On Tuesday 19 April 2011 21:10:09, Gregory Guthrie wrote:
> I am pretty new to Haskell, so need some clarification.
> I am porting some code from SML, and getting a result that surprises me.
>
> I basically have some functions which work like this:
> f1 = fa fb fc
> test1 = do print "test1:"
> f1
So f1 :: IO something
Being an IO-action, f1 can return different things in different invocations
since the world in which it runs has changed (it might read a file which
was modified between the first and the second invocation, for example).
>
> But I ran a few tests, and got odd results - so I ran the same test
> function twice, and got different results - that was my surprise. I did
> this:
> f1 = fa fb fc
> f2 = fa fb fc
> test2 = do print "test1:"
> f1
> f2
>
> and I get different results from the two executions (f1,f2), even though
> they have exactly the same definition. Reversing their order, gives the
> exact same results (i.e. the results are still different, and in the
> same original order as f2;f1). Even doing (f1;f1) gives two different
> results.
Depending on what f1 does, that may be perfectly normal or a serious bug.
We'd need to see more of the code to determine which.
>
> Seems to me that by referential transparency, I should always get the
> same result from the function(s).
>
> So, I added some Debug.trace to the argument functions which are used,
> and I get a trace from the first call(s), but none from the second
> one(s), although I do get the result from each.
Did you do it in the form
fa = trace ("fa") realFa
?
Then the trace is only evaluated the first time fa is evaluated, even if fa
is called later again.
>
> It is as if because of the laziness, it someone cached some of the
> intermediate results, so did not re-invoke the functions.
>
> Anyway, totally confused. I must be missing something significant here.
> Thanks for any clarification! (The original code is a bit long, so I
> did not include here...)
http://hpaste.org/
perhaps?
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