add basic json accessors

examples/json/Json.sig

@@ -20,6 +20,13 @@ sig
   val fromString     : string -> json list * substring
   val fromFile       : string -> json list * substring
 
+  val getObject      : json -> string -> json option
+  val getBool        : json -> bool option
+  val getString      : json -> string option
+  val getInt         : json -> int option
+  val getFloat       : json -> real option
+
+
   val pp_json : json PP.pprinter
 
 end

examples/json/Json.sml

@@ -277,4 +277,53 @@ val pp_json =
 
 val _ = PolyML.addPrettyPrinter (fn d => fn _ => fn json => pp_json json);
 
+
+(* accessors  *)
+
+fun getObject json k =
+  case json of
+      AList al => Option.map #2 (List.find (fn (key, _) => key = k) al)
+    | _ => NONE
+
+fun getBool json =
+  case json of
+      Boolean b => SOME b
+   | _ => NONE
+
+fun getString json =
+  case json of
+      String str => SOME str
+   | _ => NONE
+
+fun getInt json =
+ case json of
+     Number (Int i) => SOME i
+  | _ => NONE
+
+fun getFloat json =
+ case json of
+     Number (Float f) => SOME f
+  | _ => NONE
+
+fun foldlArray json (f: json -> 'a option) =
+    let
+        fun seq [] = SOME []
+          | seq (NONE :: _) = NONE
+          | seq (SOME x :: xs) =
+            case seq xs of
+                NONE => NONE
+              | SOME ys => SOME (x :: ys)
+    in
+        case json of
+            List l => seq (List.map f l)
+          | _ => NONE
+    end
+
+(* val json = hd (#1 (parse ([],Substring.full "{\"foo\" : [1, 23, 4], \"bar\" : 2}"))) *)
+(* val foo = getObject json "foo" *)
+(* val foo_ints = foldlArray (valOf foo) getInt *)
+(* val json_str = getString json *)
+(* val bar = getObject json "bar" *)
+
+
 end (* Json *)