Implement setting environment variables

[mtolly]

Currently using System.Environment.setEnv crashes with this:

uncaught exception in Haskell main thread: ReferenceError: h$putenv is not defined
ReferenceError: h$putenv is not defined
    at h$$mi (/Users/mtolly/git/setenv/dist/build/setenv/setenv.jsexe/all.js:38868:11)
    at h$mainLoop (/Users/mtolly/git/setenv/dist/build/setenv/setenv.jsexe/all.js:10661:25)
    at /Users/mtolly/git/setenv/dist/build/setenv/setenv.jsexe/all.js:11127:13
    at /Users/mtolly/git/setenv/dist/build/setenv/setenv.jsexe/all.js:13454:17
    at Object.wrapper [as oncomplete] (fs.js:521:5)

So I implemented that apparently missing function :)

[luite]

Thanks for implementing this. I've added some source comments.

Additionally, the return type of putenv is int though and like getenv this is using the C calling convention. int size in GHCJS is 32 bit (long long is 64), so it must return a number (0 on success) and not touch the secondary return value (h$ret1). Can you change it to do that?

[mtolly]

Sure, I'll make those edits shortly. The part about setting to empty string removing the variable is from http://hackage.haskell.org/package/base-4.7.0.1/docs/System-Environment.html#v:setEnv which says that Windows does it; thus for compatibility setEnv does it on all platforms.

[mtolly]

I amended the commit with all the edits (assuming .split('=', 1) was supposed to be .split('=', 2)).

[mtolly]

Now that you mention this is supposed to be a translation of the C function, it looks like setEnv in base is supposed to already have logic for handling the empty value case and unsetting the variable, before calling putenv at all. But as it stands the GHCJS code for setEnv does end up passing "key=" to h$putenv in the case of an empty value. Perhaps the logic should go in setEnv instead.