Use of forall as a sigil

[Iavor Diatchki]

Semantically, `forall a -> a -> Int` is the same as `forall a. a -> Int`.
The two only differ in how you use them:
  * For the first one, you have to explicitly provide the type to use for
`a` at every call site, while
  * for the second one, you usually omit the type and let GHC infer it.

So overall I think it's a pretty simple idea. For me it's hard to see that
from the text in #281, but I think a lot of the complexity there
is about a fancy notation for explicitly providing the type at call sites.

-Iavor



On Wed, Nov 18, 2020 at 9:51 AM Richard Eisenberg <rae at richarde.dev> wrote:

> Hi Bryan,
>
> First off, sorry if my first response was a bit snippy -- it wasn't meant
> to be, and I appreciate the angle you're taking in your question. I just
> didn't understand it!
>
> This second question is easier to answer. I say "forall a arrow a arrow
> Int".
>
> But I still think there may be a deeper question here left unanswered...
>
> Richard
>
> On Nov 18, 2020, at 6:11 AM, Bryan Richter <b at chreekat.net> wrote:
>
> Yeah, sorry, I think I'm in a little over my head here. :) But I think I
> can ask a more answerable question now: how does one pronounce "forall a ->
> a -> Int"?
>
> Den tis 17 nov. 2020 16:27Richard Eisenberg <rae at richarde.dev> skrev:
>
>> Hi Bryan,
>>
>> I don't think I understand what you're getting at here. The difference
>> between `forall b .` and `forall b ->` is only that the choice of b must be
>> made explicit. Importantly, a function of type e.g. `forall b -> b -> b`
>> can *not* pattern-match on the choice of type; it can bind a variable that
>> will be aliased to b, but it cannot pattern-match (say, against Int). Given
>> this, can you describe how `forall b ->` violates your intuition for the
>> keyword "forall"?
>>
>> Thanks!
>> Richard
>>
>> > On Nov 17, 2020, at 1:47 AM, Bryan Richter <b at chreekat.net> wrote:
>> >
>> > Dear forall ghc-devs. ghc-devs,
>> >
>> > As I read through the "Visible 'forall' in types of terms"
>> > proposal[1], I stumbled over something that isn't relevant to the
>> > proposal itself, so I thought I would bring it here.
>> >
>> > Given
>> >
>> >        f :: forall a. a -> a                   (1)
>> >
>> > I intuitively understand the 'forall' in (1) to represent the phrase
>> > "for all". I would read (1) as "For all objects a in Hask, f is some
>> > relation from a to a."
>> >
>> > After reading the proposal, I think my intuition may be wrong, since I
>> > discovered `forall a ->`. This means something similar to, but
>> > practically different from, `forall a.`. Thus it seems like 'forall'
>> > is now simply used as a sigil to represent "here is where some special
>> > parameter goes". It could as well be an emoji.
>> >
>> > What's more, the practical difference between the two forms is *only*
>> > distinguished by ` ->` versus `.`. That's putting quite a lot of
>> > meaning into a rather small number of pixels, and further reduces any
>> > original connection to meaning that 'forall' might have had.
>> >
>> > I won't object to #281 based only on existing syntax, but I *do*
>> > object to the existing syntax. :) Is this a hopeless situation, or is
>> > there any possibility of bringing back meaning to the syntax of
>> > "dependent quantifiers"?
>> >
>> > -Bryan
>> >
>> > [1]: https://github.com/ghc-proposals/ghc-proposals/pull/281
>> > _______________________________________________
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>>
>>
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