[Haskell-beginners] Natural functions versus existential types

[Brandon Allbery]

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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