New implementation for `ImpredicativeTypes`

Simon Peyton Jones simonpj at
Mon Sep 9 16:00:23 UTC 2019

Suppose Haskell did have existentials;

Yes, I think that's an interesting thing to work on!  I'm not sure what the implications would be.  At very least we'd need to extend System FC (GHC's intermediate language) with existential types and the corresponding pack and unpack syntactic forms.

I don't know of any work studying that question specifically, but others may have pointers.


From: Alex Rozenshteyn <rpglover64 at>
Sent: 06 September 2019 15:21
To: Simon Peyton Jones <simonpj at>
Cc: Alejandro Serrano Mena <trupill at>; GHC developers <ghc-devs at>
Subject: Re: New implementation for `ImpredicativeTypes`

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 at<mailto:simonpj at>> 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

               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.)


From: ghc-devs <ghc-devs-bounces at<mailto:ghc-devs-bounces at>> On Behalf Of Alex Rozenshteyn
Sent: 06 September 2019 03:31
To: Alejandro Serrano Mena <trupill at<mailto:trupill at>>
Cc: GHC developers <ghc-devs at<mailto:ghc-devs at>>
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:<>

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<mailto:trupill at>> 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<> 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<>.

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<>

Kind regards,
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