There seems to be an idiom whereby multiple parts generates Key->Value forms whereas for single part specifications, only a Value is returned. This however, can lead to inconsistencies as follows:

      assoc = <|"a" -> 7, b -> 8, "c" -> 9|>;
      {Query[{1, 2}][assoc], Query[1][assoc], Query[2][assoc]}
 (* -> {<|"a" -> 7, b -> 8|>, 7, 8} *)

but doesn't this contradict the documentation on multiple queries?

 Query::Docs] {op_1, op_2, ...} apply multiple operators at once to the result, yielding a list

firstly, a list is not "yielded" (returned) and secondly, each application of the operator ordinarily returns a value. A more consistent usage would therefore appear to be?

Query[{1, 2}][assoc]
(* -> {7, 8}  *)

(* not valid existing input/output - suggestive only *)

In addition, the mere presence of a range (e.g. 1 ;; 2) seems to induce individually specified parts to now return Values?

Query[{1, 2, 1 ;; 2}][assoc]
(*->  {7, 8, <|"a" -> 7, b -> 8|>} *)

1 Answer 1


In an earlier comment RunnyKine gave an example involving Key["a"] which if followed though provides an explanation. It also means that there is a subtle change in semantics using Part with an Association. Consider

 Query[{Key["a"], 2}]@assoc
 (* -> Missing["PartInvalid", {Key["a"], 2}] *)

suggesting that a list is not being interpreted as containing multiple queries but rather, it is being considered as a single query. Hence,

  Query[{1, 2}]@assoc
  (* ->  <|"a" -> 7, b -> 8|> *)

is not being parsed as a list of two queries (as the following unambiguously is)

Query[{Query@1, 2}]@assoc
(* -> {7, 8} *)

but instead is being parsed as

Query[Query[{1, 2}]]@assoc
(* -> <|"a" -> 7, b -> 8|> *)


assoc[[{1, 2}]]
(* -> <|"a" -> 7, b -> 8|> *)

but this in itself indicates a subtle semantic shift in the interpretation of Part between a List and an Association. The analogous

ls = {7, 8, 9};
ls[[{1, 2}]] 
(* ->  {7, 8} *)

has semantics - "take the list's first part -ls[[1]]- and its second part -ls[[2]]- and wrap them in a list - in other words it is the standard notational implementation of listability used throughout the Wolfram Language.

This interpretation no longer applies with an Association since assoc[[{1}]] (* -> <| a-> 7 |> *) now essentially says "wrapping parts in a list is not implementing listability for an Association but instead is simply a signal to include keys with specified parts". Personally I think it would have been more coherent to maintain a consistency with listability (especially since KeyTake can be used to return Key/Value pairs) as such consistency usually avoids the need for any disambiguation such as in this example.


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