#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: | -------------------------------------+------------------------------------- Comment (by duncan): Right, this is an initial idea and hasn't been fleshed out. Thanks for the probing example :-) So the intention is that it's a purely local thing. So in that example, the answer is no, we do not expect a caller far away to have to know anything. The idea is that evaluating "via" the noupdate field should not perform thunk updates, but I appreciate that may not match how thunk construction and update works. So how about something like this... Suppose the primitive is not on fields, but on let. This is by analogy with strict `let !_ =` versus strict constructor fields. The primitive with strictness is at use sites and a convenience for systematic use we can push it to constructor fields, which is defined in terms of constructor wrappers. So suppose the primitive is `let {-# NOUPDATE #-} x = ...`, and so then the `Yield` constructor above could perhaps be defined with a wrapper like {{{#!hs data Pipe i o r = Yield o {-# NOUPDATE #-} (Pipe i o r) yield o x = let {-# NOUPDATE #-} x' = x in Yield o x' }}} So in your `f x` example above then this would do very little (and indeed we'd want it to do precisely nothing different to the usual, by shorting out the extra let indirection). But if things are defined with `Yield (expr)` or locally ghc decides to float/push things in, then the expression would end up in the `let {-# NOUPDATE #-} x' = ...` and so there would be an effect. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12620#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler