On Tue, Feb 18, 2014 at 3:40 PM, Brandon Allbery <allbery.b@gmail.com> wrote:

On Tue, Feb 18, 2014 at 3:11 PM, Doug McIlroy <doug@cs.dartmouth.edu> wrote:

What potential evil motivates the prohibition of repeated type
parameters in an instance declaration? (Restriction removed
by FlexibleInstances)

Isn't that just related to constraints not being part of instance selection? (Assuming that `instance SomeClass a a` is interpreted as `instance (a ~ b) => SomeClass a b`, since I don't quite see how it would work otherwise given how instances work; the constraint is not necessarily obvious from looking at it. Also note that standard Haskell can't therefore describe this instance properly.)

These two instances do not actually work the same way.

instance (a ~ b) => SomeClass a b

says that any context containing 'SomeClass a b' can be reduced to a context containing (a ~ b) instead.

instance SomeClass a a

says that a 'SomeClass a b' constraint can be discharged if we enter a state in which the type checker _decides_ that a = b.

In a sense, the first instance can provide information, while the second requires it. Whether there should be such a distinction is left up to the reader.

-- Dan