Question

In a Haskell program, what’s the best way to use constants defined in C headers?

Answer 1

For this task, hsc2hs is your friend.

For a simple example, let’s get the value of INT_MAX from limits.h.

$ cat >IntMax.hsc
module Main where

#include <limits.h>

c_INT_MAX = #const INT_MAX

main = print c_INT_MAX

With hsc2hs, we can #include headers and use the values of constants with the #const directive.

Instead of building by hand, use Cabal:

$ cat >intmax.cabal
Name:          intmax
Version:       0.0
Cabal-Version: >=1.2
Build-Type:    Simple

Executable intmax
  Main-Is: IntMax.hs
  Build-Depends: base

Notice that even though the name of the main program is IntMax.hsc, the Main-Is line points to IntMax.hs. When Cabal looks for IntMax.hs but finds IntMax.hsc, it automatically feeds the latter through hsc2hs as part of the build.

$ cabal configure
Resolving dependencies...
Configuring intmax-0.0...

$ cabal build
Prerocessing executables for intmax-0.0...
Building intmax-0.0...
[1 of 1] Compiling Main             ( dist\build\intmax\intmax-tmp\IntMax.hs, dist\build\intmax\intmax-tmp\Main.o )
Linking dist\build\intmax\intmax.exe ...

$ ./dist/build/intmax/intmax
2147483647

Note that you’ll want to break up lines with multiple constants. Say you’re assembling a bitfield to pass to FormatMessage. You’ll want to write it as

flags = #const FORMAT_MESSAGE_FROM_SYSTEM
        .|.
        #const FORMAT_MESSAGE_IGNORE_INSERTS

Putting them all on one line will result in syntax errors.