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{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
{-# LANGUAGE RankNTypes, ConstraintKinds, KindSignatures, DataKinds, TypeFamilies #-}
module T9569 where
import GHC.Exts
data Proxy (c :: Constraint)
class Deferrable (c :: Constraint) where
defer :: Proxy c -> (c => a) -> a
deferPair :: (Deferrable c1, Deferrable c2)
=> Proxy (c1,c2) -> (((c1,c2) :: Constraint) => a) -> a
-- NB: ((c1,c2) :: Constraint) => blah
-- is different form
-- (c1,c2) => blah
-- The former has dict, the latter has two
deferPair _ _ = undefined
instance (Deferrable c1, Deferrable c2) => Deferrable (c1,c2) where
-- defer p f = deferPair p f -- Succeeds
defer = deferPair -- Fails
{- Notes Apr 2020.
~~~~~~~~~~~~~~~~~
Note the careful type for deferPair! You can also say
deferPair :: (Deferrable c1, Deferrable c2, d ~ (c1,c2))
=> Proxy (c1,c2) -> (d => a) -> a
but NOT
deferPair :: (Deferrable c1, Deferrable c2)
=> Proxy (c1,c2) -> ((c1,c2) => a) -> a
The point is that
(c1,c2) => a
is short for
c1 => c2 => a
-}
{-
[G] Deferrable c1, Deferrable c2
[W] Proxy (c1,c2) -> ((c1,c2) => a) -> a
~
Proxy (c1x,c2x) -> ((c1x,c2x) => ax) -> ax
[w] Deferrable c1x
[w] Deferrable c2x
-}
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