wip/absinthe: add WIP

wip/absinthe.md

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+---
+title: Absinthe
+category: Hidden
+layout: 2017/sheet
+intro: |
+  [Absinthe](http://absinthe-graphql.org/) allows you to write GraphQL servers in Elixir.
+---
+
+## Introduction
+
+### Concepts
+
+- `Schema` - The root. Defines what queries you can do, and what types they return.
+- `Resolver` - Functions that return data.
+- `Type` - A type definition describing the shape of the data you'll return.
+
+### Plug
+
+#### web/router.ex
+
+```elixir
+defmodule Blog.Web.Router do
+  use Phoenix.Router
+
+  forward "/", Absinthe.Plug,
+    schema: Blog.Schema
+end
+```
+{: data-line="4,5"}
+
+Absinthe is a Plug, and you pass it one **Schema**.
+
+See: [Our first query](http://absinthe-graphql.org/tutorial/our-first-query/)
+
+## Main concepts
+{: .-three-column}
+
+### Schema
+
+#### web/schema.ex
+
+```elixir
+defmodule Blog.Schema do
+  use Absinthe.Schema
+  import_types Blog.Schema.Types
+
+  query do
+    @desc "Get a list of blog posts"
+    field :posts, list_of(:post) do
+      resolve &Blog.PostResolver.all/2
+    end
+  end
+end
+```
+{: data-line="5,6,7,8,9,10"}
+
+This schema will account for `{ posts { ··· } }`. It returns a **Type** of `:post`, and delegates to a **Resolver**.
+
+### Resolver
+
+#### web/resolvers/post_resolver.ex
+
+```elixir
+defmodule Blog.PostResolver do
+  def all(_args, _info) do
+    {:ok, Blog.Repo.all(Blog.Post)}
+  end
+end
+```
+{: data-line="3"}
+
+This is the function that the schema delegated the `posts` query to.
+
+### Type
+
+#### web/schema/types.ex
+
+```elixir
+defmodule Blog.Schema.Types do
+  use Absinthe.Schema.Notation
+
+  @desc "A blog post"
+  object :post do
+    field :id, :id
+    field :title, :string
+    field :body, :string
+  end
+end
+```
+{: data-line="4,5,6,7,8,9"}
+
+This defines a type `:post`, which is used by the resolver.
+
+## Schema
+
+### Query arguments
+
+```
+{ user(id: "1") { ··· } }
+```
+{: .-setup}
+
+#### web/schema.ex
+
+```elixir
+query do
+  field :user, type: :user do
+    arg :id, non_null(:id)
+    resolve &Blog.UserResolver.find/2
+  end
+end
+```
+{: data-line="3"}
+
+#### Resolver
+
+```
+def find(%{id: id} = args, _info) do
+  ···
+end
+```
+{: data-line="1"}
+
+See: [Query arguments](http://absinthe-graphql.org/tutorial/query-arguments/)
+
+### Mutations
+
+```
+{
+  mutation CreatePost {
+    post(title: "Hello") { id }
+  }
+}
+```
+{: .-setup}
+
+#### web/schema.ex
+
+```elixir
+mutation do
+  @desc "Create a post"
+  field :post, type: :post do
+    arg :title, non_null(:string)
+    resolve &Blog.PostResolver.create/2
+  end
+end
+```
+
+See: [Mutations](http://absinthe-graphql.org/tutorial/mutations/)