Hi folks,
I'm using Parsec to parse a stream of tokens. The token primitive takes,
among other arguments, a function to print tokens. However, this
function is not always applied. Try the code below:
---------
import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Pos(newPos)
mytoken :: (Eq t, Show t) => t -> GenParser (SourcePos,t) () t
mytoken x = token showTok posFromTok testTok where
showTok (pos,t) = "<" ++ show t ++ ">"
posFromTok (pos,t) = pos
testTok (pos,t) = if (x == t) then Just t else Nothing
main = do
putStrLn ""
case parse the123Parser "" [(newPos "" 1 n, n) | n <- [1,2,3,4]] of
(Left err) -> putStrLn (show err)
(Right _) -> putStrLn "parsed correctly"
putStrLn ""
case parse the123Parser "" [(newPos "" 1 n, n) | n <- [1,3,4]] of
(Left err) -> putStrLn (show err)
(Right _) -> putStrLn "parsed correctly"
the123Parser = do
mytoken 1
mytoken 2
mytoken 3
eof
return 123
-------
The output I get looks like this:
(line 1, column 4):
unexpected [((line 1, column 4),4)]
expecting end of input
(line 1, column 3):
unexpected <3>
In the second parse case, it correctly uses my showTok function to show
the token. But in the first case, it just uses the regular show method.
I guess that's because the eof parser doesn't know anything about how to
show the token it sees. Any ideas on how I can get the error message in
the first case to look more like the second case?
Thanks,
Lyle
