No implicit conversion of Enumerator into Array

I have:

qs = ["all=true", "limit=-1"]
value = ["agent", "service", "token"]

This code:

qs.concat value.map do |val|
  "#{field}=#{val}"
end

ends up with the following error:

`concat': no implicit conversion of Enumerator into Array (TypeError)`

whereas this code:

values = value.map do |val|
  "field=#{val}"
end
qs.concat values

does not.

What is the difference between them?

Solution

Your issue is caused by the different precedences when providing blocks to chained method calls. In your case you use the following code:

qs.concat value.map do |val|
  "#{field}=#{val}"
end

Ruby assumes here that you mean the following:

qs.concat(value.map) do |val|
  "#{field}=#{val}"
end

That is, Ruby passes the block to the first method (i.e qs.concat) which ignores the block. Since Array#map returns an Enumerator if you don't pass a block, you get your error you saw.

To solve this, you can use the braces form of passing the block, i.e.:

qs.concat value.map { |val|
  "#{field}=#{val}"
}

In this form, the block is always passed to the "last" method, u.e. your map.

In any case, if there is any doubt about operator precedence, it is always a good idea to use explicit parenthesis or intermediate variables to make it clear both to human readers as well as the Ruby interpreter how your code is supposed to work.