Can I make a named Union of Types

General Usage
1

I am learning how to make one library compatible with another. They both have different type hierarchies for representing a file path.

One uses AbstractString, the other uses SystemPath for their abstract representation of paths. Their use of low-level interfaces like open() dispatch in a compatible way.

I don’t want to change the parameters of each high-level method to all be Union{SystemPath,AbstractString}

I’d like to give that Union Type a name and use that to type the appropriate parameters.

Can I do that? If not, how should I express this?

2

const myname = Union{...

will do

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3

And… it’s sort of distressing, because each library sort of shouldn’t have to know about each other.

I could almost make a mixin library which just contained functions to implement compatible dispatch.

Then I’d be making a patchwork. Which doesn’t seem great either.

4

I might not want the type restriction, in order to allow other pathlike types that haven’t been written yet.

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5

Just making sure I understand…
So rather than adding another type in the parameter - as the Union, remove the type specification altogether.
That seems like a good idea, Then the peer library just provides methods to make itself work in the context - just like open(::PosixPath) does already?

That leaves both libraries not needing to reference each-other.

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6

Right. Ideally julia would have a more flexible interface system so it could support something like a “pathlike” trait or specification, but that’s not currently available.

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7

if you don’t need to do specialized things, you don’t need to clamp down types

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8

we used to call that a protocol in Smalltalk. A set of method names that instances had to understand to support a functionality. Like the ReadStream protocol e.g. #(#next #next: #atEnd), which could be implemented by classes which were unrelated in the class hierarchy. Then any object expecting ReadStream could use them. No compiler support though :slight_smile:

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