[Haskell-cafe] Parsec on TeX

[Derek Elkins]

On Sun, 2008-05-04 at 20:40 -0700, Ross Boylan wrote:
> I am new to Haskell and Parsec, and am trying to understand both.  I tried
> to follow the example of how to use Parsec to parse TeX begin/end groups,
> but can't get it to run.  I'm using HUGS -98 on Debian.
> 
> When I copied the code I got errors about unknown terms (reserved and
> braces).  I've tried to get them from the lexer, but now get this error
>  :load grammar.hsl
> ERROR "grammar.hsl":21 - Type error in explicitly typed binding
> *** Term           : envEnd
> *** Type           : String -> GenParser Char a [Char]
> *** Does not match : String -> Parser ()
> 
> Can anyone help me understand what the problem is?
> 
> Here's the code the caused the above error; I believe the part after --TeX
> example is verbatim from the Parsec documentation.  I picked haskell as the
> language for to lexer "arbitrarily."
> 
> 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
> environment = do{ name <- envBegin
>                 ; environment
>                 ; envEnd name
>                 }
>               <|> return ()
> 
> envBegin :: Parser String
> envBegin     = do{ reserved "\\begin"
>                  ; braces (many1 letter)
>                  }
> 
> envEnd :: String -> Parser ()
> envEnd name = do{ reserved "\\end"
>                 ; braces (string name)
>                 }

braces returns, in this case, a string, so the type of envEnd is
String -> Parser String.  You can either change the type and add a
return () to environment after envEnd or add a return () to envEnd.