[Haskell-cafe] Question on Coerce

[Brandon Allbery]

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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