#2110: Rules to eliminate casted id's -------------------------------------+------------------------------------ Reporter: igloo | Owner: Type: feature request | Status: new Priority: lowest | Milestone: 7.6.2 Component: Compiler | Version: 6.8.2 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): I looked into this and further cleaned up the branch. But I just cannot get it to optimize `map unsafeCoerce` away, and the problem seems to be not enough eta-expansion in the simplifier. So here is the rule as desugared; it has the shape that we want it to have {{{ ------ Local rules for imported ids -------- "map/coerce" [ALWAYS] forall (@ a) (@ b) ($r$dCoercible :: a GHC.Prim.~R# b). GHC.Base.map @ a @ b ((\ (tpl :: a) -> tpl) `cast` (<a>_R -> ($r$dCoercible) :: (a -> a) ~# (a -> b))) = (\ (tpl :: [a]) -> tpl) `cast` (<[a]>_R -> [($r$dCoercible)]_R :: ([a] -> [a]) ~# ([a] -> [b])) }}} And here is the Core that we want it to match: {{{ GHC.Base.map @ GHC.Types.Int @ Test.Age ((Unsafe.Coerce.unsafeCoerce1 @ GHC.Types.Int @ Test.Age) `cast` (<GHC.Types.Int>_R -> UnivCo representational GHC.Types.Int Test.Age :: (GHC.Types.Int -> GHC.Types.Int) ~# (GHC.Types.Int -> Test.Age))) }}} And for reference, here is the information on `unsafeCoerce1`: {{{ unsafeCoerce1 :: forall a b. a -> a {- Arity: 1, HasNoCafRefs, Strictness: <S,1*U>, Unfolding: (\ @ a @ b x :: a -> x) -} }}} When trying to match, the matcher successfully matches the casts and then tries to match {{{ \ (tpl{v} [lid] :: a{tv aMF} [tv]) -> tpl{v} [lid] }}} against {{{ base:Unsafe.Coerce.unsafeCoerce1{v rKi} [gid] @ ghc-prim:GHC.Types.Int{(w) tc 3J} @ main:Test.Age{tc rM6} }}} It eta-expands both sides to match {{{ tpl{v} [lid] }}} against {{{ base:Unsafe.Coerce.unsafeCoerce1{v rKi} [gid] @ ghc-prim:GHC.Types.Int{(w) tc 3J} @ main:Test.Age{tc rM6} tpl{v} [lid] }}} and there it fails. Possible solutions: If GHC would inline `unsafeCoerce1` even though it is undersaturated, we’d be matching {{{ GHC.Base.map @ GHC.Types.Int @ Test.Age ((\ x :: GHC.Types.Int -> x) `cast` (<GHC.Types.Int>_R -> UnivCo representational GHC.Types.Int Test.Age :: (GHC.Types.Int -> GHC.Types.Int) ~# (GHC.Types.Int -> Test.Age))) }}} which would succeed (this is also the shape we get from `map Age`). But I did not manage to define `unsafeCoerce` in a way that this happens. Alternatively, the matching code could, after eta-expanding, simplify the expression, so that the unfolding would happen here. Otherwise, my code is ready for review, see branch `wip/nomeata-T2110`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2110#comment:41> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler