[Haskell-cafe] Parsec - Custom Fail

Martijn van Steenbergen martijn at van.steenbergen.nl
Tue May 5 11:10:37 EDT 2009

mwinter at brocku.ca wrote:
> Hi,
> I am using parsec to parse a small programming language. The language is typed and
> I need to do some type checking, too. I have decided to do the parsing and type checking
> simultaneously in the my parsec parser. This approach avoids to keep source code positions
> in the data type in order to produce suitable error messages during type checking. Anyhow,
> because type errors are usually detected after parsing some code I need produce error
> messages with an earlier source position. Unfortunately, there is no function that produces 
> an error taking a position as parameter. 

If you already know what position you want to report the error at, then 
why bother calling setPosition to let parsec know? Just do:

 > fail (show pos ++ ": " ++ msg)

Parsec will then result in a ParseError with its own ideas of location, 
but you can ignore that.



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