Union type list

[John Orford]

Why isn't something like

List (A | B)

possible?

[Zinggi]

It is, you just have to create a type for your union type.

e.g.

type Union a b = A a | B b

then you can use e.g.

list : List (Union Int String)

list = [A 1, B "bla"]

Is that what you wanted?

[John Orford]

Yup.  Thanks!

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[Zinggi]

No problem.

You probably also want to read the documentation on types, specifically the section about union types.

[Joey Eremondi]

The key to remember here is that Elm doesn't have what are traditionally called union types, we have "Tagged union types". There's never a time when you can have As and Bs in a list, and not know if you have an A or a B. Every value in the list will be tagged with some constructor from what ever "type" declaration you use for your union.

[Mark Hamburg]

Elm also doesn't have generalized union types even with tags. For example, one can't have a universal NoOp message type that can be included in other message type definitions and recognized and generated from generic code.

Mark

[Leroy Campbell]

I assume if it were possible to have lists with mixed types, you'd lose some algebraic guarantees. How would you use such a list? There's probably another way to accomplish the same thing while retaining type safety.

[Mark Hamburg]

Whenever you accessed the list, you would get an element of type A | B. To use if further, you would need to type case it, pass it to something that wanted A| B or A | B | C etc, or put it through some sort of type cast operator that could give you a Maybe A or a Maybe B. Elm, of course, doesn't do any of those things, so this note is just to say that one could design a language that did. The "win" in this case would be to make it easier to create composite types — e.g., a value, an error, or nothing which isn't really quite the same in usage as either a Maybe (Result e t) or a Result e (Maybe t).

Mark

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