[Haskell-cafe] Question on definition of `parse' function in Parsec library.

Christian Maeder Christian.Maeder at dfki.de
Mon Oct 10 14:27:24 CEST 2011

Am 08.10.2011 16:04, schrieb Captain Freako:
> Hi all,
> In this definition from the Parsec library:
> parse  ::  (Stream  s  Identity  t)
>        =>  Parsec  s  ()  a  ->  SourceName  ->  s  ->  Either  ParseError  a
> parse  p  =  runP  p  ()
> what's the significance of `Identity t'?
> (`t' isn't used anywhere.)


Text.Parsec.Prim contains

  class Monad m => Stream s m t | s -> t where

saying that the type s determines t (that is a functional dependency).
The instances are:

  Monad m => Stream ByteString m Char	
  Monad m => Stream ByteString m Char	
  Monad m => Stream Text m Char	
  Monad m => Stream Text m Char	
  Monad m => Stream [tok] m tok	

so usually you have a character stream. (There are lazy and strict 
version of Text and ByteString.) The last instance also works for plain 
strings (String = [Char]).

Using "Identity" for the monad m just means, that you actually do not 
need a monad (but need to supply a dummy one).

If you want to keep thinks simpler you could use packages parsec1 or parsec2

Cheers Christian


> Thanks,
> -db

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