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varmod-ifelse.mk revision 1.5 
      1 # $NetBSD: varmod-ifelse.mk,v 1.5 2020/10/23 14:24:51 rillig Exp $
      2 #
      3 # Tests for the ${cond:?then:else} variable modifier, which evaluates either
      4 # the then-expression or the else-expression, depending on the condition.
      5 #
      6 # The modifier was added on 1998-04-01.
      7 #
      8 # Until 2015-10-11, the modifier always evaluated both the "then" and the
      9 # "else" expressions.
     10 
     11 # TODO: Implementation
     12 
     13 # The variable name of the expression is expanded and then taken as the
     14 # condition.  In this case it becomes:
     15 #
     16 #	variable expression == "variable expression"
     17 #
     18 # This confuses the parser, which expects an operator instead of the bare
     19 # word "expression".  If the name were expanded lazily, everything would be
     20 # fine since the condition would be:
     21 #
     22 #	${:Uvariable expression} == "literal"
     23 #
     24 # Evaluating the variable name lazily would require additional code in
     25 # Var_Parse and ParseVarname, it would be more useful and predictable
     26 # though.
     27 .if ${${:Uvariable expression} == "literal":?bad:bad}
     28 .  error
     29 .else
     30 .  error
     31 .endif
     32 
     33 # In a variable assignment, undefined variables are not an error.
     34 # Because of the early expansion, the whole condition evaluates to
     35 # ' == ""' though, which cannot be parsed because the left-hand side looks
     36 # empty.
     37 COND:=	${${UNDEF} == "":?bad-assign:bad-assign}
     38 
     39 # In a condition, undefined variables generate a "Malformed conditional"
     40 # error.  That error message is wrong though.  In lint mode, the correct
     41 # "Undefined variable" error message is generated.
     42 # The difference to the ':=' variable assignment is the additional
     43 # "Malformed conditional" error message.
     44 .if ${${UNDEF} == "":?bad-cond:bad-cond}
     45 .  error
     46 .else
     47 .  error
     48 .endif
     49 
     50 # When the :? is parsed, it is greedy.  The else branch spans all the
     51 # text, up until the closing character '}', even if the text looks like
     52 # another modifier.
     53 .if ${1:?then:else:Q} != "then"
     54 .  error
     55 .endif
     56 .if ${0:?then:else:Q} != "else:Q"
     57 .  error
     58 .endif
     59 
     60 all:
     61 	@:;
     62