class Hash
def to_method
hash = self
Module.new do
hash.each_pair do |k,v|
define_method k.to_sym do
v
end
end
end
end
end
class Test
def initialize(hash)
self.extend hash.to_method
end
end
k = Test.new(:fun1 => 'var1')
p k.fun1
这段代码没有问题～
class Hash
def to_method
hash = self
Module.new do
hash.each_pair do |k,v|
define_method k.to_sym { v } # 就见过这里的do end, 改成了 {}
end
end
end
end
class Test
def initialize(hash)
self.extend hash.to_method
end
end
k = Test.new(:fun1 => 'var1')
p k.fun1
报错
define_hash_method.v2.rb:6:in `define_method': tried to create Proc object without a block (ArgumentError)
from define_hash_method.v2.rb:6:in `block (2 levels) in to_method'
from define_hash_method.v2.rb:5:in `each_pair'
from define_hash_method.v2.rb:5:in `block in to_method'
from define_hash_method.v2.rb:4:in `initialize'
from define_hash_method.v2.rb:4:in `new'
from define_hash_method.v2.rb:4:in `to_method'
from define_hash_method.v2.rb:14:in `initialize'
from define_hash_method.v2.rb:18:in `new'
from define_hash_method.v2.rb:18:in `<main>'
问题在于{ v }靠k.to_sym比较近 所以就被认为是to_sym的 block 了，而这样define_method就没有 block 了，这显然是不可能的，你不可能定义一个没有函数体的方法。所以加对括号把{ v }的所有权给define_method。
{}的优先级更高，所以会和它前边的先结合，而do end就不会有这种情况了：
2.1.2 :002 > 1.upto 10 { |i| p i }
SyntaxError: (irb):2: syntax error, unexpected '{', expecting end-of-input
1.upto 10 { |i| p i }
^
2.1.2 :003 > 1.upto 10 do |i| p i end
1
2
3
4
5
6
7
8
9
10
=> 1
这跟&&优先级比and高是类似的：
2.1.2 :004 > foo = "foo" && "bar" # <=> foo = ("foo" && "bar")
=> "bar"
2.1.2 :005 > foo
=> "bar"
2.1.2 :006 > foo = "foo" and "bar" # <=> (foo = "foo") and "bar"
=> "bar"
2.1.2 :007 > foo
=> "foo"
