To avoid actually realizing those boring fields in memory, use an unlifted
newtype.
type VoidUnit# :: TYPE ('TupleRep '[])
newtype VoidUnit# = VoidUnit# VoidUnit#
absurd# :: VoidUnit# -> a
absurd# (VoidUnit# v) = absurd# v
Unfortunately, GHC's pattern checker isn't smart enough to see by itself
that VoidUnit# is uninhabited, even though that's pretty obvious to a human.
TTG also has coercion problems thanks to the type families. *Sigh*
On Wed, Aug 31, 2022, 4:48 PM Olaf Klinke
Trees that grow is essentially this, but using type families.
data Foo ksi = LeftFoo !(XLeftFoo ksi) !A | RightFoo !(XRightFoo ksi) !B
type family XLeftFoo ksi :: Type type family XRightFoo ksi :: Type
Then you can define
data Both type instance XLeftFoo Both = () type instance XRightFoo Both = ()
or
data OnlyLeft type instance XLeftFoo OnlyLeft = () type instance XRightFoo OnlyLeft = Void
---
Third option is to simply have parametrized data:
data Foo b = LeftFoo !A | RIghtFoo !b
type FooBoth = Foo B type FooOnlyLeft = Foo Void
---
Sometimes I prefer a higher-kinded data approach, especially if there is very little variants needed, and they are "uniform" (e.g. second variant doesn't have some fields). Yet, sometimes simple parameterized data is simplest approach (especially as you get stuff for free by deriving Traversable!).
On the other hand, if type is big and have many uniform extension points (even if there are few different combinations), then HKD becomes boilerplate heavy as well. The drawback of TTG, is that writing polymorphic code (i.e. which work for any or some `ksi`s) is not very fun: a lot of equality constraints etc.
- Oleg
So I re-discovered Trees that Grow sans the trick of using type families to name particular combinations of parameters. Hooray Najd, and thanks Oleg for bringing this up.
Olaf
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