#15606: Don't float out lets in between lambdsa -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider {{{ f = \x. let y = <blah> in \z. let v = h x y in <stuff> }}} The full laziness pass will float out that v-binding thus {{{ f = \x. let y = <blah> v = h x y in \z. <stuff> }}} And now (if `h` is, say, imported) it'll stay like that. But suppose `<blah>` simlifies to `Just x`. Then we allow ourselves to eta-expand thus {{{ f = \x z. let y = <blah> v = h x y in <stuff> }}} Now (an early design choice in the let-floater) we never float the v-binding in between the `\x` and `\z`. This is very non-confluent: a smal change in exactly how rapidly `<blah>` simplifies can have a big, irreversible effect on the code for `f`. IDEA: extend the let-floater's design choice to not float out between two lambdas, even if they are separated by lets/cases etc. One way to say this is to ask when a lambda gets a new level number compared to its immediately enclosing lambda. Examples where `y` gets the same level number as `x` * `\x.\y. blah` * `\x. let binds in \y` * `\x. case scrut of pi -> \y.blah` Examples where `y` gets the a level number one bigger than `x` * `\x. let v = \y.rhs in blah` * `\x. f (\y.rhs)` This probably won't make a lot of difference, but it'd be worth trying -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15606 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler