How to lookup by a string key after GroupBy? Inconsistent data type handling

Question

This is relates to a bug that was introduced in 10.0.0 and fixed in 10.0.1

It also relates to an issue that has yet to be corrected.

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After applying GroupBy to a key all of whose "values" are String, is it possile to force the internal type representation to subsequently lookup by "value" rather than Key@"value"?

GroupBy modifies internal type representation:

titanic = ExampleData[{"Dataset", "Titanic"}];

(* ... Vector[Struct[List["class", "age", "sex", "survived"], 
  List[Atom[Enumeration["1st", "2nd", "3rd"]], Atom[Integer], 
   Atom[Enumeration["female", "male"]], Atom[Boolean]]], AnyLength] ... *)

(so does Normal'izing following by casting back to Dataset though AnyLength is replaced by a numerical value, eg 1309.)

Even though the keys (formerly values) are String:

titanic[GroupBy[Key@"sex"]][Keys, # -> Head[#] &]

(* {"female" -> String, "male" -> String} *)

This lookup fails:

titanic[GroupBy[Key@"sex"]]["female"]

(* ... Part female is not applicable to <|(Alternatives[Enumeration[female, \
male]] -> _List)..|> ... *)

Note: lookup query Key@"female" works. Strangely, works also without Key when KeyDrop is applied before GroupBy:

titanic[All, KeyDrop["survived"]][GroupBy[Key@"sex"]]["female"]

(* ... {<|"class" -> "1st", "age" -> 29, "sex" -> "female"|>, ... *)

Conversely, if Span substitutes KeyDrop, the data are the same

titanic[All, 1 ;; 3][GroupBy[Key@"sex"]][Keys, # -> Head[#] &]

(* {"female" -> String, "male" -> String} *)

but lookup also fails even using Key:

titanic[All, 1 ;; 3][GroupBy[Key@"sex"]][Key@"female"]

(* ... Key Key["female"] is not one of {}. ... *)

I'm not sure how to interpret that message: b/c the internal the representation is AnyType:

Assoc[Atom[AnyType], 
 Vector[Assoc[Atom[String], Atom[AnyType], 3], AnyLength], AnyLength]

How to handle these various cases uniformly?

Answer 1

The main issue here is already fixed in 10.0.1 (probably as a side-effect of fixes for some of the other typesystem bugs that have already been reported by you and others!).

The span issue is separate, I've reported that (thanks!)

Answer 2

Until this bug is fixed, a work-around is to avoid the operator form of GroupBy, e.g.:

titanic[GroupBy[#, Key@"sex"]& /* "female"]

This same bug affects many operator forms, but this work-around frequently dodges it.