[clean-list] questions

[Fergus Henderson]

On 20-Nov-2003, Tomasz Zielonka <t.zielonka@zodiac.mimuw.edu.pl> wrote:
> On Thu, Nov 20, 2003 at 05:26:36PM +0100, Jenda Krynicky wrote:
> > 
> > If e.g. I want to make sure that the stuff entered by the user is a 
> > date in YYYY-MM-DD format I definitely do not want to spend twenty 
> > cryptic lines writing a parser where
> > 	/^(\d{4})-(\d{1,2})-(\d{1,2})$/
> > suffices. (I know you need to make sure the month is between 1 and 12 
> > and the day is reasonable, but you'd do that outside the parser as 
> > well.)
> 
> date = do
>     yyyy <- count 4 digit
>     char '-'
>     mm <- count 2 digit
>     char '-'
>     dd <- count 2 digit
>     return (yyyy, mm, dd)
> 
> Is it really so long and cryptic?

No!  "\d{4}" is cryptic!  "yyyy <- count 4 digit" is elegant and reasonably
concise without being cryptic.

But if you really want to make it as concise as possible, at the expense
of readability, like the regexp, it could be a lot more concise:

	do { y <- count 4 digit; char '-'; m <- count 2 digit; char '-';
             d <- count 2 digit; return (y, m, d); }

With suitable abbreviations,

	c = char
	r = count	-- r for "repeat"
	d = digit

you could make it a one-liner:

	do{yyyy<-r 4 d;c '-';mm<-r 2 d;c '-';dd<-r 2 d;return(yyyy,mm,dd); }

But I hope you will agree that such source code compression harms readability
and that this code, *just like regexps*, is unnecessarily cryptic.

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.