Binrep.Generic.CBLen

Generically derive CBLen type family instances.

This is mostly playing around -- I've only just learned regular GHC generics, and pulling everything up to the type level is even more confusing and weird.

A CBLen instance usually indicates a type is either extremely simple or comes with size information in the type. Be careful deriving or writing instances for your own types - BLen is usually correct/sufficient.

You can attempt to derive a CBLen type family instance generically for a type via

type instance CBLen a = CBLenGeneric w a

As with deriving BLen, you must provide the type used to store the sum tag for sum types.

Then try doing something with it e.g. have GHC derive a BLen instance for you via the default method (that reifies CBLen)

deriving anyclass BLen a

Hopefully it either compiles, or you get a useful type error. If not, sorry.

Documentation

type CBLenGeneric w a = GCBLen w (Rep a)

type family GCBLen w (f :: k -> Type) :: Natural where ...

Equations

GCBLen _ U1 = 0

GCBLen _ (K1 i c) = CBLen c

GCBLen w (l :*: r) = GCBLen w l + GCBLen w r

GCBLen w (l :+: r) = CBLen w + GCBLenCaseMaybe (GCBLenSum w (l :+: r))

GCBLen _ V1 = TypeError GErrRefuseVoid

GCBLen w (M1 _ _ f) = GCBLen w f

type family GCBLenSum w (f :: k -> Type) where ...

Equations

GCBLenSum w (C1 ('MetaCons name _ _) f) = JustX (GCBLen w f) name

GCBLenSum w (l :+: r) = MaybeEq (GCBLenSum w l) (GCBLenSum w r)

type family MaybeEq a b where ...

Equations

MaybeEq (JustX n nName) (JustX m _) = If (n == m) (JustX n nName) NothingX

MaybeEq _ _ = NothingX

type family GCBLenCaseMaybe a where ...

I don't know how to pattern match in types without writing type families.

Equations

GCBLenCaseMaybe (JustX n _) = n

GCBLenCaseMaybe NothingX = TypeError ('Text "Two constructors didn't have equal constant size." :$$: 'Text "Sry dunno how to thread errors thru LOL")

data JustX a b

data NothingX