Re: [Haskell-cafe] type signature of parsec functions and how to warp them up.

17 Jun 2011

      It seems weird:

first ghci failed to load this file:
file: RunParse.hs
-------------------
module RunParse where
import System.IO
import Data.Functor.Identity (Identity)
----import Text.Parsec                 --------first, no this
line-------what about this line ???
import Text.Parsec.Prim (Parsec, parse, Stream)

runIOParse :: (Show a) => Parsec String () a -> String -> IO ()
runIOParse pa fn =
  do
    inh <- openFile fn ReadMode
    outh <- openFile (fn ++ ".parseout") WriteMode
    instr <- hGetContents inh
    let result = case parse pa fn instr of
                   Right rs -> show rs
                   Left err -> "error"
    hPutStr outh result
    hClose inh
    hClose outh
------------------

ghci tell me:
-----------------

RunParse.hs:13:23:
    Could not deduce (Stream String Identity t0)
      arising from a use of `parse'
    from the context (Show a)
      bound by the type signature for
                 runIOParse :: Show a => Parsec String () a -> String -> IO ()
      at Sim/Std/RunParse.hs:(8,1)-(18,15)
    Possible fix:
      add (Stream String Identity t0) to the context of
        the type signature for
          runIOParse :: Show a => Parsec String () a -> String -> IO ()
      or add an instance declaration for (Stream String Identity t0)
    In the expression: parse pa fn instr
    In the expression:
      case parse pa fn instr of {
        Right rs -> show rs
        Left err -> "error" }
    In an equation for `result':
        result
          = case parse pa fn instr of {
              Right rs -> show rs
              Left err -> "error" }
------------------

I just add one line of import and ghci:

import Text.Parsec

then ghci loaded it succeed!

It seems I didn't uses any functions from this import.

what goes wrong?

2011/6/17 吴兴博 <wuxb45@gmail.com>:
...
I have some different parsers of Parsec to use in a project, and I
want to make a warp function to make the testing easy.
-- 
----------------
吴兴博  Wu Xingbo