add json.ml; add test.ml; timestamp jsont

test/dune

@@ -0,0 +1,4 @@
+(test
+ (name test)
+ (modules test)
+ (libraries types json))

test/test.ml

@@ -0,0 +1,20 @@
+let () =
+  let open Json in
+  let check jsont s =
+    let encode v = encode jsont v |> Result.get_ok in
+    let decode v = decode jsont v |> Result.get_ok in
+    let ts = decode s in
+    let s' = encode ts in
+    let ts' = decode s' in
+    let s'' = encode ts' in
+    assert (String.equal s' s'')
+  in
+  let check_bad jsont s =
+    let decode = decode jsont in
+    assert (Result.is_error (decode s))
+  in
+  check Timestamp.jsont {|{"t_s": 123456780}|};
+  check Timestamp.jsont {|{"t_s": "never"}|};
+  check_bad Timestamp.jsont {|{"t_s": "123456780"}|};
+  check_bad Timestamp.jsont {|{"t_s": "agagou"}|};
+  ()