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# $NetBSD: cond-op.mk,v 1.10 2020/11/15 14:58:14 rillig Exp $
#
# Tests for operators like &&, ||, ! in .if conditions.
#
# See also:
# cond-op-and.mk
# cond-op-not.mk
# cond-op-or.mk
# cond-op-parentheses.mk
# In make, && binds more tightly than ||, like in C.
# If make had the same precedence for both && and ||, like in the shell,
# the result would be different.
# If || were to bind more tightly than &&, the result would be different
# as well.
.if !(1 || 1 && 0)
. error
.endif
# If make were to interpret the && and || operators like the shell, the
# previous condition would be interpreted as:
.if (1 || 1) && 0
. error
.endif
# The precedence of the ! operator is different from C though. It has a
# lower precedence than the comparison operators. Negating a condition
# does not need parentheses.
#
# This kind of condition looks so unfamiliar that it doesn't occur in
# practice.
.if !"word" == "word"
. error
.endif
# This is how the above condition is actually interpreted.
.if !("word" == "word")
. error
.endif
# TODO: Demonstrate that the precedence of the ! and == operators actually
# makes a difference. There is a simple example for sure, I just cannot
# wrap my head around it right now. See the truth table generator below
# for an example that doesn't require much thought.
# This condition is malformed because the '!' on the right-hand side must not
# appear unquoted. If any, it must be enclosed in quotes.
# In any case, it is not interpreted as a negation of an unquoted string.
# See CondParser_String.
.if "!word" == !word
. error
.endif
# Surprisingly, the ampersand and pipe are allowed in bare strings.
# That's another opportunity for writing confusing code.
# See CondParser_String, which only has '!' in the list of stop characters.
.if "a&&b||c" != a&&b||c
. error
.endif
# As soon as the parser sees the '$', it knows that the condition will
# be malformed. Therefore there is no point in evaluating it.
#
# As of 2020-09-11, that part of the condition is evaluated nevertheless,
# since CondParser_Expr just requests the next token, without restricting
# the token to the expected tokens. If the parser were to restrict the
# valid follow tokens for the token "0" to those that can actually produce
# a correct condition (which in this case would be comparison operators,
# TOK_AND, TOK_OR or TOK_RPAREN), the variable expression would not have
# to be evaluated.
#
# This would add a good deal of complexity to the code though, for almost
# no benefit, especially since most expressions and conditions are side
# effect free.
.if 0 ${ERR::=evaluated}
. error
.endif
.if ${ERR:Uundefined} == evaluated
. info After detecting a parse error, the rest is evaluated.
.endif
# Just in case that parsing should ever stop on the first error.
.info Parsing continues until here.
# Demonstration that '&&' has higher precedence than '||'.
.info A B C => (A || B) && C A || B && C A || (B && C)
.for a in 0 1
. for b in 0 1
. for c in 0 1
. for r1 in ${ ($a || $b) && $c :?1:0}
. for r2 in ${ $a || $b && $c :?1:0}
. for r3 in ${ $a || ($b && $c) :?1:0}
. info $a $b $c => ${r1} ${r2} ${r3}
. endfor
. endfor
. endfor
. endfor
. endfor
.endfor
all:
@:;
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