New implementation for `ImpredicativeTypes`

[Simon Peyton Jones]

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 at haskell.org> On Behalf Of Alex Rozenshteyn
Sent: 06 September 2019 03:31
To: Alejandro Serrano Mena <trupill at gmail.com>
Cc: GHC developers <ghc-devs at 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<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.reddit.com%2Fr%2Fhaskell%2Fcomments%2F3am0qa%2Fexistentials_and_the_heterogenous_list_fallacy%2Fcsdwlp2%2F%3Fcontext%3D8%26depth%3D9&data=02%7C01%7Csimonpj%40microsoft.com%7Cf5bddbe8c13b424fad8a08d732724d04%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637033338810706935&sdata=a5c3ujC0kiPlocgHg8AX%2BwIP6ZH2hnqszCpnWOiqpGQ%3D&reserved=0>

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 at gmail.com<mailto:trupill at 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<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Ftrupill%2Fghc%2Fcommit%2Fa3f95a0fe0f647702fd7225fa719a8062a4cc0a5%2Fpipelines%3Fref%3Dquick-look-build&data=02%7C01%7Csimonpj%40microsoft.com%7Cf5bddbe8c13b424fad8a08d732724d04%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637033338810716935&sdata=PP1imEsNrkIhloEV3AS52nrZBtyjen4i1e3pJTcMi6M%3D&reserved=0> 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<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Ftrupill%2Fghc&data=02%7C01%7Csimonpj%40microsoft.com%7Cf5bddbe8c13b424fad8a08d732724d04%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637033338810716935&sdata=IgtSqJP4%2BhCfy3%2FHVsqX6wpPYVMS8D1wN46aXHnFcUw%3D&reserved=0>.

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<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fmerge_requests%2F1659&data=02%7C01%7Csimonpj%40microsoft.com%7Cf5bddbe8c13b424fad8a08d732724d04%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637033338810716935&sdata=YJ5N9d7gXC2JbpqTvpqs4wjY%2F7Ev%2FikRdYIK%2Bhv4rRE%3D&reserved=0>

Kind regards,
Alejandro
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