On Sat, 15 Jan 2022, Ryan Joseph via fpc-pascal wrote:
On Jan 15, 2022, at 8:30 AM, Michael Van Canneyt via fpc-pascal
<fpc-pascal@lists.freepascal.org> wrote:
I saw a new syntax in Swift which I thought was clever and fits a pattern
I've seen before. Basically it's a case statement for class types which
lets you branch depending on which class type the class instance is at run
time.
I think Scala did it before Swift.
What did it look like? Seems like an obvious feature any OOP language should
have.
myInstance match {
case TComponent => dosomething;
case TPersistent => dosomethingelse;
}
Swift has a compound switch statement which does lots of things. It's a little messy but it accomplishes this
well. For example here they have a "case is" and "case let _ as" which tests for class
type or casts to a local variable using "as".
switch object {
case is Message:
break
case let content as MessageContent:
break
case let attachment as Attachment:
break
default: break
}
Problem for Pascal is how to handle the casting mess. C languages (and Delphi
now I guess) can do inline variable declarations to avoid the casting.
I don't see how an inline variable helps with the casting mess. You'll
always need a cast.
What I do is
Var
MyInstance : TObject;
MyNeededClass : TMyNeededClass absolute myInstance:
>
Come to think of it this a similar problem with for-loops where you want to
loop over a collection of only certain types. For example:
for monster in monsters do
if monster is TZenChan then
TZenChan(monster).Dothis;
See above for the solution.
Michael.
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