我无法向自己解释的简单代码：
puts a if a = 1
这导致了
warning: found = in conditional,should be == NameError: undefined local variable or method 'a' for main:Object
虽然,现在检查我们可以看到,它已被定义：
a #=> 1
尽管抛出了异常,为什么将其分配给1？
从docs：
The confusion comes from the out-of-order execution of the expression.
First the local variable is assigned-to then you attempt to call a
nonexistent method [a].
这部分仍然令人困惑 – 为什么解释器没有检测到已经定义的局部变量a并仍然试图调用“不存在”的方法？如果它也不检查局部变量,找到定义的局部变量a并打印1？
解决方法
让我们来看看Ruby的修饰符的抽象语法树,如果：
$ruby --dump=parsetree -e 'puts a if a = 1' # @ NODE_SCOPE (line: 1,code_range: (1,0)-(1,15)) # +- nd_tbl: :a # +- nd_args: # | (null node) # +- nd_body: # @ NODE_PRELUDE (line: 1,15)) # +- nd_head: # | (null node) # +- nd_body: # | @ NODE_IF (line: 1,15)) # | +- nd_cond: # | | @ NODE_DASGN_CURR (line: 1,10)-(1,15)) # | | +- nd_vid: :a # | | +- nd_value: # | | @ NODE_LIT (line: 1,14)-(1,15)) # | | +- nd_lit: 1 # | +- nd_body: # | | @ NODE_FCALL (line: 1,6)) # | | +- nd_mid: :puts # | | +- nd_args: # | | @ NODE_ARRAY (line: 1,5)-(1,6)) # | | +- nd_alen: 1 # | | +- nd_head: # | | | @ NODE_VCALL (line: 1,6)) # | | | +- nd_mid: :a # | | +- nd_next: # | | (null node) # | +- nd_else: # | (null node) # +- nd_compile_option: # +- coverage_enabled: false
对于标准如果：
$ruby --dump=parsetree -e 'if a = 1 then puts a end' # @ NODE_SCOPE (line: 1,24)) # +- nd_tbl: :a # +- nd_args: # | (null node) # +- nd_body: # @ NODE_PRELUDE (line: 1,24)) # +- nd_head: # | (null node) # +- nd_body: # | @ NODE_IF (line: 1,24)) # | +- nd_cond: # | | @ NODE_DASGN_CURR (line: 1,3)-(1,8)) # | | +- nd_vid: :a # | | +- nd_value: # | | @ NODE_LIT (line: 1,7)-(1,8)) # | | +- nd_lit: 1 # | +- nd_body: # | | @ NODE_FCALL (line: 1,20)) # | | +- nd_mid: :puts # | | +- nd_args: # | | @ NODE_ARRAY (line: 1,19)-(1,20)) # | | +- nd_alen: 1 # | | +- nd_head: # | | | @ NODE_DVAR (line: 1,20)) # | | | +- nd_vid: :a # | | +- nd_next: # | | (null node) # | +- nd_else: # | (null node) # +- nd_compile_option: # +- coverage_enabled: false
唯一的区别是puts的方法参数：
# | | | @ NODE_VCALL (line: 1,6)) # | | | +- nd_mid: :a
VS：
# | | | @ NODE_DVAR (line: 1,20)) # | | | +- nd_vid: :a
使用修饰符if,解析器将a视为方法调用并创建NODE_VCALL.这指示解释器进行方法调用(尽管存在局部变量a),从而产生NameError. (因为没有方法a)
使用标准if,解析器将a视为局部变量并创建NODE_DVAR.这指示解释器查找按预期工作的局部变量.
如您所见,Ruby在解析器级别识别局部变量.这就是文档说的原因:(强调添加)
the modifier and standard versions […] are not exact transformations of each other due to parse order.