Typing pattern synonyms

David Feuer david.feuer at gmail.com
Wed Sep 30 18:33:59 UTC 2015

Edward Kmett seems to have some concerns about its implications for type
checker implementations. Personally, I have little interest in programming
in a Haskell that doesn't have a feature with the power to express what
ScopedTypeVariables can. That said, there are some weird corners in the
design relating to pattern signatures and in particular to pattern bindings.
On Sep 30, 2015 2:26 PM, "Bardur Arantsson" <spam at scientician.net> wrote:

> On 09/30/2015 08:10 PM, David Feuer wrote:
> > The Eq constraint is needed to support pattern matching, the raison
> d’être
> > of pattern synonyms. I'm pretty sure the reason you need
> > ScopedTypeVariables for your second example is that GHC only allows
> pattern
> > signatures with that extension enabled. Once upon a time there was a
> > separate PatternSignatures extension, I believe, but not any more.
> Perhaps ScopedTypeVariables by default (or only?) would be relevant for
> the revamped[1] Haskell' committee?
> Would there be any actual downsides? It's always seemed to me that
> ScopedTypeVariables was the way it should have been from the start
> (given perfect foresight).
> Regards,
> [1] http://permalink.gmane.org/gmane.comp.lang.haskell.prime/3965
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