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

Anthony Clayden anthony_clayden at
Sat May 19 11:32:51 UTC 2018

On Wed, 9 May 2018 03:01 UTC, cheater00 wrote:

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

Yes absolutely. To be clear: nobody's talking about removing it. The
question is, could we get the same functionality without being so
confusing and contrary to how Haskell usually works with type
variables? (I'm not saying yea or nay, I'm trying to invite

> ...

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

The current design for ScopedTypeVariables potentially changes the
types for programs that are valid H98/H2010. (Or for programs that
were valid as at when ScopedTypeVariables was introduced ~10 years
ago, perhaps using ExplicitForAll.) Probably the effect would be that
they don't compile, but it might be more subtle.

So the explanation I've seen for the current design is it was
deliberately idiosyncratic, to minimise any disruption to existing
code. Then I'm asking whether any of that code is still around? If
not/if it's been re-factored to use ScopedTypeVariables, then any
tweak to the design could have a freer hand.

But seems there's no enthusiasm for such discussion, so I'm going to
let it die. I hear there might be moves afoot elsewhere ...

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