It seems that there are two groups here. 1) Group 1 wants Equatable (and
others) be derived automatically unless specifically overridden: similar to
auto-synthesis of properties in Objective-C. 2) The other group (Group 2) wants
Equatable (and others) be derived explicitly using a `deriving` keyword (or
something semantically equivalent). Unless I missed something, there were no
voices for keeping the status quo, and not introducing any process to
automatically derive these protocols.
I think I chose an easy strategy when proposing the `deriving` keyword. Haskell
is a mature language, and "copying" a feature from them is usually a safe
choice. For each language feature, someone has to think through all the
implications of it; this is usually far from trivial. I argue that if I take a
feature from another language, someone has probably already thought about all
the pros and cons of different solutions. This is just a plea for embracing
precedent.
There is one advantage of method 2 that (I think) hasn't been discussed so far:
when you declare a type `S`, and an `Equatable` instance is automatically
derived, there is no way to override that instance in another module. With
method 1, there is also no way to request that an `Equatable` instance should
*not* be generated. I think no one will vote for something like `struct S
@notderiving(Equateble,Hashable) { ... }`.
Also, a `deriving` keyword is less magical than just automatically deriving
`Equatable` and `Hashable` instances. I think the language rules should be as
simple as possible, by default. If you want any kind of special behavior, you
have to ask for it: `deriving Equatable`, `@IBOutlet`, `@NSManaged`.
Furthermore, I think it is good that the developer is aware that there is an
"==" method somewhere, specifically for this new type. The compiler should not
arbitrarily create methods, because someone may need them. Even if it is very
likely that you will need them. Just like in a coffee house, you are asked if
you want a coffee, even if you are visiting it every day. For example with
Objective-C, I want each developer to be aware of the difference between a
property and an iVar, and be aware of the connection between properties,
methods, and key-value-coding. The complexities of the language shouldn't be
hidden completely.
Just my two cents..
-Michael
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