#12543: Warning for duplicate language extensions -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- I recently discovered that `-XRebindableSyntax` implies `-XNoImplicitPrelude`. Since I frequently listed both extensions at the top of my file with a `LANGUAGE` pragma, this knowledge allowed me to remove many superfluous `-XNoImplicitPrelude` pragmas. I wouldn't be surprised if there are other unused/implied pragmas floating around in my code. It would be nice to have a warning when two pragmas are explicitly listed, but one implies the other. The warning format could closely follow the warning for #9939. For example: `{-# LANGUAGE NoImplicitPrelude, RebindableSyntax #-}` would produce the warning {{{ Main.hs:1:1: warning: [-Wredundant-pragmas] • Redundant pragmas: RebindableSyntax => NoImplicitPrelude • In the language pragmas: {-# LANGUAGE NoImplicitPrelude, RebindableSyntax #-} }}} Although it might be harder (impossible?) to detect, it would also be nice if there was a similar warning for unused pragmas. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12543 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler