agreed, however there were the most convenient example I could think of.
Crowdhailer
Creator of Raxx
X4lldux
I’ve created a library for doing exactly this, called disc_union. For your example it would look like this:
defmodule Result do
use DiscUnion
defunion :ok in any() | :error in atom
end
defmodule Example do
use Result
def foo() do
Result.case my_func() do # expects a %Result{} struct
:ok in value -> #continues
:error in reason -> #report the error
end
end
end
NOTE: you can also “import” a regular error tuple into the Result struct using Result.from!/1 function and it will check if what your are importing was defined as one of the variants.
Downside is that for each discriminated union there has to be a specially created case macro, in order for it to “know” what valid variants are in this union and warn you (at compile time) when you miss one or implement it with a wrong shape. I find it very useful and use it in production even though I still consider this a research project.
6
Crowdhailer
Creator of Raxx
X4lldux
Like in the tennis kata example where a PlayerPoints.case macro “understands” what it’s possible variants are - which are defined at line 13 - and programmer needs to explicitly cover all of them.
It’s basically a discriminated union concept, and even though a Elixir is a dynamic language, this library gives some compile-time warnings.
4
Tweega
Has this conversation moved on in the last couple of years? Discriminated Unions provide a good way to model messages being passed around a system. I find them very helpful in Elm and F#.
0
marciol
OvermindDL1
No. Active patterns are an entirely different concept, they are the ‘Extractor’ pattern.
A discriminated union, or a Variant, or whatever your language calls it is just a simple Sum type. You can dispatch/match/whatever based on the tag/type/head of it.
Active Patterns instead attempt to extract one datatype from another datatype, if it succeeds then you operate on the success, if it fails then it fails, and they are convenient because they can be used in a match statement to allow easy fallovers.
1
marciol
Great for the explanation @OvermindDL1. I don’t know why I found it similar at first glance, maybe I read it too fast and assumed that it was the same.
1
Adzz
@marciol
Conceptually though the active patterns allow pattern matching on abstract data - which is what the original post was talking about.
You can see that was their aim in the original paper:
So while active patterns aren’t discriminated unions they are relevant to the original post as if we implemented them in elixir we’d have what was being talked about.
3
marciol
Old topic but right now I have several patterns in a case and would be nice to have extractors, maybe it should be possible using pattern functions which returns true if they match in a cond expression, but without the extract part.
I miss semantic pattern match at this point. Let’s see how to solve it in a elegant way.
0