Improved message for GHC "No instance for" Errors

[rhapsodyv]

Hi,

I would like to propose a modification in the ghc message for "no instance for" errors. In the message, the ghc tells that there's no instance for the type you are using and it give you a hint that you can fix it adding a new instance for your type. But it don't tell me any instance alternative that already exists. So, if I don't know the correct type, I need to look at the documentation.

At least for me, I get this message when I'm using the function in the wrong way or when I forget to specify the some function signature. It's quite rare the I want to add a new instance. Of course that it's not a problem to look at the documentation, but improving it can save a significant time and help the beginers to guess the error and how fix it.

In C++ and Java, when I use g++ or javac and I make a wrong overloading, the compiler show me some alternatives. I think it help more than tell me to add a new overload to the function.

It's possible to do it in ghc?

[simonpj]

Can you give a few examples of the error message you would like to see? That is:

• A small program
• The current error message
• The error message you would prefer to have seen

That would help me understand your suggestion.

Thanks

Simon

[simonmar]

Not clear what is being proposed, no response from submitter.