Editing STG in Javascript (section)

== Sep 12, 2006 ==

To simplify handling of partial function applications, format of thunk has been changed so that instead of ''_1'', ''_2'', etc. for function argument, an array named ''_a'' is used. This array always has at least one element which is ''undefined''. Arguments start with array element indexed at 1, so to access an argument ''n'', the following needs to be used: ''this._a[n]''.

For Haskell programs executing in a web browser environment, analogous to FFI is calling external Javascript functions.
Imagine this Javascript function which prints its argument on the window status line:

<pre>
// Output an integer value into the window status line

putStatus = function (i) {window.status = i; return i;};
</pre>

To import such a function is a Haskell program, the following FFI declaration is to be used:

<pre>
foreign import ccall "putStatus" putStatus :: Int -> Int
</pre>

Note the type signature: of course it should be somewhat monadic, but for the moment, nothing has been done to support monads, so this signature is only good for testing purposes.

The current NHC98-based implementation compiles the above FFI declaration into this:

<pre>
Test2.putStatus_T=function(_1){return {_c:Test2.putStatus_C, _w:"7:1-7:56", 
                                       _a:[undefined, _1]};};
Test2.putStatus_C=function(){
return (putStatus)((this._a[1])._c());
};
</pre>

Note that like a primitive, a foreign function evaluates all its arguments before it starts executing.

A test page illustrating this can be found at:

http://www.golubovsky.org/repos/nhcjs/test2.html

When this page is loaded, the window status line should display "456" while the rest of the page remains blank. 
The Haskell source for this test page is:

http://www.golubovsky.org/repos/nhcjs/test2.hs