18 Feb
2007
18 Feb
'07
3:18 p.m.
That's completely different. The class in that case guarantees that the type has an "Eq" class, so it's okay to use the functions in the "Eq" class. You're using the guarantees supplied by the class. When you write instances, it's the other way around, the class has *requirements* that you must fulfill -- and there are multiple ways of doing it (Haskell won't guess, it will obey what you tell it -- if you don't give any class constraints it won't assume that they are there).
Hi Sebastian, 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? David Tolpin