stack build results in "output was redirected with -o, but no output will be generated because there is no Main module."

Question

Didn't see any relevant result when google searching for this error so thought I'd post it.

stack build
Building all executables for `gitchapter' once. After a successful build of all of them, only specified executables will be rebuilt.
gitchapter-0.1.0.0: build (exe)
Preprocessing executable 'app' for gitchapter-0.1.0.0..
Building executable 'app' for gitchapter-0.1.0.0..
Preprocessing executable 'test' for gitchapter-0.1.0.0..
Warning: Enabling workaround for Main module 'Main' listed in 'other-modules'
Building executable 'test' for gitchapter-0.1.0.0..
illegaly!

<no location info>: error:
    output was redirected with -o, but no output will be generated
because there is no Main module.


--  While building custom Setup.hs for package gitchapter-0.1.0.0 using:
      /home/chris/.stack/setup-exe-cache/x86_64-linux-nopie/Cabal-simple_mPHDZzAJ_2.0.1.0_ghc-8.2.2 --builddir=.stack-work/dist/x86_64-linux-nopie/Cabal-2.0.1.0 build exe:app exe:test --ghc-options " -ddump-hi -ddump-to-file -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1

It definitely is related to the 'test' executable definition - as if I remove this, it completes the stack build successfully - but without the required test executable.

gitchapter.cabal

-- This file has been generated from package.yaml by hpack version 0.28.2.
--
-- see: https://github.com/sol/hpack
--
-- hash: a3e4a735ee8304dd50f5af53a64d7b639894cbcc24ba01d0171a588e67455018

name:           gitchapter
version:        0.1.0.0
author:         Chris Stryczynski
maintainer:     Chris Stryczynski
license:        BSD3
license-file:   LICENSE
build-type:     Simple
cabal-version:  >= 1.10

executable app
  main-is: Main.hs
  other-modules:
      BlogLiterately
      Example
      FileSection
      GHCi
      GHCiSession
      Git
      GitTextPartial
      Hart
      Operations
      Operations.Parsers
      QuasiText
      Render
      Section
      Test
      Paths_gitchapter
  hs-source-dirs:
      src
  build-depends:
      HUnit
    , QuickCheck
    , base >=4.9 && <4.11
    , directory
    , extra
    , filepath
    , foldl
    , mtl
    , optparse-applicative
    , pandoc-include-code
    , parsec
    , pretty-simple
    , process
    , regex-pcre
    , regex-posix
    , safe
    , string-conversions
    , system-filepath
    , template-haskell
    , text
    , transformers
    , turtle
    , unix
    , unordered-containers
  default-language: Haskell2010

executable test
  main-is: Test.hs
  other-modules:
      BlogLiterately
      Example
      FileSection
      GHCi
      GHCiSession
      Git
      GitTextPartial
      Hart
      Main
      Operations
      Operations.Parsers
      QuasiText
      Render
      Section
      Paths_gitchapter
  hs-source-dirs:
      src
  build-depends:
      HUnit
    , QuickCheck
    , base >=4.9 && <4.11
    , directory
    , extra
    , filepath
    , foldl
    , mtl
    , optparse-applicative
    , pandoc-include-code
    , parsec
    , pretty-simple
    , process
    , regex-pcre
    , regex-posix
    , safe
    , string-conversions
    , system-filepath
    , template-haskell
    , text
    , transformers
    , turtle
    , unix
    , unordered-containers
  default-language: Haskell2010

I think you need to add `ghc-options: -main-is XXX` where `XXX` is the name of the module, e.g. `Test`. — Stéphane Laurent, Jun 02 '18 at 17:42
That was the solution! Thank you! Feel free to post it as an answer instead. — Chris Stryczynski, Jun 02 '18 at 17:54
The question is related to: https://stackoverflow.com/questions/46895199/no-output-will-be-generated-because-there-is-no-main-module — Chris Stryczynski, Oct 26 '19 at 21:01

score 7 · Accepted Answer · edited Jun 02 '18 at 18:34

7

You need to set

executable test
  ghc-options:  -main-is Test

in your cabal file.

edited Jun 02 '18 at 18:34

Chris Stryczynski

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answered Jun 02 '18 at 18:01

Stéphane Laurent

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Wow I looked at this about 4 times before realizing the `main-is` you gave is different from `main-is: Test.hs`. Thanks! – willbush Mar 13 '19 at 04:53
Changing the `.cabal` file generated by `hpack` is bad practice. Change the `package.yaml` instead. – Leif Willerts Mar 07 '21 at 08:32
@willbush But I'm actually surprised that you have to directly "inject" the information into `ghc` instead of telling cabal to pass it on. – Leif Willerts Mar 07 '21 at 09:38

stack build results in "output was redirected with -o, but no output will be generated because there is no Main module."

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