98 lines
2.7 KiB
Makefile
98 lines
2.7 KiB
Makefile
# $NetBSD: varmod-no-match.mk,v 1.3 2023/02/26 06:08:06 rillig Exp $
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#
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# Tests for the expression modifier ':N', which filters words that do not
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# match the given pattern.
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# Keep all words except for 'two'.
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.if ${:U one two three :Ntwo} != "one three"
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. error
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.endif
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# Keep all words except those starting with 't'.
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# See varmod-match.mk for the details of pattern matching.
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.if ${:U one two three four six :Nt*} != "one four six"
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. error
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.endif
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# Idiom: normalize whitespace
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#
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# The modifier ':N' can be used with an empty pattern. As that pattern never
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# matches a word, the only effect is that the string is split into words and
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# then joined again, thereby normalizing whitespace around and between the
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# words. And even though the 'N' in ':N' might serve as a mnemonic for
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# "normalize whitespace", this idiom is not used in practice, resorting to the
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# much more common ':M*' to "select all words" instead.
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.if ${:U :N} != ""
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. error
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.endif
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.if ${:U one two three :N} != "one two three"
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. error
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.endif
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.if ${:U one two three :M*} != "one two three"
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. error
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.endif
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# Idiom: single-word expression equals any of several words or patterns
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#
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# If an expression is guaranteed to consist of a single word, the modifier
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# ':N' can be chained to compare the expression to several words or even
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# patterns in a sequence. If one of the patterns matches, the final
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# expression will be the empty string.
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#
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.if ${:U word :None:Ntwo:Nthree} != ""
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# good
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.else
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. error
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.endif
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.if ${:U two :None:Ntwo:Nthree} != ""
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. error
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.else
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# good
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.endif
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#
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# The modifier ':N' is seldom used in general since positive matches with ':M'
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# are easier to grasp. Chaining the ':N' modifier is even more difficult to
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# grasp due to the many negations involved.
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#
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# The final '!= ""' adds to the confusion because at first glance, the
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# condition may look like '${VAR} != ""', which for a single-word variable is
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# always true.
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#
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# The '!= ""' can be omitted if the expression cannot have the numeric value
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# 0, which is common in practice. In that form, each ':N' can be pronounced
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# as 'neither' or 'nor', which makes the expression sound more natural.
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#
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.if ${:U word :None:Ntwo:Nthree}
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# good
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.else
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. error
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.endif
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.if ${:U two :None:Ntwo:Nthree}
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. error
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.else
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# good
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.endif
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#
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# Replacing the '${...} != ""' with '!empty(...)' doesn't improve the
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# situation as the '!' adds another level of negations, and the word 'empty'
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# is a negation on its own, thereby creating a triple negation. Furthermore,
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# due to the '!empty', the expression to be evaluated no longer starts with
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# '$' and is thus more difficult to spot quickly.
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#
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.if !empty(:U word :None:Ntwo:Nthree)
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# good
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.else
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. error
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.endif
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.if !empty(:U two :None:Ntwo:Nthree)
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. error
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.else
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# good
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.endif
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all:
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