# $NetBSD: cond-op.mk,v 1.8 2020/10/24 08:46:08 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 ||, 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 # implicit binding would be this: .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. .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. # 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 . warning 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. all: @:; |