Hi Simon,

You're exactly right, of course. My example is confusing, so let me see if I can clarify.

What I want in the ideal is map show [1, 'a', "b"]. That is, minimal syntactic overhead to mapping a function over multiple values of distinct types that results in a homogeneous list. As the reddit thread points out, there are workarounds involving TH or wrapping each element in a constructor or using bespoke operators, but when it comes down to it, none of them actually allows me to say what I mean; the TH one is closest, but I reach for TH only in times of desperation.

I had thought that one of the things preventing this was lack of impredicative instantiation, but now I'm not sure. Suppose Haskell did have existentials; would map show @(exists a. Show a => a) [1, 'a', "b"] work in current Haskell and/or in quick-look?

Tangentially, do you have a reference for what difficulties arise in adding existentials to Haskell? I have a feeling that it would make working with GADTs more ergonomic.

On Fri, Sep 6, 2019 at 12:33 AM Simon Peyton Jones <simonpj@microsoft.com> wrote:

I’m confused.   Char does not have the type (forall a. Show a =>a), so our example is iill-typed in System F, never mind about type inference.  Perhaps there’s a typo?   I think you may have ment

               exists a. Show a => a

which doesn’t exist in Haskell.  You can write existentials with a data type

 

data Showable where

   S :: forall a. Show a => a -> Showable

 

Then

               map show [S 1, S ‘a’, S “b”]

works fine today (without our new stuff), provided you say

 

               instance Show Showable where

                 show (S x) = show x

 

Our new system can only type programs that can be written in System F.   (The tricky bit is inferring the impredicative instantiations.)

 

Simon

 

From: ghc-devs <ghc-devs-bounces@haskell.org> On Behalf Of Alex Rozenshteyn
Sent: 06 September 2019 03:31
To: Alejandro Serrano Mena <trupill@gmail.com>
Cc: GHC developers <ghc-devs@haskell.org>
Subject: Re: New implementation for `ImpredicativeTypes`

 

I didn't say anything when you were requesting use cases, so I have no right to complain, but I'm still a little disappointed that this doesn't fix my (admittedly very minor) issue: https://www.reddit.com/r/haskell/comments/3am0qa/existentials_and_the_heterogenous_list_fallacy/csdwlp2/?context=8&depth=9

 

For those who don't want to click on the reddit link: I would like to be able to write something like map show ([1, 'a', "b"] :: [forall a. Show a => a]), and have it work.

 

On Wed, Sep 4, 2019 at 8:13 AM Alejandro Serrano Mena <trupill@gmail.com> wrote:

Hi all,

As I mentioned some time ago, we have been busy working on a new implementation of `ImpredicativeTypes` for GHC. I am very thankful to everybody who back then sent us examples of impredicativity which would be nice to support, as far as we know this branch supports all of them! :)

 

If you want to try it, at https://gitlab.haskell.org/trupill/ghc/commit/a3f95a0fe0f647702fd7225fa719a8062a4cc0a5/pipelines?ref=quick-look-build you can find the result of the pipeline, which includes builds for several platforms (click on the "Artifacts" button, the one which looks like a cloud, to get them). The code is being developed at https://gitlab.haskell.org/trupill/ghc.

 

Any code should run *unchanged* except for some eta-expansion required for some specific usage patterns of higher-rank types. Please don't hesitate to ask any questions or clarifications about it. A merge request for tracking this can be found at https://gitlab.haskell.org/ghc/ghc/merge_requests/1659

 

Kind regards,

Alejandro

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