On Tue, Oct 04, 2016 at 06:56:48PM +0100, Tom Ellis wrote:
On Tue, Oct 04, 2016 at 06:19:52PM +0100, Tom Ellis wrote:
My results below. Looks like there's something wrong with *> for ReaderT and StateT.
This seems to be how it executes
let loop = return () *> loop in loop in runReaderT loop () [...]
I'm adding an evaluation trace of 'forever' in terms of '>>' to this discussion because the issue has arisen in https://www.reddit.com/r/haskell/comments/5axh2j/two_methods_of_using_statet... let loop = return () >> loop in loop in runReaderT loop () -- Un-newtype let loop = return () >> loop in loop in loop () -- Un-newtype let loop = return () >> loop in loop in loop () -- Definition of >> let loop = return () >>= (\_ -> loop) in loop in loop () -- >>= for Reader in terms of >>= for m let loop = \r -> (return () r) >>= (\_ -> loop r) in loop in loop () let loop = \r -> (return () r) >>= (\_ -> loop r) in loop in (\r -> (return () r) >>= (\_ -> loop r)) () let loop = \r -> (return () r) >>= (\_ -> loop r) in loop in (return () ()) >>= (\_ -> loop ()) -- return () for ReaderT in terms of return () for m let loop = \r -> (return () r) >>= (\_ -> loop r) in loop in return () >>= (\_ -> loop ()) So for m = IO evaluation proceeds without a space leak. We evaluate 'return ()' and then proceed to evaluate 'loop ()' without creating any new thunk. 'forever' needs urgent fixing! Tom