[Haskell-cafe] Why do I have to specify (Monad m) here again?

Yitzchak Gale gale at sefer.org
Sun Feb 18 16:49:18 EST 2007

David Tolpin wrote:
>> could you please point me to a reference
>> (paper/note/something else) that explains
>> that class constraint in a class definition
>> is a guarantee with regard to a type declaration
>> but a requirement with regard to an instance
>> declaration?

Sebastian Sylvan wrote:
> Well, I guess the H98 report would be a good start.

I'm not sure that is a good place to start.

True, it is likely possible to deduce both
of those facts from the Report. But the Report
is a language spec. It assumes that you
already know about the type system, and
it is difficult to read if you don't.

> ...there are multiple tutorials on type classes
> that will cover this, most of which
> are available from haskell.org

That sounds like a much better idea.
Any particular suggestions?


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