[Haskell-cafe] coercing D a to D b

[Joachim Breitner]

Hi,

Am Mittwoch, den 13.12.2017, 21:34 -0800 schrieb Evan Laforge:
> Given:
> 
> data D a = A a | B Int Char
> 
> dmapNoCoerce :: (a -> b) -> D a -> D b
> dmapNoCoerce f (A a) = A (f a)
> dmapNoCoerce _ (B i c) = B i c
> 
> I have to reconstruct a B change it from D a to D b.  But at the lower level,
> couldn't this be implemented as a type cast?  What prevents such an
> optimization?
> 
> I can write this as
> 
> dmapCoerce :: (a -> b) -> D a -> D b
> dmapCoerce f (A a) = A (f a)
> dmapCoerce _ b@(B {}) = Unsafe.Coerce.unsafeCoerce b
> 
> From the core, it looks like dmapCoerce indeed has a cast with no allocation,
> while dmapNoCoerce allocates a new B.
> 
> It seems to work, but is it safe?  Is there a more principled way to do it?
> I can't convince Data.Coerce to cooperate, presumably because 'a' and 'b' are
> not coercible themselves, and it doesn't believe me if I try to tell it the
> type role is phantom.


this coercion is not possible in Core, because Core’s type system is
not expressive enough (at least now…). But luckily, at the STG level
this optimization is possible, see
https://git.haskell.org/ghc.git/commitdiff/19d5c7312bf0ad9ae764168132aecf3696d5410b

This is probably in 8.2.

Joachim

-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/
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