foreach/opApply is the visitor pattern

[Robert Fraser]

BCS wrote:
> Correct me if I'm wrong but I thing that D's opApply is a form of the 
> Visitor pattern where the calling function's stack frame is the visitor 
> object.
> 
> This just occurred to me. Maybe I've been missing something re the 
> visitor pattern but I think this make for a nice, cool and easy way to 
> describe it. (Also I don't remember it being described that way)

Er.... no. There's no double-dispatch (at least automatically), there's 
only one foreach body delegate. The visitor pattern as far as I know it 
uses dynamic dispatch so the visitor object can handle different objects 
in the class hierarchy differently. For example, you couldn't use a 
foreach to enumerate through a syntax tree and handle expressions and 
statements differently (well, you could, but you'd have to do it manually).