thoughts: script language for term browser

[Bert THOMPSON]

People,

I'd appreciate it if you could me thoughts on the following...

In order to display terms in as flexible a manner as possible,
we need a scripting language. This would allow the display
of the list ["this", "is", "a", "list"] as, say:

	[
	"this",
	"is",
	"a",
	"list"
	]

and the map { 1 -> "this", 2 -> "is", 3 -> "a", 4 -> "map"} as, say:

	{
	1 -> "this"
	2 -> "is"
	3 -> "a"
	4 -> "map"
	}

And, of course, nested data structures, such as lists of maps, should
also be dealt with.

We could make the term browser explicitly handle such types specially,
however it's neater to have a simple scripting language. Commonly
used types from the library could have a script that's preloaded.

I'm currently implementing such a scripting language. Initially I
considered a simple Mirandese language, but a Prolog syntax is better:
a) easier to parse (given that the technology's already in the library).
b) needs no culture-shift on the programmer's behalf.
c) Prolog interpreters are easy to implement, especially as one's
   already written.

Specifically, what I'm doing is:
	- a simple Prolog-ese interpreter
	- a library of Mercury predicates called from the Prolog
	  interpreter which do the following:
	  	- decompose terms
		- match terms against patterns
		- build frames (currently ASCII pictures)

The primitives are:
		% match term with pattern
	:- pred matches(term::in, pattern::in) is semidet.
		% extract nth arg of term
	:- pred nth_arg(term::in, int::in, term::out) is semidet.
		% name of topmost constructor (fails if not atom)
	:- pred cons_name(term::in, string::out) is semidet.
		% make a frame from a string
	:- pred frame(string::in, frame::out) is det.
		% put a visible box around a frame
	:- pred box(frame::in, frame::out) is det.
		% vertically stack two frames (left-aligned)
	:- pred stack(frame::in, frame::in, frame::out) is det.
		% horizontally juxtapose two frames (centred)
	:- pred juxtapose(frame::in, frame::in, frame::out) is det.
		% indent a frame (put some whitespace on the left)
	:- pred indent(frame::in, frame::out) is det.
	
	% A handful of others as the need becomes apparent.
	% The set is kept small enough to demo a prototype.


I'd appreciate feedback on the following:
	- is this idea fundamentally sound?
	- can anyone think of a more reasonable approach?
	- are there any things people would really like the
	  browser to do that the above seems incapable of?

I don't need the following feedback:
	- you need to add <insert primitive>:
		The above should suffice as a proof-of-concept.
	  
The prototype is in the pipeline and if anyone raises issues that
suggest this is not the way to go, I'll take a different tack.

Cheers,
Bert