#14112: bang patterns on pattern synonyms? (left vs right hand sides) -------------------------------------+------------------------------------- Reporter: carter | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 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 simonpj): Ryan has it right. The syntax is {{{ pattern P x y z = <pattern> }}} Hence bangs are allowed. For the implicitly-bidirectional case we have to turn the `<pattern>` into an expression. Currently, that conversion simply discards the bangs, which is the behaviour you are seeing. Silently ignoring the bangs is arguably rather undesirable. (The conversion also silently ignores a `~`, but that's probably correct behaviour.) It would make more sense to do one of these: A. Reject an implicitly-bidirectional pattern with bangs, on the grounds that it's not invertible (like view patterns, say); suggest using an explicitly-bidirectional pattern instead. B. Auto-generate the bang'd definition that Ryan writes above. {{{ MkPair1 !x !y = MkPair x y }}} That is, in the `ImplicitBidirectional` case, if we see `!x` in the pattern, use a `!x`, instead of `x` as the argument pattern of the builder. I think I prefer (B). It's easy to implement: see `mk_match_group` in `TcPatSyn.tcPatSynBuilderBind` . You'd need to alter `tcPatToExpr` to return the banged variables. (Ignore bangs on constructors e.g. `Just !(Just x)`; they are no-ops.) Don't forget to specify all this in the user manual. Volunteers? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14112#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler