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

17 Jun 2011

      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.

here is some of my body of parser : they all has type of "parsecT ***"
      stringSet :: ParsecT  String  u  Identity  [String]
      intSet  ::  ParsecT  String  u  Identity  [Integer]
      tupleSet ::  ParsecT  String  u  Identity  [(String, String)]

all of the returned type are instance of 'Show'.

then I write these warp function:
------------------
    import System.IO
    import Data.Functor.Identity (Identity)
    import Text.Parsec.Prim (ParsecT, runParserT, parse, Stream)
    runIOParse :: (Show a) => ParsecT String u Identity a -> String -> IO ()
    runIOParse pa fn =
      do
        inh <- openFile fn ReadMode
        outh <- openFile (fn ++ ".parseout") WriteMode
        instr <- hGetContents inh
        let result = show $ parse pa fn instr
        hPutStr outh result
        hClose inh
        hClose outh
-------------------
...
:l RunParse.hs

RunParse.hs:12:31:
    Could not deduce (u ~ ())
    from the context (Show a)
      bound by the type signature for
                 runIOParse :: Show a =>
                               ParsecT String u Identity a -> String -> IO ()
      at RunParse.hs:(7,1)-(15,15)
      `u' is a rigid type variable bound by
          the type signature for
            runIOParse :: Show a =>
                          ParsecT String u Identity a -> String -> IO ()
          at RunParse.hs:7:1
    Expected type: Text.Parsec.Prim.Parsec String () a
      Actual type: ParsecT String u Identity a
    In the first argument of `parse', namely `pa'
    In the second argument of `($)', namely `parse pa fn instr'
Failed, modules loaded: none.
-------------------

then I modify the type signature of 'runIOParse':
runIOParse :: (Show a) => ParsecT String () Identity a -> String -> IO ()
then load again
...
:l RunParse.hs

RunParse.hs:12:25:
    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 =>
                               ParsecT String () Identity a -> String -> IO ()
      at RunParse.hs:(7,1)-(15,15)
    Possible fix:
      add (Stream String Identity t0) to the context of
        the type signature for
          runIOParse :: Show a =>
                        ParsecT String () Identity a -> String -> IO ()
      or add an instance declaration for (Stream String Identity t0)
    In the second argument of `($)', namely `parse pa fn instr'
    In the expression: show $ parse pa fn instr
    In an equation for `result': result = show $ parse pa fn instr
Failed, modules loaded: none.
-------------------

I also tried some 'possible fix' in the information, but it still
failed to pass the compiler.

Main Question:
****  How can I warp a parsec function interface for do the IO test
with different 'ParsecT String u Identity a'?

-- 
----------------
吴兴博  Wu Xingbo