#16261: [] interpreted as an overloaded list in rule LHS -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.10.1 Component: Compiler | Version: 8.6.3 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 dfeuer: Old description:
Compiling `Data.Set.Internal` produces the following:
{{{ Data/Set/Internal.hs:958:11: warning: [-Winline-rule-shadowing] Rule "Set.toAscListBack" may never fire because rule "Class op fromListN" for ‘GHCExts.fromListN’ might fire first Probable fix: add phase [n] or [~n] to the competing rule | 958 | {-# RULES "Set.toAscListBack" [1] foldrFB (:) [] = toAscList #-} | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Data/Set/Internal.hs:960:11: warning: [-Winline-rule-shadowing] Rule "Set.toDescListBack" may never fire because rule "Class op fromListN" for ‘GHCExts.fromListN’ might fire first Probable fix: add phase [n] or [~n] to the competing rule | 960 | {-# RULES "Set.toDescListBack" [1] foldlFB (\xs x -> x : xs) [] = toDescList #-} | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}}
What the heck does the class op rule for `fromListN` have to do with anything anywhere near here? Note in particular that this module neither defines `fromListN` (it takes its default definition) nor uses it in code or `RULES`.
New description: Compiling `Data.Set.Internal` produces the following: {{{ Data/Set/Internal.hs:958:11: warning: [-Winline-rule-shadowing] Rule "Set.toAscListBack" may never fire because rule "Class op fromListN" for ‘GHCExts.fromListN’ might fire first Probable fix: add phase [n] or [~n] to the competing rule | 958 | {-# RULES "Set.toAscListBack" [1] foldrFB (:) [] = toAscList #-} | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Data/Set/Internal.hs:960:11: warning: [-Winline-rule-shadowing] Rule "Set.toDescListBack" may never fire because rule "Class op fromListN" for ‘GHCExts.fromListN’ might fire first Probable fix: add phase [n] or [~n] to the competing rule | 960 | {-# RULES "Set.toDescListBack" [1] foldlFB (\xs x -> x : xs) [] = toDescList #-} | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} As mpickering figured out, the trouble is that `[]` in the rule LHS is being interpreted as an overloaded list. The module does ''not'' enable `OverloadedLists`. Furthermore, even if it ''did'', I would argue that it is ''never'' correct to put one in a rule LHS, so we should always interpret `[]` as the empty list constructor in `RULES` left-hand sides. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16261#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler