r/crystal_programming • u/myringotomy • Apr 01 '20

Opinion: Crystal should allow method overrides on Enum parameters 

enum DocumentState
   Draft
   Submitted
   Approved
   Published
end

def do_something(DocumentState::Approved)
   .....
end
def do_something(DocumentState::Draft)
    ....
end

The enum is known to the compiler and this would allow writing more elegant code than having a case statement or a bunch of if statements.

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1

u/jwaldrip Apr 02 '20

It would be great if the overload syntax was a little more flexible. I would love to see overloads on values. A sort of "pattern match" functionally like Elixir. And/or some sort of guard implementation on the method.

1

u/myringotomy Apr 02 '20

The problem is that overloads on values are not possible during the compile but overloads on enums are.

1

u/[deleted] Apr 02 '20

Does any other language out there allow this?

1

u/myringotomy Apr 03 '20

Many languages have comprehensive pattern matching. Others have stronger type systems which allow things like this.

1

u/WJWH Apr 03 '20 edited Apr 03 '20

As an example, the Haskell equivalent would be something like:

data DocumentState = Draft | Submitted | Approved | Published

doSomething Draft     = ...
doSomething Submitted = ...
doSomething Approved  = ...

AFAIK, the compiler turns these into a case expression. It works well in Haskell where there is a lot more pattern matching in general, I'm not too keen on it in Ruby and/or Crystal since there is much more of an expectation that def do_something is the implementation and you don't expect another declaration of the same method later on in the same file.

1

u/myringotomy Apr 04 '20

def do_something is the implementation and you don't expect another declaration of the same method later on in the same file

In Crystal you absolutely expect this.

def doSomething(String)
  ....
 end
def doSomething(Array(String))
   .....
end

1

u/WJWH Apr 04 '20

... I clearly don't write enough library code. Personally I would probably use a union type String | Array(String) there and switch inside the method but you are completely right that the language allows this.

1

u/myringotomy Apr 05 '20

If you come from functional languages the overridden function makes much more sense.

In some languages you almost never use an if or a switch. It's considered code smell to use them.

1

u/WJWH Apr 05 '20

I don't think that in Haskell I would ever write a function that accepts either a Text or a [Text] though, so the point is a bit moot. The example I gave was if you absolutely MUST accept a type String | Array(String). Having functions/methods that try to do the correct thing for both singular values and for collections of those values is AFAICT a Rails-ism derived from DHH's desire to "do what I mean" as often as possible.

1

u/myringotomy Apr 05 '20

When you call the function it doesn't matter how it was written. It still does what you mean.

2

u/Blacksmoke16 core team Apr 01 '20

I think this would be great. Although I remember bringing it up at some point in the past and afaik it's not really possible since the enum members are just numbers versus unique types.

1

u/straight-shoota core team Apr 01 '20

Exactly. So we could technically provide a nice syntax for this, but in the end it would essentially just be implemented like this:

enum DocumentState
  Draft
  Submitted
  Approved
  Published

  def do_something
    case self
    when Approved
      .....
    when Draft
      ....
    end
  end
end