#12889: memory leak -------------------------------------+------------------------------------- Reporter: zoranbosnjak | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by rwbarton): * status: new => closed * resolution: => invalid Comment: Your program is retaining the evaluated list `[1,2,3,...]` which grows forever. You might say this is dumb, and in this case it is. But, if you had written {{{#!hs task = forM_ (filter expensiveTest [(1::Integer)..10000000]) $ \cnt -> do ... }}} then you might well have filed a bug wondering why your program is recalculating the list `filter expensiveTest [(1::Integer)..10000000]` for every call to `task`, whenever you make any of your changes 1, 2, 3. So, there is a real tension here. The normal behavior is to calculate a given expression once per time that its surrounding function is entered. Here there is no function around `[(1::Integer)..]` at all, so we calculate it just once during the lifetime of the program. If you remove the recursive call to `main` then the value is also used just once, so it can be GCed as it is generated, but normally this will lead to a space leak. The reason that the other changes cause `task` to be recalculated are more obscure, and probably not very interesting. If you want to tell GHC to always recalculate `task`, it's better to introduce an explicit function, like {{{#!hs task :: () -> ReaderT () IO () task () = forM_ [(1::Integer)..] $ \cnt -> do ... }}} (Even then GHC might decide to share the computation of `[(1::Integer)..]`, but then you have a different issue.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12889#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler