'This' type

Marek Kozieł develop4lasu at gmail.com
Sun Mar 15 03:57:43 PDT 2009

2009/3/15 Howard Lovatt <howard.lovatt at iee.org>

> I am not sure I completely follow the proposal, but if the proposal is
> something like self types in Eiffel or Scala then they can have
> problems. In the Eiffel book, I think it is ed. 3 that has self types,
> they are introduced in a Skier example. The example is organising a
> High School Ski Trip and you don't want the boys sharing with the
> girls, so you want the roommate() method to have type this (self in
> Eiffel), so that Boy's roommmate returns a Boy and Girl's roommate()
> returns a Girl. Something like:
> interface Skier { This roommate(); void setRoommate(This roomie); ... }
> class Boy implements Skier { Boy roommate() ... }
> class Girls implements Skier { Girl roommate() ... }
> Now they introduce the concept of a Ranked skier and have classes
> RankedBoy and RankedGirl, something like this:
> interface Ranked { int ranking(); }
> class RankedBoy extends Boy implements Ranking { RankedBoy roommate() ... }
> ...
> The problem is that now only RankedBoys can share with RankedBoys;
> which was not the intention in the example and is an extremely
> difficult to find bug. This shows that even *expert* programmers can
> make mistakes with self types. Therefore self types seem to solve some
> problems, but introduce others.
After some time, I found the same about this.

'This' type need lots analyze and it should be split into:

'This' type :

- work with inheritance

- can be used in generics

- but allowed only for return type (would add relation to generics)

- it would be great mistake to allow 'This' as input parameter other than <?
super This>

'Self' type :

- used as replacement current class.

I'll try to finish other specification that will allow return 'this' and
will do interfere with potential added 'This' type.

Pozdrowionka. / Regards.
Lasu aka Marek Kozieł


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