{-# LANGUAGE ImplicitParams, ConstraintKinds, GADTs, RankNTypes #-} module Main where — начинается

Question

{-# LANGUAGE ImplicitParams, ConstraintKinds, GADTs, RankNTypes #-} module Main where — начинается

имплементация рекордов

data Rec fields where
Rec :: fields => Rec fields

infixr 1 ?
(?) :: Rec fields -> (fields => r) -> r
Rec ? e = e

— готово

type RXY = Rec (?x::Int, ?y::Int)

xy, xy' :: RXY

xy = let ?x = 1; ?y = 2 in Rec

xy' = Rec where ?x = 3; ?y = 4

foo :: RXY -> Int
foo r = r ? ?x + ?y

type RZ = Rec (?z::Int)

z :: RZ
z = Rec where ?z = 42

— type RXYZ = (RXY, RZ)
type RXYZ = Rec (?x::Int, ?y::Int, ?z::Int)

xyz :: RXYZ
xyz = xy ? z ? Rec — concatenation

t1 = foo xy
t2 = foo (xyz ? Rec) — subtyping
t3 = foo (xy ? Rec)

main = print $ foo xy