[Haskell-cafe] Strange space leak

[David Feuer]

Interesting. We generally have

pure x = IO (\ s -> (# s, x #))
IO m *> n = IO (\ s -> case m s of (# new_s, a #) -> unIO ((const n) a) new_s)

so
main = let loop () = IO (\s -> (# s, () #)) *> loop () in loop ()
====
let loop () = IO (\s -> case (\s' -> (# s', () #)) s of (# new_s, a #)
-> unIO ((const (loop ()) a) new_s) in loop ()
====
let loop () = IO (\s -> case (# s, () #) of (# new_s, a #) -> unIO
(loop ()) new_s) in loop ()
====
let loop () = IO (\s -> unIO (loop ()) s) in loop ()

My only guess is that *somehow* this is getting memoized and expanding
under the lambda to

let loop () = IO (\s -> unIO (IO (\s -> unIO ......)) s) in loop ()

but I know nothing about GHCi internals, so that seems as weird to me
as it does to you.

On Fri, Nov 4, 2016 at 10:58 AM, Tom Ellis
<tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> On Fri, Nov 04, 2016 at 02:44:00PM +0000, Tom Ellis wrote:
>> Can anyone explain why this leaks space:
>>
>>     main :: IO ()
>>     main = let loop () = return () *> loop ()
>>            in loop ()
>>
>> whilst this doesn't:
>>
>>     main :: IO ()
>>     main = let loop = return () *> loop
>>            in loop
>
> More strangely, the space leak only appears in ghci (and runhaskell).  When
> compiled there is no such space leak.
>
> Tom
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