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https://github.com/kddnewton/rails-pattern_matching

Pattern matching for Rails applications
https://github.com/kddnewton/rails-pattern_matching

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Pattern matching for Rails applications

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README 

        
# rails-pattern_matching



[![Build Status](https://github.com/kddnewton/rails-pattern_matching/workflows/Main/badge.svg)](https://github.com/kddnewton/rails-pattern_matching/actions)

[![Gem](https://img.shields.io/gem/v/rails-pattern_matching.svg)](https://rubygems.org/gems/rails-pattern_matching)



This gem provides the pattern matching interface for `ActionController::Parameters`, `ActiveModel::AttributeMethods`, `ActiveRecord::Base`, and `ActiveRecord::Relation`.



That means it allows you to write code using pattern matching within your Rails applications like the following examples:



```ruby

# app/models/post.rb

class Post < ApplicationRecord

  has_many :comments

end



# app/models/comment.rb

class Comment < ApplicationRecord

  belongs_to :post

end



# app/controllers/posts_controller.rb

class PostsController < ApplicationController

  def update

    @post = Post.find(params[:id])



    case params.require(:post).permit(:title, :body, :published)

    in { published: true } if !can_publish?(@post, current_user)

      # Here we are matching against a single parameter with a guard clause.

      # This allows us to express both the value check and the authorization

      # check in a single line.

      render :edit, alert: "You are not authorized to publish this post"

    in permitted if @post.update(permitted)

      # Here we are grabbing the entire set of parameters and then using it to

      # update the post. If the update succeeds, this branch will be taken.

      redirect_to @post, notice: "Post was successfully updated"

    else

      # Here we are providing a default in case none of the above match. This

      # will be taken if the update fails, and presumably the view will render

      # appropriate error messages.

      render :edit

    end

  end

end



# app/helpers/posts_helper.rb

module PostsHelper

  def comment_header_for(post)

    case post

    in { comments: [] }

      # Here we're matching against an attribute on the post that is an

      # association. It will go through the association and then match against

      # the records in the relation.

      "No comments yet"

    in { user_id:, comments: [*, { user_id: ^user_id }, *] }

      # Here we're searching for a comment that has the same user_id as the

      # post. We can do this with the "find" pattern. This syntax is all baked

      # into Ruby, so we don't have to do anything other than define the

      # requisite deconstruct methods that are used by the pattern matching.

      "Author replied"

    in { comments: [{ user: { name: } }] }

      # Here we're extracting the first comment out of the comments association

      # and using its associated user's name in the header.

      "One comment from #{name}"

    in { comments: }

      # Here we provide a default in case none of the above match. Since we have

      # already matched against an empty array of comments and a single element

      # array, we know that there are at least two comments. We can get the

      # length of the comments association by capturing the comments association

      # in the pattern itself and then using it.

      "#{comments.length} comments"

    end

  end

end

```



## Installation



Add this line to your application's Gemfile:



```ruby

gem "rails-pattern_matching"

```



And then execute:



    $ bundle



Or install it yourself as:



    $ gem install rails-pattern_matching



## Contributing



Bug reports and pull requests are welcome on GitHub at https://github.com/kddnewton/rails-pattern_matching.



## License



The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).