Darren Duncan wrote: > Larry Wall wrote: >> >> Or going the other direction, perhaps we're missing a primitive that >> can produce a data structure with the type information stripped, and >> then eqv might be able to determine structural equivalence between >> two canonicalized values. > > Often you still want to know the declared type, though, because while > sometimes we want things to be equivalent if they are structurally > equivalent, other times we don't. A declared type implies a particular > intended interpretation of a value. For example, we could have two classes > named Weight and Distance, both of which have a single Num-typed attribute > named $v. A structural comparison of a Weight.new(3) and a Distance.new(3) > may consider them equal, but logically we would want those two to not > compare equal, because no weight is a distance or vice-versa, so every > Weight should compare as not equal to every Distance. Ignoring the declared > type only really makes sense when the two values are subtypes of a common > parent type. It only makes sense for objects of two classes to be declared > equal if you can say that a value of a type "is a" value of the other type.
Right. Still, there are times when duck-typing, flawed as it is, might be exactly what is needed to resolve the problem at hand. I forget who or in what context, but I vaguely recall someone posting an article here that proposed the use of £ in signatures as a modifier to mean "is like a" (i.e., accept anything that matches the given role's set of methods; duck-typing) rather than "is a" or "does" (which, respectively, check for inheritance or composition). (Could the original poster please repost the link here for reference purposes?) To me, this seems like a rather perlish solution: allow perl to do duck-typing, but not as the default means of type comparison; require some additional syntax to enable it. So looking for "Weight" only succeeds when the test subject inherits or composes Weight; but looking for "£ Weight" succeeds for anything that acts like a Weight. -- Jonathan "Dataweaver" Lang
