[Haskell-cafe] Code that doesn't compile - but should :)

Giuseppe Maggiore giuseppemag at gmail.com
Tue Mar 16 05:36:34 EDT 2010

The error message (obtained by loading the file with ghci) is:

GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             (
tiveMonad\HObject\Experiments\FunctorsProblems.hs, interpreted )

    Could not deduce (Selector s (f a -> a))
      from the context (Selector s (a1 -> f1 a1), Converter a1 f1)
      arising from a use of `select'
    Possible fix:
      add (Selector s (f a -> a)) to the context of
        the type signature for `f'
      or add an instance declaration for (Selector s (f a -> a))
    In the expression: select s
    In the definition of `method': method = select s
    In the expression:
        let method = select s in (\ x -> let res = ... in convert res)

    Couldn't match expected type `a1' against inferred type `f a'
      `a1' is a rigid type variable bound by
           the type signature for `f'
    In the expression: convert res
    In the expression: let res = method x in convert res
    In the expression: (\ x -> let res = method x in convert res)
Failed, modules loaded: none.

On Tue, Mar 16, 2010 at 2:31 AM, Ivan Lazar Miljenovic <
ivan.miljenovic at gmail.com> wrote:

> Giuseppe Maggiore <giuseppemag at gmail.com> writes:
> > Hi! Can anyone tell me why this code does not work? I cannot seem to
> > figure why it is broken...
> The error message (and how you got it) would help...
> > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
> FlexibleInstances,
> >   UndecidableInstances, FlexibleContexts, EmptyDataDecls,
> ScopedTypeVariables,
> >   TypeOperators, TypeSynonymInstances #-}
> You sure you have enough language extensions there? ;-)

Barely :)

> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> IvanMiljenovic.wordpress.com <http://ivanmiljenovic.wordpress.com/>

Giuseppe Maggiore
Ph.D. Student (Languages and Games)
Microsoft Student Partner
Mobile: +393319040031
Office: +390412348444
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