPosition
lbrack = parsePos $ char '[' >> return LBrack
rbrack = parsePos $ char ']' >> return RBrack
lbrace = parsePos $ char '{' >> return LBrace
rbrace = parsePos $ char '}' >> return RBrace
token = choice
[ ide
, lbrack
, rbrack
, lbrace
, rbrace
]
tokens = spaces *> many (token <* spaces)
tokenize :: SourceName -> String -> Either ParseError [TokenPos]
tokenize = runParser tokens ()
Parser:
module Parser where
import Text.Parsec as P
import Control.Monad.Identity
import Lexer
parseTest :: Show a => Parsec [TokenPos] () a -> String -> IO ()
parseTest p s =
case tokenize "test" s of
Left e -> putStrLn $ show e
Right ts' -> P.parseTest p ts'
tok :: Token -> ParsecT [TokenPos] () Identity Token
tok t = token show snd test
where test (t', _) = case t == t' of
False -> Nothing
True -> Just t
SOLUTION:
Ok, after fp4me's answer and reading Parsec's Char source more carefully, I ended up with this:
{-# LANGUAGE FlexibleContexts #-}
module Parser where
import Text.Parsec as P
import Control.Monad.Identity
import Lexer
parseTest :: Show a => Parsec [TokenPos] () a -> String -> IO ()
parseTest p s =
case tokenize "test" s of
Left e -> putStrLn $ show e
Right ts' -> P.parseTest p ts'
type Parser a = Parsec [TokenPos] () a
advance :: SourcePos -> t -> [TokenPos] -> SourcePos
advance _ _ ((_, pos) : _) = pos
advance pos _ [] = pos
satisfy :: (TokenPos -> Bool) -> Parser Token
satisfy f = tokenPrim show
advance
(\c -> if f c then Just (fst c) else Nothing)
tok :: Token -> ParsecT [TokenPos] () Identity Token
tok t = (Parser.satisfy $ (== t) . fst) <?> show t
Now I'm getting same error messages:
ghci> Parser.parseTest (choice [tok $ Ide "ok", tok $ Ide "nop"]) " asdf"
parse error at (line 1, column 1):
unexpected (Ide "asdf","test" (line 1, column 3))
expecting Ide "ok" or Ide "nop"

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"]