Darren New wrote:
> Invisible wrote:
>> The "get time" function just returns an I/O command object. You can 
>> replace that function call with the command object it returns and the 
>> meaning of the program is left completely unchanged. Thus it is 
>> referentially transparent.
> 
> So if I assign the result of "get time" to a variable and use that 
> variable in multiple places, do I get the same time each place I use it, 
> or different times?  I think that's what's confusing me.

The key is "using".

Recall that the *only* way to *execute* an I/O action is to return it 
from the "main" function. And, since you can only return a single 
action, it's how you splice each mini-action into the giant-action 
returned by "main" that counts. ;-)

If you splice in two copies of the self same action, that action gets 
executed twice. It's that simple.

>   S = get_time()
>   print(S)
>   do_long_task()
>   print(S)
> 
> I would think you *want* that to do something different from
>   print(get_time())
>   do_long_task()
>   print(get_time())
> 
> How would I write each of those in Haskell?

In Haskell, if you use the monadic "do" notation, then the syntax for 
executing an action and putting its result into a variable is

   do
     ...
     x <- do_stuff
     ...

This is technically equivilent to

   ... do_stuff >>= \x -> ...

but less messy on paper. In your case, what you're trying to do is simply

   do
     t1 <- get_time
     print t1

     do_long_task

     t2 <- get_time
     print t2

as opposed to something like

   do
     s <- get_time
     print s
     do_long_task
     print s

which just serves no useful purpose at all. Notice that it's the "<-" 
that says "execute this *now*". In the second example, you only execute 
the thing once, so it only happens once. It's really quite clear on paper.

In case you care, the desugared equivilents are:

   get_time >>= \t1 -> (print t1 >> do_long_task >> get_time >>= \t2 -> 
print t2))

and

   get_time >>= \s -> (print s >> do_long_task >> print s)

You can see why people prefer the "do" notation...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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