Merge pull request #157 from cuihtlauac/add_path

Add `path` function

CHANGES.md

@@ -5,6 +5,9 @@
 - Add Yojson.Raw.Util module to provide combinators for extracting fields
   from Yojson.Raw values. (@tmcgilchrist, #163)
 
+- Add `Util.path` function to recurse into an object through a list of keys.
+  (@cuihtlauac, @Leonidas-from-XIV, #157)
+
 ### Removed
 
 ### Changed

lib/util.ml

@@ -28,6 +28,17 @@ let member name = function
   | `Assoc obj -> assoc name obj
   | js -> typerr ("Can't get member '" ^ name ^ "' of non-object type ") js
 
+let rec path l obj =
+  match l with
+  | [] -> Some obj
+  | key :: l -> (
+      match obj with
+      | `Assoc assoc -> (
+          match List.assoc key assoc with
+          | obj -> path l obj
+          | exception Not_found -> None)
+      | _ -> None)
+
 let index i = function
   | `List l as js ->
       let len = List.length l in

lib/util.mli

@@ -85,6 +85,10 @@ val member : string -> t -> t
       object [obj], or [`Null] if [k] is not present in [obj].
       @raise Type_error if [obj] is not a JSON object. *)
 
+val path : string list -> t -> t option
+(* [path l obj] recurses the JSON object [obj] for each key in the path
+   [l] until the path is empty or there is no such key in the chain. *)
+
 val index : int -> t -> t
 (** [index i arr] returns the value at index [i] in the JSON array [arr].
       Negative indices count from the end of the list (so -1 is the last

test/fixtures.ml

@@ -8,12 +8,14 @@ let json_value =
       ("float", `Float 0.);
       ("string", `String "string");
       ("list", `List [ `Int 0; `Int 1; `Int 2 ]);
+      ("assoc", `Assoc [ ("value", `Int 42) ]);
     ]
 
 let json_string =
   "{" ^ {|"null":null,|} ^ {|"bool":true,|} ^ {|"int":0,|}
   ^ {|"intlit":10000000000000000000,|} ^ {|"float":0.0,|}
-  ^ {|"string":"string",|} ^ {|"list":[0,1,2]|} ^ "}"
+  ^ {|"string":"string",|} ^ {|"list":[0,1,2],|} ^ {|"assoc":{"value":42}|}
+  ^ "}"
 
 let unquoted_json = {|{foo: null}|}
 let unquoted_value = `Assoc [ ("foo", `Null) ]

test/test.ml

@@ -4,4 +4,5 @@ let () =
       ("equality", Test_monomorphic.equality);
       ("read", Test_read.single_json);
       ("write", Test_write.single_json);
+      ("util", Test_util.tests);
     ]

test/test_util.ml

@@ -0,0 +1,22 @@
+let path_empty () =
+  Alcotest.(check (option Testable.yojson))
+    __LOC__ (Some Fixtures.json_value)
+    (Yojson.Safe.Util.path [] Fixtures.json_value)
+
+let path_missing () =
+  Alcotest.(check (option Testable.yojson))
+    __LOC__ None
+    (Yojson.Safe.Util.path [ "does not exist" ] Fixtures.json_value)
+
+let path_traverse () =
+  Alcotest.(check (option Testable.yojson))
+    __LOC__
+    (Some (`Int 42))
+    (Yojson.Safe.Util.path [ "assoc"; "value" ] Fixtures.json_value)
+
+let tests =
+  [
+    ("empty path", `Quick, path_empty);
+    ("non-existing path", `Quick, path_missing);
+    ("traversal", `Quick, path_traverse);
+  ]

test/test_util.mli

@@ -0,0 +1 @@
+val tests : unit Alcotest.test_case list