Add JSON matcher (#20)

lib/machete.ex

@@ -29,7 +29,7 @@ defmodule Machete do
 
   At its heart, Machete provides the following two things:
 
-  * A new `~>` operator (the 'squiggle arrow') that does flexible matching of 
+  * A new `~>` operator (the 'squiggle arrow') that does flexible matching of
     its left operator with its right operator
   * A set of parametric matchers such as `string()` or `integer()` which can match
     against general types
@@ -84,6 +84,7 @@ defmodule Machete do
   * [`float()`](`Machete.FloatMatcher.float/1`) matches float values
   * [`integer()`](`Machete.IntegerMatcher.integer/1`) matches integer values
   * [`iso8601_datetime()`](`Machete.ISO8601DateTimeMatcher.iso8601_datetime/1`) matches ISO8601 formatted strings
+  * [`json()`](`Machete.JSONMatcher.json/1`) matches JSON formatted structures
   * [`is_a()`](`Machete.IsAMatcher.is_a/1`) matches against a struct type
   * [`naive_datetime()`](`Machete.NaiveDateTimeMatcher.naive_datetime/1`) matches `NaiveDateTime` instances
   * [`pid()`](`Machete.PIDMatcher.pid/1`) matches process IDs
@@ -174,6 +175,7 @@ defmodule Machete do
       import Machete.IntegerMatcher
       import Machete.IsAMatcher
       import Machete.ISO8601DateTimeMatcher
+      import Machete.JSONMatcher
       import Machete.ListMatcher
       import Machete.MapMatcher
       import Machete.MaybeMatcher

lib/machete/matchers/json_matcher.ex

@@ -0,0 +1,61 @@
+defmodule Machete.JSONMatcher do
+  @moduledoc """
+  Defines a matcher that matches JSON documents
+  """
+
+  import Machete.Mismatch
+  import Machete.Operators
+
+  defstruct matcher: nil
+
+  @typedoc """
+  Describes an instance of this matcher
+  """
+  @opaque t :: %__MODULE__{}
+
+  @doc """
+  Matches against JSON documents whose deserialization matches a provided matcher
+
+  Takes a matcher as its sole (mandatory) argument
+
+  Examples:
+
+      iex> assert "{}" ~> json(map())
+      true
+
+      iex> assert ~s({"a": 1}) ~> json(%{"a" => 1})
+      true
+
+      iex> assert "[]" ~> json(list())
+      true
+
+      iex> assert "[1,2,3]" ~> json([1,2,3])
+      true
+
+      iex> assert ~s("abc") ~> json(string())
+      true
+
+      iex> assert "123" ~> json(integer())
+      true
+
+      iex> assert "true" ~> json(boolean())
+      true
+
+      iex> assert "null" ~> json(nil)
+      true
+  """
+  @spec json(term()) :: t()
+  def json(matcher), do: struct!(__MODULE__, matcher: matcher)
+
+  defimpl Machete.Matchable do
+    def mismatches(%@for{} = a, b) when is_binary(b) do
+      Jason.decode(b)
+      |> case do
+        {:ok, document} -> document ~>> a.matcher
+        _ -> mismatch("#{inspect(b)} is not parseable JSON")
+      end
+    end
+
+    def mismatches(%@for{}, b), do: mismatch("#{inspect(b)} is not a string")
+  end
+end

mix.exs

@@ -27,6 +27,7 @@ defmodule Machete.MixProject do
 
   defp deps do
     [
+      {:jason, "~> 1.4"},
       {:ex_doc, ">= 0.0.0", only: :dev, runtime: false},
       {:credo, "~> 1.6", only: [:dev, :test], runtime: false},
       {:dialyxir, "~> 1.0", only: [:dev, :test], runtime: false}
@@ -58,6 +59,7 @@ defmodule Machete.MixProject do
           Machete.IntegerMatcher,
           Machete.IsAMatcher,
           Machete.ISO8601DateTimeMatcher,
+          Machete.JSONMatcher,
           Machete.ListMatcher,
           Machete.MapMatcher,
           Machete.MaybeMatcher,

test/machete/matchers/json_matcher_test.exs

@@ -0,0 +1,22 @@
+defmodule JSONMatcherTest do
+  use ExUnit.Case, async: true
+  use Machete
+
+  import Machete.Mismatch
+
+  doctest Machete.JSONMatcher
+
+  test "produces a useful mismatch for non strings" do
+    assert 1 ~>> json(term()) ~> mismatch("1 is not a string")
+  end
+
+  test "produces a useful mismatch for non-parseable strings" do
+    assert "%" ~>> json(term()) ~> mismatch("\"%\" is not parseable JSON")
+  end
+
+  test "produces a useful mismatch for content mismatches" do
+    assert "[1]"
+           ~>> json([])
+           ~> mismatch("List is 1 elements in length, expected 0")
+  end
+end