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POWhy do Ruby setters need "self." qualification within the class?
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copied!<p>Ruby setters—whether created by <code>(c)attr_accessor</code> or manually—seem to be the only methods that need <code>self.</code> qualification when accessed within the class itself. This seems to put Ruby alone the world of languages:</p> <ul> <li>All methods need <code>self</code>/<code>this</code> (like Perl, and I think Javascript)</li> <li>No methods require <code>self</code>/<code>this</code> is (C#, Java)</li> <li>Only setters need <code>self</code>/<code>this</code> (Ruby?)</li> </ul> <p>The best comparison is C# vs Ruby, because both languages support accessor methods which work syntactically just like class instance variables: <code>foo.x = y</code>, <code>y = foo.x</code> . C# calls them properties.</p> <p>Here's a simple example; the same program in Ruby then C#:</p> <pre><code>class A def qwerty; @q; end # manual getter def qwerty=(value); @q = value; end # manual setter, but attr_accessor is same def asdf; self.qwerty = 4; end # "self." is necessary in ruby? def xxx; asdf; end # we can invoke nonsetters w/o "self." def dump; puts "qwerty = #{qwerty}"; end end a = A.new a.xxx a.dump </code></pre> <p>take away the <code>self.qwerty =()</code> and it fails (Ruby 1.8.6 on Linux & OS X). Now C#:</p> <pre><code>using System; public class A { public A() {} int q; public int qwerty { get { return q; } set { q = value; } } public void asdf() { qwerty = 4; } // C# setters work w/o "this." public void xxx() { asdf(); } // are just like other methods public void dump() { Console.WriteLine("qwerty = {0}", qwerty); } } public class Test { public static void Main() { A a = new A(); a.xxx(); a.dump(); } } </code></pre> <p>Question: Is this true? Are there other occasions besides setters where self is necessary? I.e., are there other occasions where a Ruby method <em>cannot</em> be invoked <em>without</em> self?</p> <p>There are certainly lots of cases where self <em>becomes</em> necessary. This is not unique to Ruby, just to be clear:</p> <pre><code>using System; public class A { public A() {} public int test { get { return 4; }} public int useVariable() { int test = 5; return test; } public int useMethod() { int test = 5; return this.test; } } public class Test { public static void Main() { A a = new A(); Console.WriteLine("{0}", a.useVariable()); // prints 5 Console.WriteLine("{0}", a.useMethod()); // prints 4 } } </code></pre> <p>Same ambiguity is resolved in same way. But while subtle I'm asking about the case where </p> <ul> <li>A method <em>has</em> been defined, and</li> <li><em>No</em> local variable has been defined, and</li> </ul> <p>we encounter</p> <pre><code>qwerty = 4 </code></pre> <p>which is ambiguous—is this a method invocation or an new local variable assignment?</p> <hr> <p>@Mike Stone</p> <p>Hi! I understand and appreciate the points you've made and your example was great. Believe me when I say, if I had enough reputation, I'd vote up your response. Yet we still disagree: </p> <ul> <li>on a matter of semantics, and</li> <li>on a central point of fact</li> </ul> <p>First I claim, not without irony, we're having a semantic debate about the meaning of 'ambiguity'.</p> <p>When it comes to parsing and programming language semantics (the subject of this question), surely you would admit a broad spectrum of the notion 'ambiguity'. Let's just adopt some random notation: </p> <ol> <li>ambiguous: lexical ambiguity (lex must 'look ahead')</li> <li>Ambiguous: grammatical ambiguity (yacc must defer to parse-tree analysis)</li> <li>AMBIGUOUS: ambiguity knowing everything at the moment of execution</li> </ol> <p>(and there's junk between 2-3 too). All these categories are resolved by gathering more contextual info, looking more and more globally. So when you say,</p> <blockquote> <p>"qwerty = 4" is UNAMBIGUOUS in C# when there is no variable defined...</p> </blockquote> <p>I couldn't agree more. But by the same token, I'm saying </p> <blockquote> <p>"qwerty = 4" is un-Ambiguous in ruby (as it now exists)</p> <p>"qwerty = 4" is Ambiguous in C#</p> </blockquote> <p>And we're not yet contradicting each other. Finally, here's where we really disagree: Either ruby could or could not be implemented without any further language constructs such that,</p> <blockquote> <p>For "qwerty = 4," ruby UNAMBIGUOUSLY invokes an existing setter if there<br> is no local variable defined</p> </blockquote> <p>You say no. I say yes; another ruby could exist which behaves exactly like the current in every respect, <em>except</em> "qwerty = 4" defines a new variable when no setter and no local exists, it invokes the setter if one exists, and it assigns to the local if one exists. I fully accept that I could be wrong. In fact, a reason why I might be wrong would be interesting.</p> <p>Let me explain.</p> <p>Imagine you are writing a new OO language with accessor methods looking like instances vars (like ruby & C#). You'd probably start with conceptual grammars something like:</p> <pre><code> var = expr // assignment method = expr // setter method invocation </code></pre> <p>But the parser-compiler (not even the runtime) will puke, because even after all the input is grokked there's no way to know which grammar is pertinent. You're faced which a classic choice. I can't be sure of the details, but basically ruby does this:</p> <pre><code> var = expr // assignment (new or existing) // method = expr, disallow setter method invocation without . </code></pre> <p>that is why it's un-Ambiguous, while and C# does this:</p> <pre><code> symbol = expr // push 'symbol=' onto parse tree and decide later // if local variable is def'd somewhere in scope: assignment // else if a setter is def'd in scope: invocation </code></pre> <p>For C#, 'later' is still at compile time.</p> <p>I'm sure ruby could do the same, but 'later' would have to be at runtime, because as ben points out you don't know until the statement is executed which case applies.</p> <p>My question was never intended to mean "do I really need the 'self.'?" or "what potential ambiguity is being avoided?" Rather I wanted to know why was this particular choice made? Maybe it's not performance. Maybe it just got the job done, or it was considered best to always allow a 1-liner local to override a method (a pretty rare case requirement) ...</p> <p>But I'm sort of suggesting that the most dynamical language might be the one which postpones this decision the longest, and chooses semantics based on the most contextual info: so if you have no local and you defined a setter, it would use the setter. Isn't this why we like ruby, smalltalk, objc, because method invocation is decided at runtime, offering maximum expressiveness?</p>

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