[Haskell-cafe] Parsec on TeX

[Luke Palmer]

On Mon, May 5, 2008 at 3:40 AM, Ross Boylan
<RossBoylan at stanfordalumni.org> 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)
>                 }

My guess is the following:

  string :: String -> Parser String
  braces :: Parser a -> Parser a

Meaning

  braces (string name) :: Parser String

Which is not the same as your declared return type Parser ().  Add a
return () at the end of envEnd.

>
>
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