cabal FFI dependency

Question

I am making a small Haskell game in Windows, where I would like to respond each time the user presses a key. Because getChar behaves strangely on Windows, I use FFI to get access to getch in conio.h, as described here. The relevant code is:

foreign import ccall unsafe "conio.h getch" c_getch :: IO CInt

This works fine, when I run it in ghci, or compile it with ghc. I also want to try making a cabal package out of it, so extending from this question, I include the following in my cabal file:

...
executable noughts
  Includes:          conio.h
  Extra-libraries    conio
...

But when I run cabal configure, it tells me:

cabal: Missing dependency on a foreign library:
* Missing C library: conio

It makes sense, because in my haskell platform directory, under ...\Haskell Platform\2012.4.0.0\mingw there is a conio.h file under the include directory, but no other conio file to provide the object code.

Am I doing this the right way, and if so, how can I find out which library to include in my cabal file?

Answer 1

First off, there is not always a one-to-one mapping between C header files and libraries. In this case, the functions declared in conio.h can be found in various runtime libraries, such as crtdll (deprecated) or msvcrt (preferred, I guess).

With the Haskell Platform on Windows, Cabal will look for these libraries in .\mingw\lib (under your Haskell Platform directory): if you ask for msvcrt, it will look for .\mingw\lib\libmsvcrt.a. This specific library should already be shipped with your Haskell Platform. (If you want to point to other directories with lib*.a files, you can use Cabal's --extra-lib-dirs option.)

A tiny example of this would be as follows; this is Main.hs:

{-# LANGUAGE ForeignFunctionInterface #-}
import Foreign.C.Types
foreign import ccall unsafe "conio.h _putch" c_putch :: CInt -> IO ()

main :: IO ()
main = do
    c_putch . toEnum . fromEnum $ '!'
    c_putch . toEnum . fromEnum $ '\n'

And this would be something-awesome.cabal:

name:                something-awesome
version:             0.1.0.0
build-type:          Simple
cabal-version:       >=1.8

executable yay
  main-is:             Main.hs
  build-depends:       base ==4.5.*

  includes:            conio.h
  extra-libraries:     msvcrt

This should work fine:

c:\tmp\something-awesome> dir /B
Main.hs
something-awesome.cabal

c:\tmp\something-awesome> cabal configure
Resolving dependencies...
Configuring something-awesome-0.1.0.0...

c:\tmp\something-awesome> cabal build
Building something-awesome-0.1.0.0...
Preprocessing executable 'yay' for something-awesome-0.1.0.0...
[1 of 1] Compiling Main             ( Main.hs, dist\build\yay\yay-tmp\Main.o )
Linking dist\build\yay\yay.exe ...

c:\tmp\something-awesome> dist\build\yay\yay.exe
!