Deref2.purs

module Deref where

import Prelude hiding (map, apply, pure)

import Record as Record
import Prelude as Prelude
import Prim.Row as Row
import Data.Symbol (SProxy(..), class IsSymbol)

data Deref r1 (r2 :: # Type) a = Deref ({ | r1 } -> a)

data DerefOf a (sym :: Symbol) = DerefOf

deref :: forall a b. DerefOf a b
deref = DerefOf

map ::
  forall r1 r2 rx sym a b.
  IsSymbol sym =>
  Row.Cons sym a rx r1 =>
  Row.Cons sym a () r2 =>
  (a -> b) -> DerefOf a sym -> Deref r1 r2 b
map f _ = Deref (Prelude.map f (Record.get (SProxy :: _ sym)))

apply ::
  forall sproxy r1 r2 r3 rx sym a b.
  IsSymbol sym =>
  Row.Cons sym a rx r1 =>
  Row.Cons sym a r2 r3 =>
  Deref r1 r2 (a -> b) -> DerefOf a sym -> Deref r1 r3 b
apply (Deref a) _ = Deref (Prelude.apply a (Record.get (SProxy :: _ sym)))

test = ado
  a <- deref :: _ "a"
  b <- deref :: _ "b"
  in a + b

runDeref :: forall r a. Deref r r a -> { | r } -> a
runDeref (Deref k) = k

result = runDeref test { a: 1, b: 2 }