#15885: Enhancing COMPLETE pragma to support pattern synonyms with polymorphic (output) types -------------------------------------+------------------------------------- Reporter: Shayan-Najd | Owner: (none) Type: task | Status: new Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.6.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- On our work on the [https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow new front-end AST for GHC] based on [https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/TreesThatGro... TTG], we would like to use [https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/HandlingSour... a pattern synonym] similar to the following: {{{#!hs pattern LL :: HasSrcSpan a => SrcSpan -> SrcSpanLess a -> a pattern LL s m <- (decomposeSrcSpan -> (m , s)) where LL s m = composeSrcSpan (m , s) }}} We know that any match on `LL` patterns, makes the pattern matching total, as it uses a view pattern with a total output pattern (i.e., in `decomposeSrcSpan -> (m , s)`, the pattern `(m , s)` is total). As far as I understand, currently COMPLETE pragmas cannot be used with such a polymorphic pattern synonym. I believe we need to enhance COMPLETE pragmas to support such pattern synonyms. This can be done either syntactically, or (preferably) type-directed. For example, we should be able to write `{-# COMPLETE LL #-}` or `{-# COMPLETE LL :: HasSrcSpan a => a #-}`. In the type-directed approach a. the totality checker *may* need to track, at least, the set of required constraints of pattern synonyms mentioned in a COMPLETE pragma; and b. the order of pattern synonyms mentioned in a pragma should be taken into account (as noted by @carter). For example, in the case of `LL`, `HasSrcSpan a` is a required type constraint. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15885 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler