[snip]

instance Foo Int where foo = show

instance Bar Int where bar a = (show a) ++ " bar!"

instance Foo Char where foo = show

instance Foo a => Bar a where bar = foo

Notice: this last instance derives another instance of Bar Int, etc.

If I try to compile this via GHC it complains loudly unless I feed it "-fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances", in which case it seems to do the right thing. However, the warnings (and the names of the flags - "undecidable instances" doesn't sound good) make me nervous, and confused. Why is there a problem with saying "every instance of Foo is also an instance of Bar, and here's how"?

The problem is the sublanguage in which you are working. The class constraint simplification starts fairly simply (oh well) in Haskell, but the more powerful you make it the more powerful is the engine that does the job. Eventually you have full-blown Prolog programming at that level, and hence undecidability. The class checker for the above is like a little Prolog program: foo(int). bar(int). foo(char). bar(X) :- foo(X). --- Stefan Kahrs