[Haskell-cafe] Question on Coerce

[David Feuer]

Optimizations don't seem to make the problem much *harder* (Generics manage
to work out "decided strictness", including unpacking), but they do make it
less stable: whether two things have the same representation in memory may
depend on optimization flags.

On Wed, Dec 2, 2020, 12:19 PM Brandon Allbery <allbery.b at gmail.com> wrote:

> coerce works when one type is a newtype wrapper for another, not when two
> types happen to share the same runtime representation. I don't think ghc
> can currently prove the latter except in the case of newtypes, especially
> in the presence of optimizations.
>
> On Wed, Dec 2, 2020 at 11:33 AM Brent Walker <brenthwalker at gmail.com>
> wrote:
>
>> My question was on how to use coerce -- not whether this is worth doing
>> or not.  If we were discussing that, then I would say it's not 'just a
>> leaf' -- it's ALL the leaves of the tree -- and for a tree of depth n,
>> there are 2^n of them so depending on what 'n' is, it could potentially be
>> something to worry about.
>>
>>
>>
>> On Wed, Dec 2, 2020 at 6:00 PM Henning Thielemann <
>> lemming at henning-thielemann.de> wrote:
>>
>>>
>>> On Wed, 2 Dec 2020, Brent Walker wrote:
>>>
>>> > In the following code, function fmap does not compile because variable
>>> > 'y' on line marked <***> has type (Expr a) where an (Expr b) is
>>> > expected.  The code can be fixed simply by returning (Val x) on the
>>> rhs
>>> > of the function but then we are allocating a new object for something
>>> we
>>> > could potentially reuse since (Val n) has the same runtime
>>> > representation in (Expr a) and (Expr b) (it has no dependence on the
>>> > type variable).
>>>
>>> Is it worth the trouble? It is only a leaf of the tree.
>>
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>
>
>
> --
> brandon s allbery kf8nh
> allbery.b at gmail.com
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