Re: How do I extend an enum?
On Saturday, 19 March 2016 at 17:40:27 UTC, Lass Safin wrote:
Why:
enum Base {
A,
B,
}
enum Derived : Base {
C, // Gives error, says it can't implicitly convert
expression to Base.
D = 1, // Same error
E = cast(Base)294, // Finally works. Can only be
cast(Derived) instead.
}
void func(Derived d) {}
func(Derived.E); // works.
func(Derived.A); // Gives error, says it can't call function
with Base.A.
func(cast(Derived)Derived.A); // Works.
So, what's the proper way of extending an enum?
Look at the grammar:
https://dlang.org/spec/enum.html
There's no inheritance system for the enums. after the ":" can
only be specified the type of the members, aka the "EnumBaseType".
"
EnumDeclaration:
enum Identifier EnumBody
enum Identifier : EnumBaseType EnumBody
"
So when you write
enum Derived : Base {}
It just means that "Derived" members must be of type "Base"
So actually the only thing you can do is to reduce the members
count:
enum Base {A,B}
enum Derived : Base {C = Base.A}
Re: How do I extend an enum?
On Saturday, 19 March 2016 at 17:40:27 UTC, Lass Safin wrote:
Why:
enum Base {
A,
B,
}
enum Derived : Base {
C, // Gives error, says it can't implicitly convert
expression to Base.
D = 1, // Same error
E = cast(Base)294, // Finally works. Can only be
cast(Derived) instead.
}
void func(Derived d) {}
func(Derived.E); // works.
func(Derived.A); // Gives error, says it can't call function
with Base.A.
func(cast(Derived)Derived.A); // Works.
So, what's the proper way of extending an enum?
There is no way to extend an enum. When you think about it, it's
actually the opposite of what you'd generally want. Given two
classes:
class A {}
class B : A {}
Every instance of B is a valid A. That is, given a variable of
type A, you could assign any B to it.
Now consider enums:
enum A { x, y, z }
enum B : A {}
Which values could you put in B? Only those that would be valid
for A. That is, only x, y and z. Imagine that we could:
enum B : A { w }
A foo = B.w;
foo now holds a value that is not valid for its type. Hence, you
simply cannot.
Are there cases where you want to define a new enum that contains
all the items in a 'base' enum in addition to some new items?
Absolutely, and D lacks a good way to do that. But subtyping
would in any case not be the correct way to do it.
Are there cases where you want to extend an enum by making a
subtype with more items? I would argue that's a strong code smell
in D, but I can see why you'd want to.
--
Simen
Re: How do I extend an enum?
On Saturday, 19 March 2016 at 17:41:29 UTC, Lass Safin wrote:
On Saturday, 19 March 2016 at 17:40:27 UTC, Lass Safin wrote:
Why:
enum Base {
A,
B,
}
enum Derived : Base {
C, // Gives error, says it can't implicitly convert
expression to Base.
D = 1, // Same error
E = cast(Base)294, // Finally works. Can only be
cast(Derived) instead.
}
void func(Derived d) {}
func(Derived.E); // works.
func(Derived.A); // Gives error, says it can't call function
with Base.A.
func(cast(Derived)Derived.A); // Works.
So, what's the proper way of extending an enum?
Meant "Can also be cast(Derived) instead."
"enum B : A" doesn't mean "B extends A", but rather "enum B
containing members of type A". Not specifying a type makes it
implicitly convertible to int, I think.
If you're looking to extend a named enum, I think you have to
create a new one. It will become a new type too, though that
might not matter.
enum Foo { first=123, second=456, third=789 } // int type inferred
enum Bar : int { // the ": int" here is important
first = Foo.first, // implicit cast to int
second = Foo.second,
third = Foo.third,
fourth = 42,
fifth = 0
}
If you don't define Bar as having members of type int, it will
guess that you want Foo (because we're assigning members with
values of Foo's). They would be limited to the range of values
Foo offers, and Bar.fourth = 42 is irreconcilabe with that.
Re: How do I extend an enum?
On Saturday, 19 March 2016 at 17:40:27 UTC, Lass Safin wrote:
Why:
enum Base {
A,
B,
}
enum Derived : Base {
C, // Gives error, says it can't implicitly convert
expression to Base.
D = 1, // Same error
E = cast(Base)294, // Finally works. Can only be
cast(Derived) instead.
}
void func(Derived d) {}
func(Derived.E); // works.
func(Derived.A); // Gives error, says it can't call function
with Base.A.
func(cast(Derived)Derived.A); // Works.
So, what's the proper way of extending an enum?
Meant "Can also be cast(Derived) instead."
How do I extend an enum?
Why:
enum Base {
A,
B,
}
enum Derived : Base {
C, // Gives error, says it can't implicitly convert
expression to Base.
D = 1, // Same error
E = cast(Base)294, // Finally works. Can only be
cast(Derived) instead.
}
void func(Derived d) {}
func(Derived.E); // works.
func(Derived.A); // Gives error, says it can't call function with
Base.A.
func(cast(Derived)Derived.A); // Works.
So, what's the proper way of extending an enum?
