[Haskell-cafe] Functional Parsers in GHCI

[Ivan Lazar Miljenovic]

Amiruddin Nagri <amir.nagri at gmail.com> 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 at gmail.com
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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
IvanMiljenovic.wordpress.com