[Haskell-beginners] Natural functions versus existential types

[Darren Grant]

Interesting. So is it the case that existentials are only meant to hide
internals? Is there another useful application for them?




On Sun, Jan 13, 2013 at 5:50 PM, Brandon Allbery <allbery.b at gmail.com>wrote:

> On Sun, Jan 13, 2013 at 8:34 PM, Darren Grant <therealkludgy at gmail.com>wrote:
>
>> On Wed, Jan 9, 2013 at 1:45 PM, Aleksandar Dimitrov <
>> aleks.dimitrov at gmail.com> wrote:
>>
>>> > Sometimes I read an argument that existentials are required due to
>>> unknown
>>>
>>  > constraints at compile time, but do not have an intuition for this
>>> > situation yet.  For instance, I read that IO requires existentials.
>>> Still
>>> > working on that one. :)
>>>
>>> Where did you read that? The *definition* of the IO type does not use
>>> existentials [2]. But I'm not very familiar with the internals.
>>
>>
>> You're right!  I'm not sure what I saw, but I would guess that I
>> conflated some application of IO with the definition.
>>
>
> The only thing I'm aware of offhand is that an existential is used to hide
> the internals, not because this is in some sense necessary for IO to work,
> but to allow the compiler to ensure that nothing is violating IO's
> "contract":  you can't do anything with something you don't know the type
> of, so it's impossible for normal code to do anything out of bounds with an
> IO type.  You could just as well write an implementation of IO without any
> existentials or magic internals, but it'd be trivial to "cheat".  (GHC's,
> with the magic stripped away, is just state:  sequential execution is
> guaranteed by passing a state "baton" between IO actions, and the magic
> just makes sure you can't stash a copy of the baton or manufacture one
> yourself.  There's some slight additional magic that functions as an
> optimization.)
>
> --
> brandon s allbery kf8nh                               sine nomine
> associates
> allbery.b at gmail.com
> ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>
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