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

Olaf Klinke olf at aatal-apotheke.de
Tue Aug 30 13:30:39 UTC 2022

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

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


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