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{-# LANGUAGE NoImplicitPrelude #-}
module Compiler.ST (ST, runST, fixST)
where
import Compiler.Base -- for seq
import Data.Tuple -- for snd
import Data.Function -- for $
import Data.Functor
import Control.Applicative
import Control.Monad
data ST s a = ST { runST :: s -> (s, a) }
instance Functor (ST s) where
fmap f t = ST (\state1 -> let (state2, result) = runST t state1
in (state2, f result)
)
instance Applicative (ST s) where
pure x = ST (\s -> (s, x))
t1 <*> t2 = ST (\state1 -> let (state2,f) = runST t1 state1
(state3,x) = runST t2 state2
in (state3, f x))
instance Monad (ST s) where
f >>= g = ST (\state1 -> let (state2,x) = runST f state1 in x `seq` runST (g x) state2)
unsafeInterleaveIO f = ST (\state -> (state, snd $ runST f state))
fixST f = ST (\state1 -> let result@(state2, x) = runST (f x) state1 in result)
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