1# $NetBSD: cond-late.mk,v 1.3 2020/11/15 14:07:53 rillig Exp $ 2# 3# Using the :? modifier, variable expressions can contain conditional 4# expressions that are evaluated late, at expansion time. 5# 6# Any variables appearing in these 7# conditions are expanded before parsing the condition. This is 8# different from many other places. 9# 10# Because of this, variables that are used in these lazy conditions 11# should not contain double-quotes, or the parser will probably fail. 12# 13# They should also not contain operators like == or <, since these are 14# actually interpreted as these operators. This is demonstrated below. 15# 16 17all: cond-literal 18 19COND.true= "yes" == "yes" 20COND.false= "yes" != "yes" 21 22# If the order of evaluation were to change to first parse the condition 23# and then expand the variables, the output would change from the 24# current "yes no" to "yes yes", since both variables are non-empty. 25cond-literal: 26 @echo ${ ${COND.true} :?yes:no} 27 @echo ${ ${COND.false} :?yes:no} 28 29VAR+= ${${UNDEF} != "no":?:} 30.if empty(VAR:Mpattern) 31.endif 32