[Haskell-cafe] Still no joy with parsec

[Derek Elkins]

On Tue, 2008-05-06 at 09:52 -0700, Ross Boylan wrote:
> I kept getting parse failures when I ran my little Parsec TeX snippet on
> a sample code.  Seeing that ghc6.8 had debugging, I upgraded to it, only
> to discover that I can't even get the code to compile.
> 
> $ ghci
> GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
> Loading package base ... linking ... done.
> Prelude> :load g
> [1 of 1] Compiling Main             ( g.hs, interpreted )
> 
> g.hs:11:19:
>     Couldn't match expected type `t1 -> GenParser Char () t'
>            against inferred type `CharParser st ()'
>     In the expression: reserved "\\begin" 1
>     In a 'do' expression: reserved "\\begin" 1
>     In the expression:
>         do reserved "\\begin" 1
>            braces (many1 letter)
> Failed, modules loaded: none.

Look at the expression.

> Do parsec and 6.8 just not get along?
> 
> More generally, how can I go about diagnosing such problems?  Since I
> can't load it, I can't debug it or get :info on the types.
> 
> It looks as if maybe it's expecting a Monad, but getting a parser.  But
> I don't know why that would have changed vs using 6.6.
> 
> More questions about the error messages.  Where is the expected type,
> and where is the inferred type, coming from?  I'm guessing the expected
> type is from the function signature and the position inside a do (or
> perhaps from the argument following the ; in the do?) and the inferred
> type is what I would just call the type of reserved "begin".
> 
> And what is the 1 that appears after 'reserved "\\begin"'?  An indicator
> that all occurrences of the text refer to the same spot in the program?
> Nesting level?

Look at your code.

> Source:
> import Text.ParserCombinators.Parsec
> import qualified Text.ParserCombinators.Parsec.Token as P
> import Text.ParserCombinators.Parsec.Language(haskell)
> reserved = P.reserved haskell
> braces = P.braces haskell
> 
> 
> -- TeX example
> 
> envBegin :: Parser String
> envBegin     = do{ reserved "\\begin"
> 1                 ; braces (many1 letter)
>                  }