After reading this proposal, I agree that StandaloneKindSignatures ought
to be encouraged in the codebases, and I vote that we mention them in
the coding style¹.
Cheers,
Hécate
———
¹ https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/coding-style
Cheers,
Hécate.
Le 18/05/2021 à 19:58, Baldur Blöndal a écrit :
Discussion to permit use of StandaloneKindSignatures in the GHC coding
style guide. I believe it increases the clarity of the code,
especially as we move to fancier kinds.
It is the only way we have for giving full signatures to type
synonyms, type classes, type families and others. An example:
type Cat :: Type -> Type
type Cat ob = ob -> ob -> Type
type Category :: forall ob. Cat ob -> Constraint
class Category cat where
id :: cat a a ..
type Proxy :: forall k. k -> Type
data Proxy a = Proxy
type Some :: forall k. (k -> Type) -> Type
data Some f where
Some :: f ex -> Some f
-- | The regular function type
type (->) :: forall {rep1 :: RuntimeRep} {rep2 :: RuntimeRep}.
TYPE1 rep1 -> TYPE rep2 -> Type
type (->) = FUN 'Many
This is in line with function definitions that are always given a
top-level, standalone type signature (1) and not like we currently
define type families/synonyms (2) by annotating each argument or not
at all. Using -XStandaloneKindSignatures (3) matches (1)
-- (1)
curry :: ((a, b) -> c) -> (a -> b -> c)
curry f x y = f (x, y)
-- (2)
type Curry (f :: (a, b) -> c) (x :: a) (y :: b) = f '(x, y) :: c
-- (3)
type Curry :: ((a, b) -> c) -> (a -> b -> c)
type Curry f x y = f '(x, y)
It covers an edgecase that `KindSignatures` don't. The only way for
deriving to reference datatype arguments is if they are quantified by
the declaration head -- `newtype Bin a ..`. StandaloneKindSignatures
allows us to still provide a full signature. We could write `newtype
Bin a :: Type -> Type` without it but not `newtype Bin :: Type -> Type
-> Type`
type Bin :: Type -> Type -> Type
newtype Bin a b = Bin (a -> a -> b)
deriving (Functor, Applicative)
via (->) a `Compose` (->) a
Let me know what you think
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