#14112: bang patterns on pattern synonyms? (left vs right hand sides) -------------------------------------+------------------------------------- Reporter: carter | Owner: RyanGlScott Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1 Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): They might be ignored for data constructors, yes. But what about pattern synonyms? For instance, we have that: {{{ λ> pattern Id a = a λ> let f (Id _) = "foo" λ> f undefined "foo" λ> let g !(Id _) = "bar" λ> g undefined "*** Exception: Prelude.undefined CallStack (from HasCallStack): error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err undefined, called at <interactive>:19:3 in interactive:Ghci9 }}} That is to say, bang patterns aren't no-ops for //every// conlike, as they matter in the presence of pattern synonyms. This awkwardly melds with implicitly bidirectional pattern synonyms, since if you define this: {{{ λ> pattern Foo a = Just !(Id a) λ> case Foo undefined of Just _ -> "wat" "wat" λ> case Just undefined of Foo _ -> "wat" "*** Exception: Prelude.undefined CallStack (from HasCallStack): error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err undefined, called at <interactive>:22:11 in interactive:Ghci11 }}} Notice that the bang pattern doesn't kick in when `Foo` is used as an expression, only as a pattern. In order to make a `Foo` expression strict as a pattern, you'd have to dig underneath the `Id` pattern synonym to figure out whether //its// bindings are strict or not. All of this sounds terribly fiddly in contrast to option (A), which is simple to describe, implement, and even gives nice error messages. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14112#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler