> On Mar 26, 2021, at 8:41 PM, Alexis King <lexi.lam...@gmail.com> wrote:
>
> If there’s a single principal type that makes my function well-typed and
> exhaustive, I’d really like GHC to pick it.
I think this is the key part of Alexis's plea: that the type checker take into
account exhaustivity in choosing how to proceed.
Another way to think about this:
> f1 :: HList '[] -> ()
> f1 HNil = ()
>
> f2 :: HList as -> ()
> f2 HNil = ()
Both f1 and f2 are well typed definitions. In any usage site where both are
well-typed, they will behave the same. Yet f1 is exhaustive while f2 is not.
This isn't really about an open-world assumption or the possibility of extra
cases -- it has to do with what the runtime behaviors of the two functions are.
f1 never fails, while f2 must check a constructor tag and perhaps throw an
exception.
If we just see \HNil -> (), Alexis seems to be suggesting we prefer the f1
interpretation over the f2 interpretation. Why? Because f1 is exhaustive, and
when we can choose an exhaustive interpretation, that's probably a good idea to
pursue.
I haven't thought about how to implement such a thing. At the least, it would
probably require some annotation saying that we expect `\HNil -> ()` to be
exhaustive (as GHC won't, in general, make that assumption). Even with that,
could we get type inference to behave? Possibly.
But first: does this match your understanding?
Richard
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