#8581: Pattern synonym used in an expression context could have different constraints to pattern used in a pattern context -------------------------------------+------------------------------------- Reporter: cactus | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 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: | -------------------------------------+------------------------------------- Description changed by mpickering: Old description:
The two directions of an explicitly-bidirectional pattern might have utterly different class constraints. After all, the two directions are specified by quite different code. Suppose that
* Pattern `P` (used in a pattern) requires constraints `CR`, and provides constraints `CP` * Constructor `P` (used in an expression) requires constraints `CE`
Then I think the only required relationship is this: `CP` must be provable from `CE` (since CP is packaged up in a P-object).
Currently, `CE := CP + CR.
The two directions of an explicitly-bidirectional pattern might have utterly different class constraints. After all, the two directions are specified by quite different code. Suppose that
* Pattern `P` (used in a pattern) requires constraints `CR`, and
* Constructor `P` (used in an expression) requires constraints `CE`
Then I think the only required relationship is this: `CP` must be
New description: provides constraints `CP` provable from `CE` (since CP is packaged up in a P-object). Currently, `CE := CP + CR`. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8581#comment:31 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler