lang: core, funcs: Port some functions to CallableFunc API

Some modern features of our function engine and language might require
this new API, so port what we can and figure out the rest later.

lang/funcs/structs/if.go

@@ -115,12 +115,14 @@ func (obj *IfFunc) Stream(ctx context.Context) error {
 			}
 			obj.last = input // store for next
 
-			var result types.Value
+			args, err := interfaces.StructToCallableArgs(input) // []types.Value, error)
+			if err != nil {
+				return err
+			}
 
-			if input.Struct()["c"].Bool() {
-				result = input.Struct()["a"] // true branch
-			} else {
-				result = input.Struct()["b"] // false branch
+			result, err := obj.Call(ctx, args) // get the value...
+			if err != nil {
+				return err
 			}
 
 			// skip sending an update...
@@ -141,3 +143,13 @@ func (obj *IfFunc) Stream(ctx context.Context) error {
 		}
 	}
 }
+
+// Call this function with the input args and return the value if it is possible
+// to do so at this time.
+// XXX: Is is correct to implement this here for this particular function?
+func (obj *IfFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+	if c := args[0].Bool(); c {
+		return args[1], nil // true branch
+	}
+	return args[2], nil
+}