Scoped Type Variables discussion forum [was: open up the issues tracker on ghc-proposals]

cheater00 cheater00 cheater00 at
Wed May 9 03:01:59 UTC 2018

I couldn't live without ScopedTypeVariables. For me it's an essential tool
when I want to figure out

1. if the type being inferred is the one I expect
2. what type a specific thing in code I am working with is

Also useful for adding that one bit the inferer is missing without
immediately modifying a complex type sig. I can do that later, but the
proof of concept should be quick and easy.

Backwards compat: Isn't this what we have Haskell 98, Haskell 2010, etc?

On Sat, 5 May 2018 04:35 Anthony Clayden, <anthony_clayden at>

> This thread is a discussion about discussions, not the discussion itself
> ;-)
> I'm cc'ing to the cafe; but I'd prefer replies to come to
> glasgow-haskell-users.
> >> I can volunteer to at least scrape together all the objections to
> ScopedTypeVariables as currently. It's not yet a proposal, so not on
> github. Start a wiki page? A cafe thread? (It'll get lost.) A ghc-users
> thread? (It'll get ignored.)
> > ... don’t care what forums or list or whatever. As long as it’s
> collated and such
> > It could even be on the prime issue tracker for prime proposals. Just
> that it’s written down :)
> Thanks Carter, but I understand Haskell Prime to be to assess
> mature/stable proposals (preferably already delivered as extensions). This
> discussion is at first going to be more exploratory:
> * likes and dislikes about ScopedTypeVariables as currently.
> * confusions experienced by users (especially newbies)
> -- although absolute newbies wouldn't be using it(?), so intermediates?
> * feedback from those teaching Haskell.
> * wild ideas for possible alternative designs.
> * possible improvements to the current design.
> * I think we're all agreed that ScopedTypeVariables should have been in
> Haskell from the beginning;
> but it wasn't, so now we have to worry about backwards compatibility for
> programs that worked around the omission.
> Or do we? What code would break? How much pain would that cause?
> * anything else?
> > We have lots of forums, but your point is that certain sorts of
> discussions never get going with the right audience – you especially point
> to “confused beginners”.
> > ... It’s quite a challenge because beginners tend not to be vocal, and
> yet they are a crucial set of Haskell users. Every Haskell user started as
> a beginner.
> On this particular topic, there's plenty of confused people asking
> questions on StackOverflow. (Heads up: they're especially asking why they
> need explicit `forall` whereas in reguar Haskell that 'intermediates' see,
> the forall is implicit.)
> Can other people point me to questions/likes/dislikes on other forums?
> Reddit for example.
> If you've read this far, you now understand what we're trying to cover.
> It's going to be random/varied thoughts at first, then perhaps coalescing
> to an approach or two. At that point a formal proposal on github proper;
> and the random stuff might be interesting background but will essentially
> get archived/thrown away.
> I do agree with David's suggestion that github Issue tracker looks like a
> suitable solution. We can write formatted code and text. We can add links
> and references. What do others think? Joachim has opened up Issues tracker,
> as a try-out. If using it doesn't work out, that's fine and in keeping with
> my "thrown away" above.
> Also where else should I post links to this message to 'advertise' the
> thread? I don't reddit much, so if that's suitable, please someone post
> there.
> Thank you
> AntC
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