[mercury-users] Leveraging Perl

[Ralph Becket]

> From: Peter Schachte [mailto:schachte at cs.mu.OZ.AU]
> 
> You can do anything in Mercury.  I think you should ask what you can't
do *as
> easily* in Mercury.

Sorry, that's what I meant to say.

> One thing you can't do as easily in Mercury is write a simple text
file
> filter.  For example, take a .html file containing cities and their
current
> weather observations as a big table, and extract and print out the
> information for a particular city.  This is the kind of thing that's
pretty
> quick and easy to hack together in Perl.
> 
> I think Mercury could be good at this sort of thing with the right
libraries.
> I don't really know what these libraries would be, but I'd like to
have
> them....
> 
> Another thing Mercury is not as good at as Perl is rapid prototyping.
If
> you were writing a text processing application in a speed programming
> contest, would you choose Perl or Mercury?  I'd take Perl every time.

I'm an Awk man myself, but either way Mercury is not a domain specific
language so I doubt we'd ever manage to be quite as concise as Awk/Perl
for programs below a page of code or two. 

I've often thought I could use a sensible, simple text file munging
library.  We probably have most of what we need already.  The
following will turn a string (say, a line read in from a file) into
an array with the standard Awk form ($0 for the whole line, $1, $2 etc.
for the `words' in the line):

	array([String | string__words(char__is_whitespace, String)])

A library with something like the following would give you the standard 
Awk file processing mechanism:

	process_file(P) -->
		io__read_line_as_string(Result),
		(	{ Result = error(_) }, ...
		;	{ Result = eof }
		;	{ Result = ok(Line),
			{ Words = array([Line | string__words(...)]) },
			P(Words),
			process_file(P)
		).

The extras/lex library could be used to write a standard regexp string 
parser.

This probably wouldn't be too much work.  The regexp string parser would
be non-trivial, but not too taxing.

I'm too busy right now, but I could knock something together in the near
future unless someone else beats me to it.

-- Ralph
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