Use of forall as a sigil
Richard Eisenberg
rae at richarde.dev
Wed Nov 18 17:50:38 UTC 2020
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 <mailto: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 <mailto: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 <https://github.com/ghc-proposals/ghc-proposals/pull/281>
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