Splat for concatenation. Clean up custom rails helpers with concat and capture

Splat for concatenation

In Ruby, the splat operator (*) is used for any-length argument lists:

def make_into_array(*args)
  args
end

make_into_array(1, 2)  # => [ 1, 2 ]

It can be used as a list flattener, though.

You can use it for nicer array concatenation, like so:

foos = [ 1, 2 ]

[ *foos, "more" ]
# => [ 1, 2, "more" ]

[ "more", *foos, *foos, "even more" ]
# => [ "more", 1, 2, 1, 2, "even more" ]

It can also be used for argument concatenation:

COMMON_ACCESSIBLE = [ :name, :email ]
attr_accessible *COMMON_ACCESSIBLE, as: :user

attr_accessible *COMMON_ACCESSIBLE, :is_admin, as: :admin

Basically, splat lets array or argument list can be put anywhere inside another array or argument list in a "flat" way - making the element part of that list itself.

Clean up custom rails helpers with concat and capture

A normal way to concatenate built-in rails helpers, such as content_tag or link_to is using + or <<:

module MyHelper
  def widget
    content_tag(:p, class: "widget") do
      link_to("Hello", hello_path) << " " << link_to("Bye", goodbye_path)
    end
  end
end

Rails provide a handy function concat to concatenate those helpers:

module MyHelper
  def widget
    content_tag(:p, class: "widget") do
      concat link_to("Hello", hello_path)
      concat " "
      concat link_to("Bye", goodbye_path)
    end
  end
end

The concat method is used to output text in a non-output block (i.e <% %>), similar to the output block (<%= %>):

<%
concat "hello"
# is the equivalent of <%= "hello" %>
%>

When using concat outside a block helper (e.g link_to or content_tag), because the behavior of concat, it can be output directly in a non-output block. Thus, when using it with output block, it will render twice. Example with widget method from MyHelper module from the previous example:

<% widget %> #This will render

<%= widget %> #This will render tiwce

This is because, inside a block helper of an element, concat appends only to that element's content. But when using it outside, concat will return and append directly to the content.

In order to use concat without output directly to the output buffer, simply wrap it with a helper block, which does output only concatenated thing inside it. And that what capture helper does:

module MyHelper
  def widget
    capture do
      concat link_to("Hello", hello_path)
      concat " "
      concat link_to("Bye", goodbye_path)
    end
  end
end

This technique is very crucial when rendering a complex helper with a lot of logic influencing the choice of elements or attributes.

Sources: thepugautomatic, api rubyonrails