Re: [Haskell-cafe] Functional Parsers in GHCI

23 May 2010


      Amiruddin Nagri  writes:
...
Hi All,
I am a newbie to Haskell, and have been using Programming in Haskell along
with Eric Meijer videos covering the book syllabus. I am currently on
Chapter 8 Functional Parsers, and trying to implement few of the examples
given in the book.
Following is the example :
item = \inp -> case inp of
        [] -> []
        (x:xs) -> [(x,xs)]
p = do  x <- item
        y <- item
        z <- item
        return (x,y,z)
=========
When I compile it using GHCI, I get the following error :
[1 of 1] Compiling Main             ( parser.hs, interpreted )
parser.hs:5:8:
    No instance for (Monad ((->) [t]))
      arising from a do statement at parser.hs:5:8-16
Try importing Control.Monad.Instances and see if that gets it to work.
...
Possible fix: add an instance declaration for (Monad ((->) [t]))
    In a stmt of a 'do' expression: x <- item
    In the expression:
        do x <- item
           y <- item
           z <- item
           return (x, y, z)
    In the definition of `p':
        p = do x <- item
               y <- item
               z <- item
               ....
Failed, modules loaded: none.
=========
I tried googling for samples, above error message etc. But not getting to
solution. Can anyone guide me as to what am I doing wrong and how can I fix
this.
Regards,
Amiruddin Nagri,
India
GTalk : amir.nagri@gmail.com
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