[Haskell-cafe] Data structure containing elements which are instances of the same type class

[Alexander Solla]

On Thu, Aug 16, 2012 at 8:07 PM, wren ng thornton <wren at freegeek.org> wrote:

> On 8/15/12 2:55 PM, Albert Y. C. Lai wrote:
>
>> On 12-08-15 03:20 AM, wren ng thornton wrote:
>>
>>> (forall a. P(a)) -> Q <=> exists a. (P(a) -> Q)
>>>
>>
>> For example:
>>
>> A. (forall p. p drinks) -> (everyone drinks)
>> B. exists p. ((p drinks) -> (everyone drinks))
>>
>> In a recent poll, 100% of respondents think A true, 90% of them think B
>> paradoxical, and 40% of them have not heard of the Smullyan drinking
>> paradox.
>>
>
> :)
>
> Though bear in mind we're discussing second-order quantification here, not
> first-order.


Can you expand on what you mean here?  I don't see two kinds of
quantification in the type language (at least, reflexively, in the context
of what we're discussing).  In particular, I don't see how to quantify over
predicates for (or sets of, via the extensions of the predicates) types.

Is Haskell's 'forall' doing double duty?
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