#14112: bang patterns on pattern synonyms? (left vs right hand sides) -------------------------------------+------------------------------------- Reporter: carter | Owner: RyanGlScott Type: bug | Status: patch 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): Phab:D3896 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => patch * differential: => Phab:D3896 Comment: I've implemented (A) in Phab:D3896. Replying to [comment:10 carter]:
so what i'm sensing is the point that
normal / "strict" data types are only strict on construction, but lazy on matching (except perhaps when dealing with unpacked/unboxed style internal tricks eg mapping Int to Int# etc).
In contrast, we have here a way that can express lazy construction and strict matching?
am i missing something?
I'd summarize the point by saying that if you were to hypothetically allow an implicitly bidirectional pattern synonym with a bang pattern in the RHS, you'd want both the pattern and the builder expression to be strict. However, in practice GHC was making the pattern strict, but not the builder. We pondered whether it would be possible to desugar a builder that figured out the right strictness, but this turns out to be an awkward thing to do in the presence of other pattern synonyms in the RHS. It's much easier to simply disallow bang patterns in the RHS altogether, since this avoids the need to explain away a special syntactic rule that only applies for expressions in implicitly bidirectional pattern synonyms. (In Phab:D3896, I've tailored the error message so that if you do try typing something like `data StrictJust a = Just !a`, it recommends using an explicitly bidirectional pattern synonym instead.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14112#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler