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[X10-core] Handling .equals
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From: Vijay S. <vi...@sa...> - 2009-07-08 05:29:02
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Problem: Given a two-headed type system -- with <tt>struct Struct</tt>
and <tt>class Object</tt> as the two heads -- how does one provide a
signature for the <tt>equals</tt> method?<br>
<br>
Solution: <br>
<br>
(0) Distinguish between two relationships on types: <br>
<ul>
<li> <tt>_ is a subtype of _</tt> (written: <tt>_ <: _</tt>).
If <tt>S <: T</tt>, then in every context a value of type <tt>S</tt>
can substitute for a value of type <tt>T</tt>. Thus a value of type <tt>S</tt>
can be assigned to a variable of type <tt>T</tt>. <br>
</li>
<li> <tt>_ behaves as _</tt> (written: <tt>_ <| _</tt>). If <tt>S</tt>
behaves as <tt>T</tt> then all methods available on <tt>T</tt> are
available on <tt>S</tt>, but it is not necessarily the case that a
value of type s can be assigned to a variable of type T. For instance,
S may be a struct and I an interface.</li>
<li><tt>S <: T</tt> implies <tt>S <| T</tt><br>
</li>
</ul>
(1) Introduce <tt>x.Type</tt>. For any struct or object x, <tt>x.Type</tt>
is the struct or class of which x is an instance. <br>
<br>
(2) Assert (and ensure) that the X10 type system validates: <tt><br>
</tt>
<blockquote><tt>for all x. (x.Type <: Struct or x.Type <: Object)</tt><br>
</blockquote>
<br>
(3) Define <br>
<br>
<tt>interface Equals {<br>
def equals[T](x:T){this.Type <: T}:boolean;<br>
}</tt><br>
<br>
(4) Have Struct implement Equals:<br>
<br>
<tt>package x10.lang;<br>
abstract struct Struct implements Equals {}</tt><br>
<br>
Hence <tt>Struct <| Equals</tt><br>
<br>
(5) Have Object implement Equals:<br>
<br>
<tt>package x10.lang;<br>
class Object implements Equals {<br>
def equals[T](x:T){this.Type <: T} = this == x;<br>
...<br>
}<br>
</tt>Hence <tt>Object <| Equals<br>
<br>
</tt>(5) Hence the type system can establish: <tt>for all x. x.Type
<| Equals. </tt>Therefore in the body of a class G with a type
parameter X, and x:X, it is legal to invoke <tt>x.equals(y)</tt> as
long as it can be established that <tt>y:Y</tt> and <tt>X <: Y</tt>.<br>
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