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{-# LANGUAGE ImplicitParams, RankNTypes #-}
-- Tests impredivative polymorphism with left-to-right
-- flow information; see the uses of "$"
module TestIP where
import Control.Monad.ST
import Data.STRef
-- Here's a use of runST with ($)
foo = runST $ (do { v <- newSTRef 0; readSTRef v })
-- Here's a use of implicit parameters with ($)
type PPDoc = (?env :: Int) => Char -> Char
f :: PPDoc -> PPDoc
f c = g $ c
-- Fully annotated version of f, as compiled by GHC 6.4.2
--
-- f ?env c = $ (C->C) (C->C)
-- (\(x:C->C). g ?env (\?env. x))
-- (c ?env)
--
-- The subsumption test needed from the call to $ is this:
-- ?env => (?env => C -> C) -> C -> C <= a->b
-- (?env => C -> C) -> C -> C <= a->b
-- (a) C->C <= b
-- (b) a <= (?env => C -> C)
-- And perhaps surprisingly (b) succeeds!
g :: PPDoc -> PPDoc
g d = d
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