Obscure ar error ultimately from missing module header

[quasicomputational]

$ cabal new-build all
Build profile: -w ghc-8.6.3 -O1
In order, the following will be built (use -v for more details):
 - foo-1 (lib) (first run)
Configuring library for foo-1..
Preprocessing library for foo-1..
Building library for foo-1..
[1 of 1] Compiling Main             ( Foo.hs, /tmp/foo/dist-newstyle/build/x86_64-linux/ghc-8.6.3/foo-1/build/Main.o )
/usr/bin/ar: /tmp/foo/dist-newstyle/build/x86_64-linux/ghc-8.6.3/foo-1/build/Foo.o: No such file or directory
$ cat foo.cabal 
cabal-version: 2.4
name: foo
version: 1

library
  default-language: Haskell2010
  exposed-modules: Foo
  build-depends: base

$ cat Foo.hs 
main = return ()
$ cabal --version
cabal-install version 2.4.1.0
compiled using version 2.4.1.0 of the Cabal library 

The user's problem here is that Foo.hs needs a module Foo where header, but it's not at all obvious how to figure that out from the ar error.

[phadej]

The error comes from GHC. I don't think there's anything we can easily do, otherwise than moving out from using `ghc --make`, i.e. don't use GHC's built-in "build system features" (there's issue about that, cannot lookup on mobile). `-v3` output would help. If cabal gives modules (not filepaths) than GHC does something silly (as it's not Foo module in Foo.hs) Similar problem is having library and test-suite sources in a same directory. Currently it's not a good idea; but it doesn't need to be. (Currently library dependency from test-suite won't be used, as GHC will *discover* library modules as sources)

…

Sent from my iPhone

On 3 Feb 2019, at 11.00, quasicomputational ***@***.***> wrote: $ cabal new-build all Build profile: -w ghc-8.6.3 -O1 In order, the following will be built (use -v for more details): - foo-1 (lib) (first run) Configuring library for foo-1.. Preprocessing library for foo-1.. Building library for foo-1.. [1 of 1] Compiling Main ( Foo.hs, /tmp/foo/dist-newstyle/build/x86_64-linux/ghc-8.6.3/foo-1/build/Main.o ) /usr/bin/ar: /tmp/foo/dist-newstyle/build/x86_64-linux/ghc-8.6.3/foo-1/build/Foo.o: No such file or directory $ cat foo.cabal cabal-version: 2.4 name: foo version: 1 library default-language: Haskell2010 exposed-modules: Foo build-depends: base $ cat Foo.hs main = return () $ cabal --version cabal-install version 2.4.1.0 compiled using version 2.4.1.0 of the Cabal library The user's problem here is that Foo.hs needs a module Foo where header, but it's not at all obvious how to figure that out from the ar error. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub, or mute the thread.