No output will be generated because there is no Main module

Question

I wrote a simple module called Test:

module Test (main) where

main =
  putStrLn "Hello"

However, when i try to compile it via the following command line:

ghc Test.hs -o my-program

I get the following error:

[1 of 1] Compiling Test             ( Test.hs, Test.o )

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

Answer 1

ghc will assume that the main is located in a module called Main (like the compiler says).

ghc however has an option -main-is where you can specify the name of the module where the main function is located. So you can compile with:

ghc -main-is Test Test.hs -o my-program

Answer 2

I had the same problem, but in my case I had caused it by adding a module name to the test source file, test/Spec.hs:

module Tests where

main :: IO ()
main = hspec $ do ...

The solution was simply removing the first line:

main :: IO ()
main = hspec $ do ...

This is also the default when you create a project with stack - e.g., stack new myproject.