switching component for ghc-mod

I have a project whose main cabal component is a library

library
  hs-source-dirs:      src
  ...

and which also defines

executable todo
   if !flag(example)
        Buildable: False
   ghc-options: -Wall
   cpp-options: -DGHCJS_BROWSER

   default-language: Haskell2010
   hs-source-dirs: example/todo
   other-modules: TodoDispatcher
                  TodoStore
                  TodoComponents


   main-is: Main.hs

Now if I try to type check one file I get the following error

Configuring react-flux-1.0.3...
EXCEPTION: types:
           Could not find module ‘TodoDispatcher’
           Use -v to see a list of the files searched for.

And if I do so from the command line I get

 $ ghc-mod  --ghcOpt=-v7 type example/todo/TodoViews.hs 29 28                                            not sandboxed
EXCEPTION: types:
           Could not find module ‘TodoDispatcher’
           Locations searched:
             .stack-work/dist/x86_64-osx/Cabal-1.22.8.0/build/TodoDispatcher.hs
             .stack-work/dist/x86_64-osx/Cabal-1.22.8.0/build/TodoDispatcher.lhs
             .stack-work/dist/x86_64-osx/Cabal-1.22.8.0/build/TodoDispatcher.hsig
             .stack-work/dist/x86_64-osx/Cabal-1.22.8.0/build/TodoDispatcher.lhsig
             src/TodoDispatcher.hs
             src/TodoDispatcher.lhs
             src/TodoDispatcher.hsig
             src/TodoDispatcher.lhsig
             .stack-work/dist/x86_64-osx/Cabal-1.22.8.0/build/autogen/TodoDispatcher.hs
             .stack-work/dist/x86_64-osx/Cabal-1.22.8.0/build/autogen/TodoDispatcher.lhs
             .stack-work/dist/x86_64-osx/Cabal-1.22.8.0/build/autogen/TodoDispatcher.hsig
             .stack-work/dist/x86_64-osx/Cabal-1.22.8.0/build/autogen/TodoDispatcher.lhsig

So it seems to ignore the hs-source-dirs setting of the component which the current file I try to typecheck belongs to, and instead consider the one from the first component of the package, just like cabal repl would do by default

By default cabal repl loads the first component in a package.

How can I change that nice bit of global state ?

Solution

OK so here's what you should do in order to get this to work:

You need to edit the cabal file and change the flag of the component to
```
`Buildable : True`
```

You can then type check

 $ ghc-mod --ghcOpt=-v7 type example/todo/TodoViews.hs 29 28                                            not sandboxed
 29 24 29 34 "[Char]"
 27 25 33 12 "TextInputArgs"
 27 9 33 12 "ReactElementM [

In Emacs:

You need to get out of Emacs maybe
You need to open your file and haskell-process-load-file (aka C-c C-l, aka Space m s b)

It should have loaded the correct "target", which is haskell-mode parlance for "component" in cabal. If that works then ghc-mod should also. (?)

http://haskell.github.io/haskell-mode/manual/latest/Interactive-Haskell.html

14.8.1 Changing REPL target

With haskell-session-change-target you can change the target for REPL session.

After REPL session started, in haskell-interactive-mode buffer invoke the haskell-session-change-target and select from available targets for

Testing

Benchmark

Executable

Library

Answer “yes” to restart the session and run your tests, benchmarks, executables.