151 lines
4.4 KiB
Makefile
151 lines
4.4 KiB
Makefile
# $NetBSD: cond-func-empty.mk,v 1.8 2020/09/23 08:11:28 rillig Exp $
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#
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# Tests for the empty() function in .if conditions, which tests a variable
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# expression for emptiness.
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#
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# Note that the argument in the parentheses is indeed a variable name,
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# optionally followed by variable modifiers. This is like the defined()
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# function.
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#
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.undef UNDEF
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EMPTY= # empty
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SPACE= ${:U }
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WORD= word
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# An undefined variable is empty.
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.if !empty(UNDEF)
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. error
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.endif
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# An undefined variable has the empty string as the value, and the :M
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# variable modifier does not change that.
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#
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.if !empty(UNDEF:M*)
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. error
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.endif
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# The :S modifier replaces the empty value with an actual word, and
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# after that the expression is no longer empty. Because the variable
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# was undefined in the first place, the expression has the flag VAR_JUNK
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# but not VAR_KEEP, therefore it is still considered undefined.
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# Only very few variable modifiers turn an undefined variable expression
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# into a defined variable expression. The :U and :D modifiers belong to
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# that group, but :S doesn't (see VAR_KEEP).
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#
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# XXX: This is hard to explain to someone who doesn't know these
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# implementation details.
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#
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.if !empty(UNDEF:S,^$,value,W)
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. error
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.endif
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# The :U modifier modifies expressions based on undefined variables
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# (VAR_JUNK) by adding the VAR_KEEP flag, which marks the expression
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# as "being interesting enough to be further processed".
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#
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.if empty(UNDEF:S,^$,value,W:Ufallback)
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. error
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.endif
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# And now to the surprising part. Applying the following :S modifier to the
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# undefined variable makes it non-empty, but the marker VAR_JUNK is preserved
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# nevertheless. The :U modifier that follows only looks at VAR_JUNK to decide
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# whether the variable is defined or not. This kind of makes sense since the
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# :U modifier tests the _variable_, not the _expression_.
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#
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# But since the variable was undefined to begin with, the fallback value is
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# used in this expression.
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#
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.if ${UNDEF:S,^$,value,W:Ufallback} != "fallback"
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. error
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.endif
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# The variable EMPTY is completely empty (0 characters).
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.if !empty(EMPTY)
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. error
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.endif
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# The variable SPACE has a single space, which counts as being empty.
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.if !empty(SPACE)
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. error
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.endif
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# The variable .newline has a single newline, which counts as being empty.
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.if !empty(.newline)
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. error
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.endif
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# The empty variable named "" gets a fallback value of " ", which counts as
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# empty.
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#
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# Contrary to the other functions in conditionals, the trailing space is not
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# stripped off, as can be seen in the -dv debug log. If the space had been
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# stripped, it wouldn't make a difference in this case.
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#
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.if !empty(:U )
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. error
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.endif
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# Now the variable named " " gets a non-empty value, which demonstrates that
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# neither leading nor trailing spaces are trimmed in the argument of the
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# function. If the spaces were trimmed, the variable name would be "" and
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# that variable is indeed undefined. Since ParseEmptyArg calls Var_Parse
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# without VARE_UNDEFERR, the value of the undefined variable is returned as
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# an empty string.
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${:U }= space
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.if empty( )
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. error
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.endif
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# The value of the following expression is " word", which is not empty.
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.if empty(:U word)
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. error
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.endif
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# The :L modifier creates a variable expression that has the same value as
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# its name, which both are "VAR" in this case. The value is therefore not
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# empty.
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.if empty(VAR:L)
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. error
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.endif
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# The variable WORD has the value "word", which does not count as empty.
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.if empty(WORD)
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. error
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.endif
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# The expression ${} for a variable with the empty name always evaluates
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# to an empty string (see Var_Parse, varUndefined).
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.if !empty()
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. error
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.endif
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# Ensure that variable expressions that appear as part of the argument are
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# properly parsed. Typical use cases for this are .for loops, which are
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# expanded to exactly these ${:U} expressions.
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#
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# If everything goes well, the argument expands to "WORD", and that variable
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# is defined at the beginning of this file. The surrounding 'W' and 'D'
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# ensure that the parser in ParseEmptyArg has the correct position, both
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# before and after the call to Var_ParsePP.
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.if empty(W${:UOR}D)
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. error
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.endif
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# There may be spaces at the outside of the parentheses.
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# Spaces inside the parentheses are interpreted as part of the variable name.
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.if ! empty ( WORD )
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. error
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.endif
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${:U WORD }= variable name with spaces
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# Now there is a variable named " WORD ", and it is not empty.
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.if empty ( WORD )
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. error
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.endif
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all:
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@:;
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