Maybe it is possible to do something In a google summer of code. Nothing as sophisticated as the Helium paper ("Scripting the Type Inference Process", but maybe a partial implementation of the techniques mentioned, so that the development can be enhanced in the future.

Maybe some kind of library that permits postprocessing of GHC errors and/or the identification of points in the current type checker where some kind of rules can be defined by the programmer can be the first step.

2013/4/23 Brent Yorgey <byorgey@seas.upenn.edu>

On Tue, Apr 23, 2013 at 12:49:59PM +0200, Alberto G. Corona wrote:
> Hi
>
> I ever was worried about the barrier that the complexity of the Haskell
> errors impose to users of DSLs. Many DSLs look so simple that even someone
> without knowledge of Haskell can make use of them for some domains.
>
> However when the program is compiled then al the monsters of the
> deep appear in the surface: polymorphisms, undefined instances, type errors
> reported in a line produced by a type assumption in another, etc. This is
> a problem for an industrial use of Haskell in the large scale. For obvious
> reasons.

Indeed. For example, in my experience this is a big problem for diagrams.

>
>
> The question: Is it possible to develop a GHC extension that attach (or
> prepend) such an explanation to the ghc error?
>
> Or any other alternative that carry out the same functionality.

Surely it is possible. I have wanted this too. I guess the majority
of the work would just be in coming up with a good, general design
which is useful but not too difficult to implement. If anyone wanted
to undertake such a project I would be happy to contribute some ideas.

-Brent

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--
Alberto.