[Haskell-cafe] is closing a class this easy?

[Miguel Mitrofanov]

On 18 Jul 2009, at 16:49, Wolfgang Jeltsch wrote:

> Am Samstag, 18. Juli 2009 08:58 schrieb Miguel Mitrofanov:
>> Oops... Sorry, wrong line. Should be
>>
>> isAB :: forall p. p A -> p B -> p x
>
> Is this a well-known approach for closing classes?

I have an impression that this is kinda folklore. But I can't provide  
any references; I don't even remember if I invented it myself or not.