How to overcome long-circuit evaluation

[Ursego]

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by Ursego » 19 Feb 2013, 21:39

Unfortunately, the PowerScript programming language doesn't support the short-circuit evaluation of Boolean expressions - the evaluation is always "long-circuit" . Let's suppose we have Boolean functions uf_is_bird() and uf_is_dinosaur(); the first one is very light and quick, but the second is very heavy and slow (because so are dinosaurs it goes to the database). If you have come to PowerBuilder from a programming language which supports the short-circuit evaluation (and almost all the languages support it!), you can mistakenly write:

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lb_is_bird_or_dinosaur = (uf_is_bird() or uf_is_dinosaur())

or (if you are returning a value from a Boolean function)

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return (uf_is_bird() or uf_is_dinosaur())

You can suppose, that uf_is_dinosaur() will not be called if uf_is_bird() has returned true, but you are wrong: uf_is_dinosaur() will be executed ALWAYS. So, express yourself in one of the following ways:

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lb_is_bird_or_dinosaur = uf_is_bird()
if not lb_is_bird_or_dinosaur then lb_is_bird_or_dinosaur = uf_is_dinosaur()

or

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lb_is_bird_or_dinosaur = false
if uf_is_bird() then lb_is_bird_or_dinosaur = uf_is_dinosaur()

or (if you are returning a value from a Boolean function)

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if uf_is_bird() then return true
if uf_is_dinosaur() then return true
return false

It was an OR relation, but the same problem exists with an AND. In the following example, the condition ll_row > 0 will not protect against failure when ll_row = 0 (as a Java or C# developer could think):

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lb_from_usa = (ll_row > 0 and dw_addr.object.country[ll_row] = "US")

To use ll_row > 0 as a protection, write this way:

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lb_from_usa = false
if ll_row > 0 then lb_from_usa = (dw_addr.object.country[ll_row] = "US")

Remember about that when you are trying to make you code safer using the IsValid() function. The following example, written in the Java/C# style, is dangerous - if the object has not been created, you will get a run-time error:

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if IsValid(ids_XXX) and ids_XXX.RowCount() > 0 then...

The correct solution is obvious:

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if IsValid(ids_XXX) then
   if ids_XXX.RowCount() > 0 then...
end if