I'm trying to wrap my head around Cont and callCC, by reducing this function:
s0 = (flip runContT) return $ do
(k, n) <- callCC $ \k -> let f x = k (f, x)
in return (f, 0)
lift $ print n
if n < 3
then k (n+1) >> return ()
else return ()
I've managed to reach this point:
s21 = runContT (let f x = ContT $ \_ -> cc (f, x) in ContT ($(f,0))) cc where
cc = (\(k,n) -> let
iff = if n < 3 then k (n+1) else ContT ($())
in print n >> runContT iff (\_ -> return ()))
And at this point i have no idea what to do with recursive definition of f
What is the best way to finish this reduction?
You can proceed as follows.
s21 = runContT (let f x = ContT $ \_ -> cc (f, x) in ContT ($(f,0))) cc where
cc = (\(k,n) -> let
iff = if n < 3 then k (n+1) else ContT ($())
in print n >> runContT iff (\_ -> return ())
-- runContT is the opposite of ContT
s22 = (let f x = ContT $ \_ -> cc (f, x) in ($(f,0))) cc
where
cc = (\(k,n) -> let
iff = if n < 3 then k (n+1) else ContT ($())
in print n >> runContT iff (\_ -> return ())
-- reordering
s23 = ($(f,0)) cc
where
f x = ContT $ \_ -> cc (f, x)
cc = (\(k,n) -> let
iff = if n < 3 then k (n+1) else ContT ($())
in print n >> runContT iff (\_ -> return ())
s24 = cc (f,0)
where ...
-- beta
s25 = let iff = if 0 < 3 then f (0+1) else ContT ($())
in print 0 >> runContT iff (\_ -> return ())
where ...
-- if, arithmetics
s26 = let iff = f 1
in print 0 >> runContT iff (\_ -> return ())
where ...
s27 = print 0 >> runContT (f 1) (\_ -> return ())
where ...
s28 = print 0 >> runContT (ContT $ \_ -> cc (f, 1)) (\_ -> return ())
where ...
s29 = print 0 >> (\_ -> cc (f, 1)) (\_ -> return ())
where ...
s30 = print 0 >> cc (f, 1)
where ...
-- repeat all the steps s24..s30
s31 = print 0 >> print 1 >> cc (f, 2)
where ...
-- etc.
s32 = print 0 >> print 1 >> print 2 >> cc (f, 3)
where ...
s33 = print 0 >> print 1 >> print 2 >>
let iff = if 3 < 3 then f (3+1) else ContT ($())
in print 3 >> runContT iff (\_ -> return ())
where ...
s34 = print 0 >> print 1 >> print 2 >> print 3 >>
let iff = ContT ($())
in runContT iff (\_ -> return ()))
where ...
s35 = print 0 >> print 1 >> print 2 >> print 3 >>
runContT (ContT ($())) (\_ -> return ())
where ...
s36 = print 0 >> print 1 >> print 2 >> print 3 >>
($()) (\_ -> return ())
where ...
s37 = print 0 >> print 1 >> print 2 >> print 3 >>
return ()