[generics-rep] No instance for Unit?

[athanclark]

Why isn't there a Generic Unit rep instance? Is it because there's no data constructors? If so, why would it not be technically trivial to generate a value of Unit given the fact that one exists? I'm not sure if I follow the technical reasoning why there isn't an instance, but I could see it because of how weird Unit is for purescript.

[garyb]

I don't know why there isn't an instance here - it could be because of the Unit representation, but I'm not sure if that matters so much with -generic-rep. Maybe someone else knows better?

I assume the instance would be

instance genericUnit ∷ Generic Unit (Constructor "Unit" NoArguments) where
  to _ = unit
  from _ = Constructor NoArguments

[athanclark]

This is probably asked a lot too - but why stop at Unit? Why not also have instances for all elementary types? Is it because it's supposed to only represent the structure of algebraic data types? I feel like there could pretty easily be constants for supporting types like Int etc:

data GenericInt = GenericInt Int

instance genericInt :: Generic Int GenericInt where
  to (GenericInt x) = x
  from x = GenericInt x

instance genericShowGenericInt :: GenericShow GenericInt where
  genericShow' (GenericInt x) = show x

Now if I make my own fancy type deriving generic:

data Foo a = Bar | Baz a
derive instance genericFoo :: Generic a a' => Generic (Foo a) _
instance showFoo :: (Show a, Generic a a') => Show (Foo a) where
  show = genericShow

then this wouldn't cause a type error:

show (Foo 1) -- "Foo 1"

[hdgarrood]

That's not really how Generic is meant to be used. Unlike the previous (now-deprecated) Generic class from purescript-generics, this one is intended to only 'peel off' one layer of your data type, so that you aren't locked in to using Generic all the way down your entire data structure. So when you use it, you're supposed to use it together with type classes which have instances for the 'leaves' of your data types already.

See for example the EncodeRep and EncodeRepArgs classes in argonaut-generic: https://pursuit.purescript.org/packages/purescript-argonaut-generic/5.0.0/docs/Data.Argonaut.Encode.Generic.Rep

EncodeRep delegates to EncodeRepArgs for arguments of data constructors; see the instance (IsSymbol name, EncodeRepArgs a) => EncodeRep (Constructor name a). Then, EncodeRepArgs itself delegates to the standard EncodeJson class for data constructor arguments; see the instance (EncodeJson a) => EncodeRepArgs (Argument a).

I suspect that in your case, the issue is the constraint on your genericFoo instance. You should just write:

data Foo a = Bar | Baz a
derive instance genericFoo :: Generic (Foo a) _
instance showFoo :: (Show a) => Show (Foo a) where
  show = genericShow

[JordanMartinez]

Should this issue be closed?

[hdgarrood]

Yes, I think so.