[Haskell-cafe] Haskell spec vs FlexibleInstances

[Dan Doel]

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

>
> On Tue, Feb 18, 2014 at 3:11 PM, Doug McIlroy <doug at 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
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