Skip to content
Snippets Groups Projects

unused-top-binds wrongly warns about underscore-prefixed field names when DuplicateRecordFields enabled

    • Closed Issue created by jml @trac-jml

    Given this code:

    module Main (main) where

data Foo
  = Foo { _foo :: String
        , _bar :: String
        } deriving (Eq, Show)


main :: IO ()
main = do
  let x = Foo "apple" "bear"
  putStrLn $ "x = " ++ show x

    Saved as unused-fields.hs

    Then with ghc 8.0.1:

    $ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.0.1

    The following command will compile without error or warning:

    $ ghc -Wall -Werror -o unused-fields ./unused-fields.hs

    But if DuplicateRecordFields is enabled, then:

    $ ghc -Wall -Werror -o unused-fields ./unused-fields.hs -XDuplicateRecordFields
[1 of 1] Compiling Main             ( unused-fields.hs, unused-fields.o ) [flags changed]

unused-fields.hs:21:11: warning: [-Wunused-top-binds]
    Defined but not used: ‘_foo’

unused-fields.hs:22:11: warning: [-Wunused-top-binds]
    Defined but not used: ‘_bar’

<no location info>: error:
Failing due to -Werror.

    I would have no warnings, since _foo and _bar have underscore prefixes, which is a documented way of selectively disabling this warning. (c.f. https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/using-warnings.html#ghc-flag--Wunused-top-binds).

    Details requested by https://ghc.haskell.org/trac/ghc/wiki/ReportABug:

    $ gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 8.0.0 (clang-800.0.38)
Target: x86_64-apple-darwin15.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

    GOOD (without -XDuplicateRecordFields)

    $ ghc -dcore-lint -v -Wall -Werror -o unused-fields ./unused-fields.hs
Glasgow Haskell Compiler, Version 8.0.1, stage 2 booted by GHC version 7.10.3
Using binary package database: /nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/package.conf.d/package.cache
loading package database /nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/package.conf.d
wired-in package ghc-prim mapped to ghc-prim-0.5.0.0
wired-in package integer-gmp mapped to integer-gmp-1.0.0.1
wired-in package base mapped to base-4.9.0.0
wired-in package rts mapped to rts
wired-in package template-haskell mapped to template-haskell-2.11.0.0
wired-in package ghc mapped to ghc-8.0.1
wired-in package dph-seq not found.
wired-in package dph-par not found.
Hsc static flags:
loading package database /nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/package.conf.d
wired-in package ghc-prim mapped to ghc-prim-0.5.0.0
wired-in package integer-gmp mapped to integer-gmp-1.0.0.1
wired-in package base mapped to base-4.9.0.0
wired-in package rts mapped to rts-1.0
wired-in package template-haskell mapped to template-haskell-2.11.0.0
wired-in package ghc mapped to ghc-8.0.1
wired-in package dph-seq not found.
wired-in package dph-par not found.
*** Chasing dependencies:
Chasing modules from: *unused-fields.hs
!!! Chasing dependencies: finished in 0.68 milliseconds, allocated 0.777 megabytes
Stable obj: [Main]
Stable BCO: []
Ready for upsweep
  [NONREC
      ModSummary {
         ms_hs_date = 2016-09-22 08:27:24 UTC
         ms_mod = Main,
         ms_textual_imps = [(Nothing, Prelude)]
         ms_srcimps = []
      }]
*** Deleting temp files:
Deleting:
compile: input file unused-fields.hs
*** Checking old interface for Main:
[1 of 1] Skipping  Main             ( unused-fields.hs, unused-fields.o )
Upsweep completely successful.
*** Deleting temp files:
Deleting:
link: linkables are ...
LinkableM (2016-09-22 08:45:48 UTC) Main
   [DotO unused-fields.o]
Linking unused-fields ...
Created temporary directory: /var/folders/6q/jpykk8bn6dq68ynzm6c1r7c40000gn/T/ghc62639_0
*** C Compiler:
/nix/store/a5f6qqgzgmmvw0xkvlfdsjzaprmnlz2s-clang-wrapper-3.7.1/bin/cc -m64 -fno-stack-protector -DTABLES_NEXT_TO_CODE -c /var/folders/6q/jpykk8bn6dq68ynzm6c1r7c40000gn/T/ghc62639_0/ghc_1.c -o /var/folders/6q/jpykk8bn6dq68ynzm6c1r7c40000gn/T/ghc62639_0/ghc_2.o -fno-common -U__PIC__ -D__PIC__ -I/nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/include
*** Linker:
/nix/store/a5f6qqgzgmmvw0xkvlfdsjzaprmnlz2s-clang-wrapper-3.7.1/bin/cc -m64 -fno-stack-protector -DTABLES_NEXT_TO_CODE -m64 -o unused-fields -Wl,-no_compact_unwind unused-fields.o -L/nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/base-4.9.0.0 -L/nix/store/8dkzvp7d7vpprc8cblmjcyyyzyy9940i-libiconv-osx-10.9.5/lib -L/nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/integer-gmp-1.0.0.1 -L/nix/store/8b7big21k9ipi44mqlnp8njbkncwhh27-gmp-6.1.0/lib -L/nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/ghc-prim-0.5.0.0 -L/nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/rts /var/folders/6q/jpykk8bn6dq68ynzm6c1r7c40000gn/T/ghc62639_0/ghc_2.o -Wl,-u,_ghczmprim_GHCziTypes_Izh_static_info -Wl,-u,_ghczmprim_GHCziTypes_Czh_static_info -Wl,-u,_ghczmprim_GHCziTypes_Fzh_static_info -Wl,-u,_ghczmprim_GHCziTypes_Dzh_static_info -Wl,-u,_base_GHCziPtr_Ptr_static_info -Wl,-u,_ghczmprim_GHCziTypes_Wzh_static_info -Wl,-u,_base_GHCziInt_I8zh_static_info -Wl,-u,_base_GHCziInt_I16zh_static_info -Wl,-u,_base_GHCziInt_I32zh_static_info -Wl,-u,_base_GHCziInt_I64zh_static_info -Wl,-u,_base_GHCziWord_W8zh_static_info -Wl,-u,_base_GHCziWord_W16zh_static_info -Wl,-u,_base_GHCziWord_W32zh_static_info -Wl,-u,_base_GHCziWord_W64zh_static_info -Wl,-u,_base_GHCziStable_StablePtr_static_info -Wl,-u,_ghczmprim_GHCziTypes_Izh_con_info -Wl,-u,_ghczmprim_GHCziTypes_Czh_con_info -Wl,-u,_ghczmprim_GHCziTypes_Fzh_con_info -Wl,-u,_ghczmprim_GHCziTypes_Dzh_con_info -Wl,-u,_base_GHCziPtr_Ptr_con_info -Wl,-u,_base_GHCziPtr_FunPtr_con_info -Wl,-u,_base_GHCziStable_StablePtr_con_info -Wl,-u,_ghczmprim_GHCziTypes_False_closure -Wl,-u,_ghczmprim_GHCziTypes_True_closure -Wl,-u,_base_GHCziPack_unpackCString_closure -Wl,-u,_base_GHCziIOziException_stackOverflow_closure -Wl,-u,_base_GHCziIOziException_heapOverflow_closure -Wl,-u,_base_ControlziExceptionziBase_nonTermination_closure -Wl,-u,_base_GHCziIOziException_blockedIndefinitelyOnMVar_closure -Wl,-u,_base_GHCziIOziException_blockedIndefinitelyOnSTM_closure -Wl,-u,_base_GHCziIOziException_allocationLimitExceeded_closure -Wl,-u,_base_ControlziExceptionziBase_nestedAtomically_closure -Wl,-u,_base_GHCziEventziThread_blockedOnBadFD_closure -Wl,-u,_base_GHCziWeak_runFinalizzerBatch_closure -Wl,-u,_base_GHCziTopHandler_flushStdHandles_closure -Wl,-u,_base_GHCziTopHandler_runIO_closure -Wl,-u,_base_GHCziTopHandler_runNonIO_closure -Wl,-u,_base_GHCziConcziIO_ensureIOManagerIsRunning_closure -Wl,-u,_base_GHCziConcziIO_ioManagerCapabilitiesChanged_closure -Wl,-u,_base_GHCziConcziSync_runSparks_closure -Wl,-u,_base_GHCziConcziSignal_runHandlersPtr_closure -Wl,-search_paths_first -lHSbase-4.9.0.0 -lHSinteger-gmp-1.0.0.1 -lHSghc-prim-0.5.0.0 -lHSrts -lCffi -liconv -lgmp -lm -ldl
link: done
*** Deleting temp files:
Deleting: /var/folders/6q/jpykk8bn6dq68ynzm6c1r7c40000gn/T/ghc62639_0/ghc_2.o /var/folders/6q/jpykk8bn6dq68ynzm6c1r7c40000gn/T/ghc62639_0/ghc_1.c
*** Deleting temp dirs:
Deleting: /var/folders/6q/jpykk8bn6dq68ynzm6c1r7c40000gn/T/ghc62639_0

    BAD

    $ ghc -dcore-lint -v -Wall -Werror -o unused-fields ./unused-fields.hs -XDuplicateRecordFields
Glasgow Haskell Compiler, Version 8.0.1, stage 2 booted by GHC version 7.10.3
Using binary package database: /nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/package.conf.d/package.cache
loading package database /nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/package.conf.d
wired-in package ghc-prim mapped to ghc-prim-0.5.0.0
wired-in package integer-gmp mapped to integer-gmp-1.0.0.1
wired-in package base mapped to base-4.9.0.0
wired-in package rts mapped to rts
wired-in package template-haskell mapped to template-haskell-2.11.0.0
wired-in package ghc mapped to ghc-8.0.1
wired-in package dph-seq not found.
wired-in package dph-par not found.
Hsc static flags:
loading package database /nix/store/vvn40k4257a2f6dlps52jnff6qa41ph3-ghc-8.0.1/lib/ghc-8.0.1/package.conf.d
wired-in package ghc-prim mapped to ghc-prim-0.5.0.0
wired-in package integer-gmp mapped to integer-gmp-1.0.0.1
wired-in package base mapped to base-4.9.0.0
wired-in package rts mapped to rts-1.0
wired-in package template-haskell mapped to template-haskell-2.11.0.0
wired-in package ghc mapped to ghc-8.0.1
wired-in package dph-seq not found.
wired-in package dph-par not found.
*** Chasing dependencies:
Chasing modules from: *unused-fields.hs
!!! Chasing dependencies: finished in 1.05 milliseconds, allocated 0.777 megabytes
Stable obj: [Main]
Stable BCO: []
Ready for upsweep
  [NONREC
      ModSummary {
         ms_hs_date = 2016-09-22 08:27:24 UTC
         ms_mod = Main,
         ms_textual_imps = [(Nothing, Prelude)]
         ms_srcimps = []
      }]
*** Deleting temp files:
Deleting:
compile: input file unused-fields.hs
*** Checking old interface for Main:
[1 of 1] Compiling Main             ( unused-fields.hs, unused-fields.o ) [flags changed]
*** Parser [Main]:
!!! Parser [Main]: finished in 1.27 milliseconds, allocated 0.517 megabytes
*** Renamer/typechecker [Main]:
!!! Renamer/typechecker [Main]: finished in 128.27 milliseconds, allocated 49.950 megabytes
*** Desugar [Main]:
Result size of Desugar (after optimization)
  = {terms: 141, types: 94, coercions: 0}
*** Core Linted result of Desugar (after optimization):
!!! Desugar [Main]: finished in 5.14 milliseconds, allocated 1.194 megabytes

unused-fields.hs:22:11: warning: [-Wunused-top-binds]
    Defined but not used: ‘_foo’

unused-fields.hs:23:11: warning: [-Wunused-top-binds]
    Defined but not used: ‘_bar’

<no location info>: error:
Failing due to -Werror.
Upsweep partially successful.
*** Deleting temp files:
Deleting:
link(batch): upsweep (partially) failed OR
   Main.main not exported; not linking.
*** Deleting temp files:
Deleting:
*** Deleting temp dirs:
Deleting:
    Trac metadata
    Trac field Value
    Version 8.0.1
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC
    Operating system
    Architecture
    To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information

    Child items ...

    • Activity

    Please register or sign in to reply