How to use macro constants

[nh2]

I simply want to use a macro constant defined like

#define MYCONSTANT 1

in a .chs file, like

x :: CInt
x = MYCONSTANT

How do I do this?

It would be nice if that was part of the documentation; it explains all kind of complicated things like enum define etc, but I can't figure this simple one out.

[ian-ross]

The way that C2HS is set up at the moment makes this a little less transparent to do than you might hope. Here's what you say:

#include "blah.h"

{#enum define Consts {MYCONSTANT as MyConstant} deriving (Eq, Ord)#}

x :: Int
x = fromEnum MyConstant

Why is it like this? Not conpletely sure, but I suspect that part of it is based on the fact that the rules for the naming of C preprocessor identifiers are different from the rules for the naming of Haskell identifiers. In your case, MYCONSTANT is a valid Haskell constructor name, but not a valid variable name. If instead in your header you had #define myconstant 1 (which is perfectly legal C), then it might make sense to replace occurrences of myconstant in the CHS file with the definition from the header, but because the naming rules are different, some sort of translation scheme is needed. That's what the enum define hook provides.

Now, I'm not saying that it's pretty, and I'm not saying it's the best way to do things, but it does work...

[nh2]

MYCONSTANT is a valid Haskell constructor name, but not a valid variable name

I wasn't really suggesting that the CPP should replace my in-haskell identifiers - something like x = {#const MYCONSTANT #} would be totally fine as well.

I understand the enum solution now. It seams quite overheady to me, and relying on the enum instance gives it a somewhat shady taste.

In any way, it would be great if the user guide could mention that this is the way to do this.

[ian-ross]

That's a good idea (the {#const MYCONSTANT#} thing). I can't think of anything that that would break, although I can't yet quite think of how to implement it either... I'll add an example to the user guide showing how to use {#enum define ...#} and I'll see if I can figure out how to implement this idea.

[nh2]

I think hsc2hs has a #const keyword as well.

[ian-ross]

@nh2 I've implemented a {#const ...#} hook, which seems to work, at least in my simple test case. You just say {#const MYCONSTANT#} to pick up a value defined via #define MYCONSTANT whatever in an included C header. Let me know if it doesn't do what you wanted!

[nh2]

@ian-ross Thanks!