Optional parens in Ruby for method with uppercase start letter?

问题

I just started out using IronRuby (but the behaviour seems consistent when I tested it in plain Ruby) for a DSL in my .NET application - and as part of this I'm defining methods to be called from the DSL via define_method.

However, I've run into an issue regarding optional parens when calling methods starting with an uppercase letter.

Given the following program:

class DemoClass
    define_method :test do puts "output from test" end
    define_method :Test do puts "output from Test" end

    def run
        puts "Calling 'test'"
        test()
        puts "Calling 'test'"
        test
        puts "Calling 'Test()'"
        Test()
        puts "Calling 'Test'"
        Test
    end
end

demo = DemoClass.new
demo.run

Running this code in a console (using plain ruby) yields the following output:

ruby .\test.rb
Calling 'test'
output from test
Calling 'test'
output from test
Calling 'Test()'
output from Test
Calling 'Test'
./test.rb:13:in `run': uninitialized constant DemoClass::Test (NameError)
    from ./test.rb:19:in `<main>'

I realize that the Ruby convention is that constants start with an uppercase letter and that the general naming convention for methods in Ruby is lowercase. But the parens are really killing my DSL syntax at the moment.

Is there any way around this issue?

回答1:

This is just part of Ruby's ambiguity resolution.

In Ruby, methods and variables live in different namespaces, therefore there can be methods and variables (or constants) with the same name. This means that, when using them, there needs to be some way to distinguish them. In general, that's not a problem: messages have receivers, variables don't. Messages have arguments, variables don't. Variables are assigned to, messages aren't.

The only problem is when you have no receiver, no argument and no assignment. Then, Ruby cannot tell the difference between a receiverless message send without arguments and a variable. So, it has to make up some arbitrary rules, and those rules are basically:

for an ambiguous token starting with a lowercase letter, prefer to interpret it as a message send, unless you positively know it is a variable (i.e. the parser (not(!) the interpreter) has seen an assignment before)
for an ambiguous token starting with an uppercase letter, prefer to interpret it as a constant

Note that for a message send with arguments (even if the argument list is empty), there is no ambiguity, which is why your third example works.

test(): obviously a message send, no ambiguity here
test: might be a message send or a variable; resolution rules say it is a message send
Test(): obviously a message send, no ambiguity here
self.Test: also obviously a message send, no ambiguity here
Test: might be a message send or a constant; resolution rules say it is a constant

Note that those rules are a little bit subtle, for example here:

if false
  foo = 'This will never get executed'
end

foo # still this will get interpreted as a variable

The rules say that whether an ambiguous token gets interpreted as a variable or a message send is determined by the parser and not the interpreter. So, because the parser has seen foo = whatever, it tags foo as a variable, even though the code will never get executed and foo will evaluate to nil as all uninitialized variables in Ruby do.

TL;DR summary: you're SOL.

What you could do is override const_missing to translate into a message send. Something like this:

class DemoClass
  def test; puts "output from test" end
  def Test; puts "output from Test" end

  def run
    puts "Calling 'test'"
    test()
    puts "Calling 'test'"
    test
    puts "Calling 'Test()'"
    Test()
    puts "Calling 'Test'"
    Test
  end

  def self.const_missing(const)
    send const.downcase
  end
end

demo = DemoClass.new
demo.run

Except this obviously won't work, since const_missing is defined on DemoClass and thus, when const_missing is run, self is DemoClass which means that it tries to call DemoClass.test when it should be calling DemoClass#test via demo.test.

I don't know how to easily solve this.

来源：https://stackoverflow.com/questions/2950956/optional-parens-in-ruby-for-method-with-uppercase-start-letter

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ruby

ironruby