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# $NetBSD: cond-late.mk,v 1.3 2020/11/15 14:07:53 rillig Exp $
#
# Using the :? modifier, variable expressions can contain conditional
# expressions that are evaluated late, at expansion time.
#
# Any variables appearing in these
# conditions are expanded before parsing the condition. This is
# different from many other places.
#
# Because of this, variables that are used in these lazy conditions
# should not contain double-quotes, or the parser will probably fail.
#
# They should also not contain operators like == or <, since these are
# actually interpreted as these operators. This is demonstrated below.
#
all: cond-literal
COND.true= "yes" == "yes"
COND.false= "yes" != "yes"
# If the order of evaluation were to change to first parse the condition
# and then expand the variables, the output would change from the
# current "yes no" to "yes yes", since both variables are non-empty.
cond-literal:
@echo ${ ${COND.true} :?yes:no}
@echo ${ ${COND.false} :?yes:no}
VAR+= ${${UNDEF} != "no":?:}
.if empty(VAR:Mpattern)
.endif
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