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      1  1.3  rillig # $NetBSD: varmod-no-match.mk,v 1.3 2023/02/26 06:08:06 rillig Exp $
      2  1.1  rillig #
      3  1.3  rillig # Tests for the expression modifier ':N', which filters words that do not
      4  1.3  rillig # match the given pattern.
      5  1.3  rillig 
      6  1.3  rillig 
      7  1.3  rillig # Keep all words except for 'two'.
      8  1.3  rillig .if ${:U one two three :Ntwo} != "one three"
      9  1.3  rillig .  error
     10  1.3  rillig .endif
     11  1.3  rillig 
     12  1.3  rillig # Keep all words except those starting with 't'.
     13  1.3  rillig # See varmod-match.mk for the details of pattern matching.
     14  1.3  rillig .if ${:U one two three four six :Nt*} != "one four six"
     15  1.3  rillig .  error
     16  1.3  rillig .endif
     17  1.3  rillig 
     18  1.3  rillig 
     19  1.3  rillig # Idiom: normalize whitespace
     20  1.3  rillig #
     21  1.3  rillig # The modifier ':N' can be used with an empty pattern.  As that pattern never
     22  1.3  rillig # matches a word, the only effect is that the string is split into words and
     23  1.3  rillig # then joined again, thereby normalizing whitespace around and between the
     24  1.3  rillig # words.  And even though the 'N' in ':N' might serve as a mnemonic for
     25  1.3  rillig # "normalize whitespace", this idiom is not used in practice, resorting to the
     26  1.3  rillig # much more common ':M*' to "select all words" instead.
     27  1.3  rillig .if ${:U :N} != ""
     28  1.3  rillig .  error
     29  1.3  rillig .endif
     30  1.3  rillig .if ${:U one    two three :N} != "one two three"
     31  1.3  rillig .  error
     32  1.3  rillig .endif
     33  1.3  rillig .if ${:U one    two three :M*} != "one two three"
     34  1.3  rillig .  error
     35  1.3  rillig .endif
     36  1.3  rillig 
     37  1.3  rillig 
     38  1.3  rillig # Idiom: single-word expression equals any of several words or patterns
     39  1.3  rillig #
     40  1.3  rillig # If an expression is guaranteed to consist of a single word, the modifier
     41  1.3  rillig # ':N' can be chained to compare the expression to several words or even
     42  1.3  rillig # patterns in a sequence.  If one of the patterns matches, the final
     43  1.3  rillig # expression will be the empty string.
     44  1.3  rillig #
     45  1.3  rillig .if ${:U word :None:Ntwo:Nthree} != ""
     46  1.3  rillig #  good
     47  1.3  rillig .else
     48  1.3  rillig .  error
     49  1.3  rillig .endif
     50  1.3  rillig .if ${:U two :None:Ntwo:Nthree} != ""
     51  1.3  rillig .  error
     52  1.3  rillig .else
     53  1.3  rillig #  good
     54  1.3  rillig .endif
     55  1.3  rillig #
     56  1.3  rillig # The modifier ':N' is seldom used in general since positive matches with ':M'
     57  1.3  rillig # are easier to grasp.  Chaining the ':N' modifier is even more difficult to
     58  1.3  rillig # grasp due to the many negations involved.
     59  1.3  rillig #
     60  1.3  rillig # The final '!= ""' adds to the confusion because at first glance, the
     61  1.3  rillig # condition may look like '${VAR} != ""', which for a single-word variable is
     62  1.3  rillig # always true.
     63  1.3  rillig #
     64  1.3  rillig # The '!= ""' can be omitted if the expression cannot have the numeric value
     65  1.3  rillig # 0, which is common in practice.  In that form, each ':N' can be pronounced
     66  1.3  rillig # as 'neither' or 'nor', which makes the expression sound more natural.
     67  1.3  rillig #
     68  1.3  rillig .if ${:U word :None:Ntwo:Nthree}
     69  1.3  rillig #  good
     70  1.3  rillig .else
     71  1.3  rillig .  error
     72  1.3  rillig .endif
     73  1.3  rillig .if ${:U two :None:Ntwo:Nthree}
     74  1.3  rillig .  error
     75  1.3  rillig .else
     76  1.3  rillig #  good
     77  1.3  rillig .endif
     78  1.3  rillig #
     79  1.3  rillig # Replacing the '${...} != ""' with '!empty(...)' doesn't improve the
     80  1.3  rillig # situation as the '!' adds another level of negations, and the word 'empty'
     81  1.3  rillig # is a negation on its own, thereby creating a triple negation.  Furthermore,
     82  1.3  rillig # due to the '!empty', the expression to be evaluated no longer starts with
     83  1.3  rillig # '$' and is thus more difficult to spot quickly.
     84  1.3  rillig #
     85  1.3  rillig .if !empty(:U word :None:Ntwo:Nthree)
     86  1.3  rillig #  good
     87  1.3  rillig .else
     88  1.3  rillig .  error
     89  1.3  rillig .endif
     90  1.3  rillig .if !empty(:U two :None:Ntwo:Nthree)
     91  1.3  rillig .  error
     92  1.3  rillig .else
     93  1.3  rillig #  good
     94  1.3  rillig .endif
     95  1.1  rillig 
     96  1.1  rillig 
     97  1.1  rillig all:
     98