#12620: Allow the user to prevent floating and CSE -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #9520, #8457 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by edsko): * related: => #9520, #8457 Comment: Right, so a lot of the thinking that led to this ticket came from trying to understand memory leaks in conduit code. See my recent blog post http://www.well-typed.com/blog/2016/09/sharing-conduit/ where these issues are described in great detail; this should also serve, I hope, as one "compelling use case". That said, I like the idea of a "noupdate" much better than a "nofloat". It would seem to me that its semantics would be easier to specify; and if it means I don't have to think so hard about what exactly the optimizer is doing to my code in order to understand why I do or do not have a memory leak, that would very welcome. I really like @duncan 's suggestion of having a type annotation on a type; though we might also want some adhoc way of saying "make ''this'' thunk not-updateable". An easyish experiment perhaps might be to declare a magic datatype {{{#!hs data DontUpdate a = DontUpdate a }}} with the property that any code that looks at the thunk in the payload of `DontUpdate` doesn't cause that thunk to be updated. Then in @duncan 's example we could define {{{#!hs data Pipe i o r = Yield o (DontUpdate (Pipe i o r)) }}} That said, I'm not sure exactly what DontUpdate should do for the lambda; but this is a question about @duncan's proposal too. I ''think'' what we want to happen is that the thunks in the function closure never get updated (this, in a nutshell, is what is causing memory leaks in conduit code; see the blog post); but that's already more magical than just saying "don't update this thunk". -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12620#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler