On 9/2/21 11:04 PM, Richard Eisenberg wrote:
On Sep 2, 2021, at 2:56 PM, john.ericson
<john.ericson@obsidian.systems <mailto:john.ericson@obsidian.systems>>
wrote:
Does the most basic e.g.
newtype Some f where
MkSome :: forall a. f a -> Some f
Have one of those problematic equalities?
No. That's not a GADT -- the constructor doesn't restrict anything
about `f`.
Morally, sure, but GHC doesn't know about this.
I tried, and -XGADTSyntax + -XExistenialTypes = -XGADTs it seems.
I think you're after newtype existentials. I think these should indeed
be possible, because what you propose appears to be the same as
newtype Some f = MkSome (exists a. f a)
We can probably support the syntax you wrote, too, but I don't want to
commit to that right now.
The syntax I wrote is already basically valid?
data Some f = forall a. Some (f a)
data Some f where MkSome :: forall a f. f a -> Some f
Is accepted
newtype Some f = forall a. Some (f a)
newtype Some f where MkSome :: forall a f. f a -> Some f
Is not with "A newtype constructor cannot have existential type variables"
I propose we teach GHC how these "GADTs" in fact merely have existential
variables, and not the FC constraints that require the extra evaluation
for soundness. Than we can get the operational/runtime benefits of what
you propose for cheap. Don't get me wrong -- the other aspects in the
paper this doesn't address are still quite valuable, but I think this is
a useful stepping stone / removal of artificial restrictions we should
do first.
This sort of thing is brought up in #1965, where it is alleged this is
in fact more difficult than it sounds. All more reason it is a good
stepping stone, I say!
John
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