#7994: Make foldl into a good consumer -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by nomeata): So, this is what we want to happen, somehow. We want to transform code of the form {{{ let f = λ x. let h = f (g1 x) in λ y... h (g2 y) ... in ...f 1 2..... f 3 4 ... }}} into {{{ let f = λ x y. let h = f (g1 x) in ... h (g2 y) ... in ...f 1 2..... f 3 4 ... }}} If `g1` is cheap, this is already done (see `[Arity analysis]` in `CoreArity`). But in order to implement `foldl` in terms of `foldr` and get list fusion for it (which would be nice), this needs to work also when `g1` is expensive. In a slightly more general setting, the transformation would not be ok, e.g. in {{{ let f = λ x. let h = f (g1 x) in λ y... h (g2 y) ... in ...map (f 1).... }}} or {{{ let f = λ x. let h = f (g1 x) in λ y... map h... in ...f 1 2..... f 3 4 ... }}} because the expensive `g1` would no longer be shared. So we want an analysis that * looks at the call-sites of `f` * notices that the demand put on `f` from the body of the let, `C(C¹(S))`, is actually better than the vanilla demand `C(S)`, * makes sure that with this demand put on `f`, its definition (the rhs) now also has this demand (i.e. no sharing lost in the right hand sid), * uses this to mark the `λ y` as one-shot, which will presumably allow the desired optimization. The demand analyser currently analyses the function before the body. One could do a second round, but there is a risk of exponential blow-up. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7994#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler