From: timr <timrandg@...>
Date: 2010-11-12T13:25:20+09:00
Subject: Re: Scheme's (cond ((assertion) (value))...(else (value))) statement implemented in ruby?

On Nov 11, 5:51 pm, Clifford Heath <n...@spam.please.net> wrote:
> timr wrote:
> > Hi Rubyists,
> > I love ruby, but am learning scheme and kinda like the (cond
> > ((assertion) (value))...(else (value))) syntax for what would normally
> > look like this in ruby:
>
> > if (assertion 1)
> >   value 1
> > elsif (assertion 2)
> >   value 2
> > else
> >   default
> > end
>
> > I find scheme's cond syntax more quickly digestible since it reads
> > like a table.
> > (cond
> >     ((assertion 1) (value 1))
> >     ((assertion 2) (value 2))
> >                 (else (default)))
>
> What's wrong with this?
>
> x = 3
> case
> when x == 2; puts 'two'
> when x == 3; puts 'three'
> else puts 'other'
> end
>
> Clifford Heath.

Case interrogates a single variable for its state--whether it falls in
a certain range, or is equal to such and such a value. The cond
statement is much more flexible, in that each assertion can be
completely unrelated to the other assertions--i.e. they can cover more
than the state of a single variable.

cond l{Store.open_time..Store.close_time === Time.now}
=>l{Store.access = true},
     l{after_hours_users.include? User.name}
=>l{Store.access = true},
     cond_end
=>l{Store.access = false; User.rain_check(1)}

In the above code access is given to anyone during store hours, but if
it is not during store hours and the user is listed as an
after_hours_user, they are allowed acces. If they can't get in because
it is after hours and they are not listed as an after_hours_user, they
are denied access but given a rain_check.

You can't do this kind of complex functionality with case, which only
inquires with regards to one variable.
Tim