#14806: Officially sanction certain unsafeCoerce applications with unboxed unary tuples -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.1 Component: Documentation | Version: 8.2.2 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: -------------------------------------+------------------------------------- It seems that it should be okay to `unsafeCoerce` between types that wrap certain things in unboxed unary tuples and ones that do not. For example, {{{#!hs unsafeCoerce :: (A -> B) -> A -> (# B #) unsafeCoerce :: ((# A #) -> B) -> A -> B }}} Generally, I believe `unsafeCoerce :: E1 -> E2` should be okay when the only differences are in what is wrapped in an unboxed unary tuple and both of the following hold: 1. Each new unary tuple wrapper in `E2` is in a positive position. 2. Each new unary tuple wrapper in `E1` is in a negative position. Semantically, {{{#!hs unsafeCoerce :: (A -> B) -> A -> (# B #) = \f a -> let !fa = f a in (# fa #) unsafeCoerce :: ((# A #) -> B) -> A -> B = \f !a -> f a }}} Am I correct in this interpretation? If so, is this something the developers would be willing to commit to and document? The first version in particular (a new unary tuple wrapper in positive position in the result) would be very useful for reducing both source code and generated code size in libraries supporting both strict and lazy operations. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14806 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler