The method_missing macro expand to ..., however, it should only expand to a single def

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1

Hello.

Following is the spec output for one of my shards, i consider it can describe my issue quite directly.

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

In spec/hashr_spec.cr:22:11

 22 | value.foo.should eq h
            ^--
Error: wrong method_missing expansion

The method_missing macro expanded to:

    1 |     def foo
    2 |       value = @obj["foo"]
    3 | 
    4 |       Hashr.new(value)
    5 |     end
    6 | 
    7 |     def foo=(value)
    8 |       @obj["foo"] = value
    9 |     end
   10 |   

However, it should only expand to a single def

so, method_missing can only create a single method, right?

If i want to create two method at one-off i method missing macro, how can i do that?

Thanks.

2

Can you create a minimal example of what your code currently does versus what you want it to do? It’s not entirely clear from just looking at this error.

3

Please check this.

4

Ahh, well good question. I tried just using two method_missing calls and got:

The method_missing macro expanded to:

   1 |     def blah=(value)
   2 |       pp value
   3 |     end
   4 |   

However, the generated method won't be found by the original call invocation

So, guess you just can’t? :person_shrugging:.

Related: Remove method_missing macro hook · Issue #4701 · crystal-lang/crystal · GitHub

5

You can create each def by itself casing on the usage.

class A
  @hash = Hash(String, Int32).new

  macro method_missing(call)
    {% if call.name.ends_with? "=" %}
      def {{call.name}}(value)
        @hash[{{call.name.gsub(/=/, "").stringify}}] = value
      end
    {% else %}
      def {{call.name}}
        @hash[{{call.name.stringify}}]
      end
    {% end %}
  end
end

a = A.new
a.value = 5
p a.value # => 5

EDIT: quick hack, please add the corresponding checks to make sure it won’t blow out in your face!

2 Likes
6

Not work for me when test on 1.9.2, raise following error when a.phone = “xxx”, :grinning:

There was a problem expanding macro 'method_missing'

Called macro defined in src/hashr.cr:19:3

 19 | macro method_missing(key)

Which expanded to:

 > 1 |     
 > 2 |       def phone = _arg0(value)
 > 3 |         @obj["phone"] = value
 > 4 |       end
 > 5 |     
 > 6 |   
Error: unexpected token: "="
7

Well, given the error I would assume your code is not entirely faithful to what I posted, but I can’t help much without further context :slight_smile: You can print/debug the macro code with something like {% p! call.name %} to know what you’re getting.

8

Oops, my fault, i am not try your code directly, instead of, i follow your’s ideas, change code directly on my shard, and run spec, it failed because i misunderstood the call.name with call.id, some part of my code use call.id instead, which cause issue.

It works now like a charm! i release v0.3 hashr just now, it can also be used as a models object now.

thank you very much!

1 Like