[Haskell-cafe] selectively allow alternatives in a sum type

David Feuer david.feuer at gmail.com
Tue Aug 30 15:09:12 UTC 2022

One downside of this *particular* formulation is that it doesn't play well
with Coercible.

On Tue, Aug 30, 2022, 9:31 AM Olaf Klinke <olf at aatal-apotheke.de> wrote:

> Dear Café,
> Is there prior art/existing packages for the following? Is it
> maybefunctional programming folklore? Is it a sign of bad program design?
> Sometimes I feel the need to selectively allow or disallow alternatives
> in a sum type. That is, suppose we have a sum type
> data Foo = LeftFoo !A | RightFoo !B
> and at some places in the program we want the type system to enforce
> that only the constructor LeftFoo can be used. My solution would be to
> use a strict version of Data.Functor.Const and make the type higher
> rank:
> newtype Const' a b = Const' !a
> -- Const' Void b ~ Void
> -- Const' ()   b ~ ()
> data Foo' f = LeftFoo' !A | RightFoo' !(f B)
> type Foo = Foo' Identity
> type LeftFoo = Foo' (Const' Void) -- can not construct a RightFoo'
> The advantage over defining LeftFoo as an entirely different type is
> that Foo and LeftFoo can share functions operating entirely on the left
> option.
> Olaf
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