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?