- From: Robert O'Callahan <robert@ocallahan.org>
- Date: Sat, 28 Sep 2013 23:20:29 +1200
- To: Domenic Denicola <domenic@domenicdenicola.com>
- Cc: Simon Pieters <simonp@opera.com>, www-style <www-style@w3.org>, Boris Zbarsky <bzbarsky@mit.edu>, "public-script-coord@w3.org" <public-script-coord@w3.org>
- Message-ID: <CAOp6jLYoKPQx2WouCo8E50Woj0TUHJCmr-ygezxH_AzAtzQ9MA@mail.gmail.com>
On Sat, Sep 28, 2013 at 10:24 AM, Robert O'Callahan <robert@ocallahan.org>wrote: > You need to read up more about that. What I have proposed is exactly what > that principle would suggest. > (Sorry about being terse, it's the weekend here and I had things to do.) In that proposal, a DOMRect has a current x/y/width/height, provides read-only access to that state, and some other useful read-only functionality defined in terms of that state (e.g. right/bottom). It offers no way to mutate the state, and says nothing about whether the state is immutable or not. A DOMRectImmutable extends DOMRect with the invariant that the state never changes. It is Liskov-Wing substitutable for a DOMRect, because all clients can do with a DOMRect is read the current state, and that works on a DOMRectImmutable. All invariants that a DOMRect satisfies are also satisfied by DOMRectImmutable. A DOMRectMutable extends DOMRect with methods to modify the current state. It too is Liskov-Wing substitutable for a DOMRect. I know this can be a little confusing, but it's totally OK for a LW-substitutable subclass to offer invariants that the superclass doesn't offer. The reverse is forbidden (since a client could rely on such an invariant in the superclass and be broken when the subclass is substituted). In "Square extends Rectangle", this is actually OK if the Rectangle interface doesn't expose mutability. It fails when the Rectangle interface lets you mutate width and height independently. It fails because Rectangle then offers an invariant that Square can't offer --- namely, that after you set "width" to W and "height" to H, then reading "width" will give you W and reading "height" will give you H, for all non-negative W and H. Rob -- Jtehsauts tshaei dS,o n" Wohfy Mdaon yhoaus eanuttehrotraiitny eovni le atrhtohu gthot sf oirng iyvoeu rs ihnesa.r"t sS?o Whhei csha iids teoa stiheer :p atroa lsyazye,d 'mYaonu,r "sGients uapr,e tfaokreg iyvoeunr, 'm aotr atnod sgaoy ,h o'mGee.t" uTph eann dt hwea lmka'n? gBoutt uIp waanndt wyeonut thoo mken.o w * *
Received on Saturday, 28 September 2013 11:20:56 UTC