Why i can't compile with GHC if code contain module definition?

Question

I'am trying to compile a very small haskell code with ghc:

module Comma where

import System.IO

main = do  
    contents <- getContents  
    putStr (comma contents)  

comma input = 
  let allLines = lines input
      addcomma [x]    =   x
      addcomma (x:xs)   = x ++ "," ++ (addcomma xs)
      result = addcomma allLines
  in result

The command i'm using to compile is :

ghc --make Comma.hs

And i'm getting this answer:

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

No file is generated, and there is no warning or errors messages.

If i comment the "module Comma where" line from code it compiles correctly:

[1 of 1] Compiling Main ( Comma.hs, Comma.o ) Linking Comma ...

I don't understand what is happening. I'm using ghc 7,4,1 (Glasgow Haskell Compiler, Version 7.4.1, stage 2 booted by GHC version 7.4.1) and ubuntu linux.

I appreciate if anyone could tell why doesn't compile with the module definition

Answer 1

GHC compiles the function Main.main to be the entry point of an executable. When you omit the module declaration, Module Main where is implicitly inserted for you.

But when you explicitly name it something other than Main ghc doesn't find an entry point.

My usual workflow is to use ghci (or ghci + emacs) instead for these snippets which let's you bypass this issue entirely. Alternatively, you could compile with -main-is Comma to explicitly tell ghc to use the Comma module.

Answer 2

No file is generated

Are you sure? I would expect that at least Comma.o and Comma.hi are generated. The former contains the compiled code ready to be linked into an executable, and the latter contains interface information that ghc uses to typecheck modules that import the module Comma.

However, ghc will only link the compiled modules into an executable if there is a main function. By default, that means a function named main in a module named Main. If you don't put an explicit module name, the name Main is assumed, and that's why your test works when you delete the module Comma where line.

To compile and link the Comma.hs file you can either use module Main where instead of module Comma where, or you can use the -main-is flag to tell ghc that Comma.main is to be the main function:

ghc --make -main-is Comma Comma.hs

Or:

ghc --make -main-is Comma.main Comma.hs