Re: Use of forall as a sigil

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 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 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 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 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 _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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