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Apply 'and/'or to a list of lists

FormerMember
FormerMember

Hello,

I have a function, which generates a list of results. The results are lists, which can also be nil in case of an error.

I want to check if, all results are valid with 'and' or if some are invalid with 'or'. However, when I use the 'apply' function, I get an error because the lists are tried to be evaluated. Here is an example with 'and' but the same happens with 'or'

and( '(1) '(2) '(3)) -> '(3)
and( '(1) '(2) nil) -> nil

apply('and '((1) (2) (3))) -> *Error* eval: not a function - 1

I can achieve the same result with a 'forall' loop but it requires more code and is probably slower? Also, it requires an ugly inversion, to emulate the 'or' behavior.

Is there a way to prevent the evaluation of the arguments in an 'apply' call with 'and' or 'or'? Also, is there a way to know, which functions cause this? So far, I only observed this with and/or.

Best regards
Paul

  • Paul,

    That's because and is a syntax form - usually it selectively evaluates its arguments - this allows "expression short-circuiting" which means that the later arguments are only evaluated if the earlier arguments are true. So that allows things like:

    boundp('someVar) && someVar

    to work - if the variable is not bound, it will never get to the second expression.

    As an alternative, you could use:

    forall(x yourList x)

    which will return t if all values in yourList were non-nil.

    Andrew

  • in reply to Andrew Beckett

    Andrew gave you the equivalent for (apply 'and ...) with forall

    The equivalent for (apply 'or ...) is exists

    (exists x yourList x)

    Cheers

    Aurélien

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