[ghc-steering-committee] #380 GHC2021: What's wrong with Functional dependencies

Simon Peyton Jones simonpj at microsoft.com
Fri Dec 4 14:07:38 UTC 2020

We’re back to “what is GHC2021 for?”   Maybe we need to refine the criteria.

Arnaud seems to be saying “anything that is guarded by its own syntax, and is widely used, should go in”.   But GHC2020 is also saying “these are blessed extensions that are well understood and we expect to be part of the language forever”.

You could argue that fundeps are in that class.   But let’s not forget the “expect to be part of the language forever”.   For me, standalone kind signatures are definitely “forever”.  But fundeps – because they don’t carry evidence, as they stand – feel less solidly rooted.


From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Spiwack, Arnaud
Sent: 04 December 2020 12:44
To: GHC Steering Committee <ghc-steering-committee at haskell.org>
Subject: [ghc-steering-committee] #380 GHC2021: What's wrong with Functional dependencies

Dear all,

It seems that a number of us have voted against including FunctionalDependencies and  TypeFamilyDependencies.

I don't understand why. They are properly guarded by syntax. I don't know any complaint against them FunctionalDependencies, at least, is as standard as it gets (there are functional dependencies all over the mtl).

Is it an oversight? Or do some of us really believe that we ought to keep these out, and why?

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