Count on 'if' control structure's lazy evaluation?

[Eoghan]

Hi everyone - sorry if this is a very basic question, but I wondered if there's an official position on this.

Can we count on LiveCode's 'if' control structure always doing lazy evaluation?

... or would the following code be bad practice?

Code: Select all

if (tMyArray is not empty) and (tMyArray[tMyKey] is not empty) then
   # do something with tMyArray[tMyKey]
end if

LazyEval_Test.livecode.zip

Stack showing lazy evaluation

(1.21 KiB) Downloaded 194 times

In the above stack, the evaluation of the 'if' is lazy (if the 'firstTest value' is 'false', then the secondTest() function isn't called) - but I'd better not rely on that functionality if LiveCode might change to eager evaluation at some point.

Thanks
Eoghan

[Klaus]

HI Eoghan,

not sure I understand the problem!?

If the first expression = FALSE why check the other expression at all, lazy evaluation or not?
Does not make any logical sense to me


Best

Klaus

[Klaus]

P.S.

In your case you only need to check one condition:

Code: Select all

if tMyArray[tMyKey] <> empty then
   # do something with tMyArray[tMyKey]
end if

[Eoghan]

Hi Klaus,

When I was studying Comp Sci (nearly 20 years ago ...), they always said that the following code was the "Wally trap":

Code: Select all

if (tMyArray is not empty) and (tMyArray[tMyKey] is not empty) then

They told us to test each part of that 'if' statement separately, in case we came across a programming language that would execute both parts, even if the first part was false.

As you say, logically the above code should be fine - but I wondered if it was officially stated anywhere that it is alright to use in LiveCode.

Ah, thanks for your second comment! So 'tMyArray[tMyKey] <> empty' won't cause problems if tMyArray is empty (it won't try and do a C-type dereference of a null pointer)?

Cheers,
Eoghan.

[Klaus]

Hi Eoghan,

Ah, thanks for your second comment! So 'tMyArray[tMyKey] <> empty' won't cause problems if tMyArray is empty (it won't try and do a C-type dereference of a null pointer)?

no it won't


Best

Klaus

[Eoghan]

Excellent, thanks Klaus!

Cheers,
Eoghan.

[jacque]

The engine has worked this way since the beginning and I don't expect it to change. I rely on the behavior all the time and I think a lot of code would break if it didn't. The original author of MetaCard said he did it on purpose in the interest of efficiency.

[Thierry]

Hello,

In fact, this is not really a Lazy evaluation,
but a short circuit boolean evaluation,
which most of modern languages have!

( Ada, Java have 2 differents operators to use or not this feature because
of some side-effect )

http://en.wikipedia.org/wiki/Short-circuit_evaluation

Regards,
Thierry

[Eoghan]

Thanks for your comments Jacqueline & Thierry, very useful to know.

Cheers,
Eoghan.