Add JsonExtractor utility class

The extractor wraps standard boilerplate for working with JsonType
structures into a utility class that makes for easier reading
code when pulling values of known types from a parsed Json structure.

json/json_extractor.pony

@@ -0,0 +1,191 @@
+use "collections"
+
+class val JsonExtractor
+  """
+  Utility class for working with JSON structures.
+
+  Given the following JSON:
+  ```json
+{
+  "environments": [
+    {
+      "name": "corral-env",
+      "user": "sean",
+      "image": "an-image:latest",
+      "shell": "fish",
+      "workdir": "/workspace",
+      "workspace": "/home/sean/ponylang/corral",
+      "mounts": [
+        {"source":"/var/run/docker.sock","target":"/var/run/docker.sock", "type": "bind"}
+      ]
+    }
+  ]
+}
+  ```
+
+  We can use the following code to extract our values:
+
+  ```pony
+  primitive Parser
+    fun apply(json: JsonType val): Array[Environment] val ? =>
+      recover val
+        let envs = JsonExtractor(json)("environments")?.as_array()?
+        let result: Array[Environment] = Array[Environment]
+        for e in envs.values() do
+          let obj = JsonExtractor(e).as_object()?
+          let name = JsonExtractor(obj("name")?).as_string()?
+          let user = JsonExtractor(obj("user")?).as_string()?
+          let image = JsonExtractor(obj("image")?).as_string()?
+          let shell = JsonExtractor(obj("shell")?).as_string()?
+          let workdir = JsonExtractor(obj("workdir")?).as_string()?
+          let workspace = JsonExtractor(obj("workspace")?).as_string()?
+          let mounts = recover trn Array[Mount] end
+          for i in JsonExtractor(obj("mounts")?).as_array()?.values() do
+            let m = MountParser(i)?
+            mounts.push(m)
+          end
+
+          let environment = Environment(name, user, image, shell, workdir, workspace, consume mounts)
+          result.push(environment)
+        end
+        result
+      end
+
+  primitive MountParser
+    fun apply(json: JsonType val): Mount ? =>
+      let obj = JsonExtractor(json).as_object()?
+      let source = JsonExtractor(obj("source")?).as_string()?
+      let target = JsonExtractor(obj("target")?).as_string()?
+      let mtype = JsonExtractor(obj("type")?).as_string()?
+
+      Mount(source, target, mtype)
+  ```
+
+  The JsonExtractor creates a lot of intermediate objects, but it makes the code
+  easier to read and understand. We suggest not using it in critical paths where
+  performance is a concern.
+  """
+  let _json: JsonType val
+
+  new val create(json: JsonType val) =>
+    """
+    Create a new JsonExtractor from a JSON structure.
+    """
+    _json = json
+
+  fun val apply(idx_or_key: (String | USize)): JsonExtractor val ? =>
+    """
+    Extract an array or object by index or key and return a new JsonExtractor.
+    """
+    match (_json, idx_or_key)
+    | (let a: JsonArray val, let idx: USize) =>
+      JsonExtractor((a.data)(idx)?)
+    | (let o: JsonObject val, let key: String) =>
+      JsonExtractor((o.data)(key)?)
+    else
+      error
+    end
+
+  fun val size(): USize ? =>
+    """
+    Return the size of the JSON structure.
+
+    Results in an error for any structure that isn't a `JsonArray` or `JsonObject`.
+    """
+    match _json
+    | let a: JsonArray val =>
+      a.data.size()
+    | let o: JsonObject val =>
+      o.data.size()
+    else
+      error
+    end
+
+  fun val values(): Iterator[JsonType val] ? =>
+    """
+    Return an iterator over the values of the JSON structure.
+
+    Results in an error for any structure that isn't a `JsonArray`.
+    """
+    match _json
+    | let a: JsonArray val =>
+      a.data.values()
+    else
+      error
+    end
+
+  fun val pairs(): Iterator[(String, JsonType val)] ? =>
+    """
+    Return a pairs iterator over the values of the JSON structure.
+
+    Results in an error for any structure that isn't a `JsonArray`.
+    """
+    match _json
+    | let o: JsonObject val =>
+      o.data.pairs()
+    else
+      error
+    end
+
+  fun val as_array(): Array[JsonType] val ? =>
+    """
+    Extract an Array from the JSON structure.
+    """
+    match _json
+    | let a: JsonArray val =>
+      a.data
+    else
+      error
+    end
+
+  fun val as_object(): Map[String, JsonType] val ? =>
+    """
+    Extract a Map from the JSON structure.
+    """
+    match _json
+    | let o: JsonObject val =>
+      o.data
+    else
+      error
+    end
+
+  fun val as_string(): String ? =>
+    """
+    Extract a String from the JSON structure.
+    """
+    _json as String
+
+  fun val as_none(): None ? =>
+    """
+    Extract a None from the JSON structure.
+    """
+    _json as None
+
+  fun val as_f64(): F64 ? =>
+    """
+    Extract a F64 from the JSON structure.
+    """
+    _json as F64
+
+  fun val as_i64(): I64 ? =>
+    """
+    Extract a I64 from the JSON structure.
+    """
+    _json as I64
+
+  fun val as_bool(): Bool ? =>
+    """
+    Extract a Bool from the JSON structure.
+    """
+    _json as Bool
+
+  fun val as_string_or_none(): (String | None) ? =>
+    """
+    Extract a String or None from the JSON structure.
+    """
+    match _json
+    | let s: String => s
+    | let n: None => n
+    else
+      error
+    end