#14318: TH shadowing bind statement triggers -Wunused-matches -------------------------------------+------------------------------------- Reporter: lyxia | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Template Haskell | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by lyxia: Old description:

{{{ $(newName "x" >>= \x -> newName "f" >>= \f -> lamE [varP f, varP x] (doE [ bindS (varP x) (listE [varE f `appE` varE x]) , noBindS (varE x)]) ) }}}

generates the following expression:

{{{ \f x -> do x <- f x x }}}

and `-Wunused-matches` complains that `x` is not used, while both bound occurrences are in fact used.

New description: {{{ {-# LANGUAGE TemplateHaskell #-} module Test where import Language.Haskell.TH m :: (a -> [b]) -> a -> [b] m = $(newName "x" >>= \x -> newName "f" >>= \f -> lamE [varP f, varP x] (doE [ bindS (varP x) (listE [varE f `appE` varE x]) , noBindS (varE x)]) ) }}} The splice generates the following expression: {{{ \f x -> do x <- [f x] x }}} and `-Wunused-matches` complains that `x` is not used, while both bound occurrences are in fact used (the two uses have different types so that's quite certain). -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14318#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14318: TH shadowing bind statement triggers -Wunused-matches -------------------------------------+------------------------------------- Reporter: lyxia | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Template Haskell | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by mgsloan): * owner: mgsloan => (none) Comment: I've looked into this a bit. It looks like names created by `newName` are turned into GHC names via `mkSystemNameAt`, and integer making the name unique is used directly. See `Convert.thRdrName`. I'm guessing that code involving warnings is expecting that these unique names are unique per binding site, whereas in this case the same unique name is bound multiple times. I'm not sure what the correct approach is to fixing this. It could either be to fix the warning code, or, alternatively, have TH ensure that generated code has per-binding uniqueness. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14318#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler