[Haskell-cafe] Why can't I bind unlifted values at the top level?

[David Feuer]

That's what ML calls the "value restriction", right?

On Sun, Jan 22, 2023, 1:11 AM Ben Gamari <ben at smart-cactus.org> wrote:

> Indeed that is the ticket.
>
> There is one wrinkle that I should have mentioned earlier: not even all
> data constructor applications are admissible at the top level. In
> particular, constructors with strict fields may require computation to
> construct. The validity check would need to account for this, either by
> rejecting such constructors altogether or by having some of syntactic WHNF
> check.
>
> Cheers,
>
> - Ben
>
> On January 21, 2023 5:06:13 PM EST, Tom Ellis <
> tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
>>
>> On Sat, Jan 21, 2023 at 10:52:33PM +0100, Jaro Reinders wrote:
>>
>>> On 21-01-2023 20:00, Ben Gamari wrote:
>>>
>>>> However, I suspect you do have a point when it comes to unlifted data
>>>> constructors. I think it would be fine to allow an application of a
>>>> data constructor of an unlifted type on the top-level:
>>>>
>>>>    type UMaybe :: Type -> UnliftedType
>>>>    data UMaybe a = UNothing | UJust a
>>>>
>>>>    x :: UMaybe Int
>>>>    x = UJust 42
>>>>
>>>> Perhaps you could open a ticket for this?
>>>>
>>>
>>> This ticket seems related: #17521 Consider top-level unlifted bindings [1].
>>>
>>> Or do you think it needs a separate ticket?
>>>
>>
>> Thanks Jaro.  That ticket contains the observation
>>
>> "there are other cases where unlifted types are desireable at the
>>  top-level (e.g. saturated data constructor applications). In
>>  principle it would be fairly straightforward to incorporate a
>>  validity check that admits top-level constructor applications which
>>  rejecting function applications if we wanted."
>>
>> So I think that ticket subsumes my question, and filing another one
>> would be redundant.
>>
>> Tom
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