#10744: Allow oneShot to work with unboxed types -------------------------------------+------------------------------------- Reporter: akio | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): I think it'd be fine to allow `oneShot` to accept unboxed types; just a matter of using `openAlphaTyVar` etc in `MkId.oneShot`. Give it a try and submit a patch. Moreover, the worst that can happen if you mis-use it, by applying it to a function that is called many times, is that computation may be repeated. For example {{{ f xs y = let x = expensive_fn y in map (oneShot (\v. x+v)) xs }}} The `oneShot` claims (falsely unless `xs` has length at most one) that `(\v. x+v)` is called at most once. So GHC will move the call to `expensive_fn` inside the lambda thus: {{{ f xs y = map (oneShot (\v. expensive_fn y + v)) xs }}} That will work just fine, but it'll call `expensive_fn` once per element of `xs`. So, no seg-faults at least! It would be good if someone felt like extending the Haddock comments for `GHC.Magic.oneShot` to explain this. Thanks Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10744#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler