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forall() always returns t when the valueList is empty

Quesar

Hi,

As the subject describes, the forall() function will always return t when the valueList is nil. For example:

forall(x nil (stringp x))  ---> t

forall(x nil (numberp x))  ---> t

This behaviour is mystifying to me, since forall() is only supposed to return t if every member of the valueList fulfills the predicate expression. If the valueList is empty (nil), then the example expressions above should be false, thus the return value should be nil.

For all other cases where the valueList is non-nil, forall() seems to work as expected. This isn't a particularly big problem, but I don't understand the rationale behind this behaviour when the valueList is empty.

Does anyone know the reason behind this?

Thanks.

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  • Andrew Beckett

    It's perfectly logical to me. Everything in the list did fulfil the predicate - there was nothing in the list though. The documentation puts it another way, that it "Verifies that an expression remains true for every element in a list" - well, it was never false, so the result should be true still. In fact to me I can't really see any sense in it returning nil because the predicate expressions in your examples are never evaluated - so nothing ever returns nil within the loop.

    Honestly though, I guess you could argue it either way for this boundary case, but this is the way it is. You can easily get what you want by doing:

    theList && forall(x theList numberp(x))

    Andrew

  • Quesar
    in reply to Andrew Beckett

    Hi Andrew, thanks for replying so quickly!

    Well, I guess that's one way of looking at it, both true and false can be correct depending on how the function should be interpreted for this case. Since the list is empty, my first instinct is that the result should be false (because nothing is evaluated in the first place), but I guess that wasn't how the developers looked at it.

    Thanks for explaining the other side of the coin  Slight smile

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  • Quesar
    in reply to Andrew Beckett

    Hi Andrew, thanks for replying so quickly!

    Well, I guess that's one way of looking at it, both true and false can be correct depending on how the function should be interpreted for this case. Since the list is empty, my first instinct is that the result should be false (because nothing is evaluated in the first place), but I guess that wasn't how the developers looked at it.

    Thanks for explaining the other side of the coin  Slight smile

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