Get super and ancestor class name

1

New to Crystal with years Ruby experience.

I could not find a way to get the super or ancestors of a class in Crystal. In ruby, we have superclass and ancestor methods. What are the equivalent in Crystal lang?

Thanks.

2

They also exist in crystal but are macro (compile time) level methods, i.e. you can’t get that information at runtime. See TypeNode.ancestors and TypeNode.superclass.

EDIT: Add an example

abstract class One; end
abstract class Two < One; end
class Three < Two; end

{{pp Three.superclass}} # => Two
{{pp Three.ancestors}} # => [Two, One, Reference, Object]
3

Thanks, Blacksmoke16. Very helpful.

But I am still not sure how to handle my use case. Essentially, I have an object, for which I want to return its ancestors. Something like:

a = MyCalss.new

ancestors = get_ancestors(a)

Based on the documentation, I tried the following, but I seemed to not able to get across the boundary of Macro and regular code:

macro add_describe_methods

def describe

"Class is: " + {{ @type.stringify }}

end

def self.describe

"Class is: " + {{ @type.stringify }}

end

def self.get_ancestors

"Ancestors are: " + {{@type.superclass}}

end

end

class Foo

add_describe_methods

end

a = Foo.new

pp a.describe # => this works

pp Foo.describe # => this works too

pp Foo.get_ancestors

The compiler complains:

Showing last frame. Use --error-trace for full trace.

There was a problem expanding macro ‘add_describe_methods’

Code in tmp.cr:16:3

16 | add_describe_methods

^

Called macro defined in tmp.cr:1:1

1 | macro add_describe_methods

Which expanded to:

8 |

9 | def self.get_ancestors

10 | "Ancestors are: " + Reference

^

Error: no overload matches ‘String#+’ with type Reference.class

Overloads are:

  • String#+(other : self)
  • String#+(char : Char)

How should I fix it? Thanks.

4

Use three backticks for code blocks like that next time, makes it a bit easier to read.

The error you’re seeing is just as it says, there is no method on String that allows you to add a Reference.class. Your options would be {{@type.superclass.stringify}} so the macro result is turned into a string. Or Just do like "Ancestors are: {{@type.superclass}}" to add the value directly into the string.

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