SplitBy problem with Associations

Question

Association keys are supposed to be transparent, but in this reduced example SplitBy fails:

ds = <|t1 -> <|"a" -> 1|>, t2 -> <|"a" -> 1|>, t3 -> <|"a" -> 2|> |>

This works:

ds // Values // SplitBy[#, #a &]&

{{<|"a" -> 1|>, <|"a" -> 1|>}, {<|"a" -> 2|>}}

But the following fail for the same reason:

ds // SplitBy[#, #a &] &

ds // Map[Normal] // SplitBy[#, #a &]&

Split::normal: "Nonatomic expression expected at position 1 in !(Split[<|t1 -> {\"a\" -> 1}, t2 -> {\"a\" -> 1}, t3 -> {\"a\" -> 2}|>, ((#a &))[#1] === ((#a &))[#2] &]). "

Is there a workaround that doesn't involve injecting keys t1, t2 etc as key-value pairs in each row?

Answer 1

SplitBy is implemented in terms of Split which does not work on associations:

Split[ds]

During evaluation of Split::normal: Nonatomic expression expected at position 1 in Split[<|t1-><|a->1|>,t2-><|a->1|>,t3-><|a->2|>|>]. >>

So I think you will have to convert to a list

SplitBy[List @@ ds, #a &]

(* {{<|"a" -> 1|>, <|"a" -> 1|>}, {<|"a" -> 2|>}} *)

If you need to retain the keys you could use something like:

SplitBy[Normal@ds, #a& @* Last]

(* {{t1 -> <|"a" -> 1|>, t2 -> <|"a" -> 1|>}, {t3 -> <|"a" -> 2|>}} *)