[fpc-pascal] Case statement for class introspection

Ryan Joseph genericptr at gmail.com
Sat Jan 15 03:14:39 CET 2022

> On Jan 15, 2022, at 8:30 AM, Michael Van Canneyt via fpc-pascal <fpc-pascal at 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.

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:
                case let content as MessageContent:
                case let attachment as Attachment:
                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. 

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

Swift does something similar as the switch which would look kind of like this:

for case monster as TZenChan in monsters do

That syntax is not so nice but I like they're trying to help us manage class introspection using existing language constructs.

	Ryan Joseph

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