[Haskell-beginners] haskell variables - or not

[Kyle Murphy]

On Sat, Jul 25, 2009 at 8:32 AM, <dukeofperl at ml1.net> wrote:

> On Fri, 24 Jul 2009, Duke Normandin wrote:
>
> > Hello....
> >
> > I'm 2 days into Haskell. NOT new to Perl, PHP, M(umps), TCL, newLISP. I'm
> > using "YAHT.pdf" as an intro to Haskell. Here's a quote:
> >
> > [quote]
> > ... if you have a value x, you must not think of x as a register, a
> > memory location or anything else of that nature. x is simply a name, just
> as
> > Hal  is my name. You cannot arbitrarily decide to store a different
> person in
> > my name any more than you can arbitrarily decide to store a different
> value in
> > x. This means that code that might look like the following C code is
> invalid
> > (and has no counterpart) in Haskell:
> >
> > int x = 5;
> > [/quote]
> >
> > So is fair to say that an expression in Haskell like:
> >
> > x = 5
> >
> > is more like a constant in imperative languages - say Perl? E.g.
> >
> >  use constant SECONDS_PER_DAY => 86400;
> > --
> > duke
>
> Thanks to all the responders!!
>
> --Thomas F.
>
> > You will get the idea in a couple of days.  I am sure.
>
> Maybe next week!  I'm going fly-fishing for trout on the weekend, as is my
> usual
> practice here in the Alberta (Canada) foothills ;)
>
> -- Kyle M. & Thomas D.
>
> What I gather from your replies is that the only "true" variables in
> Haskell are the args passed to a function. Is that correct?
>
> ...and further, that what sometimes appears to be a variable to an
> "unwashed" imperative programmer", is really a function with no arguments,
> returning a "constant" value. How did I do?
> --
> duke
>

You got it. It's one of the hardest things to wrap your head around (well,
that and Monads) but once you do the language starts to make a whole lot
more sense. I can't claim I'm a master of it or anything as I'm pretty new
to the language myself but it helps explain why what another commenter on
this list tried to do didn't work. He was attempting to keep track of the
number of calls to a function by doing:
a = a + 1which once you realize a is a function and not a variable the stack
overflow he was receiving makes sense as he created an infinitely recursive
function (which incidentally isn't a problem until you attempt to evaluate
it which he did on the very next line when he tried to print it). Haskell
can support something like a variable outside of functions using these weird
little monads like MVars and IORefs (among others), but those are
manipulated as something like blackboxes using various functions to store
and retrieve values out of them and to understand how they work you need to
understand how Monads work (something I'm not entirely clear on myself).

-Kyle Murphy
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/beginners/attachments/20090725/a992b185/attachment.html