Did 10.0.2 break some forms for named slots?

Question

Bug introduced in 10.0.2 and fixed in 10.2.0

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Since 10.0.2 some code that previously worked using named slots fail in very particular circumstances.

Attchd screenshot shows the issue w/ a particular Dataset which involves multiple pre-processing steps to construct, so cannot display here.

Lookup using form #name& fails, while the more normal forms or in Key form work.

However, it's difficult to reproduce this issue in a minimal example. When I try to project out a few rows and columns from this table to copy-paste it as text, subsequent re-casting to Dataset works w/ all forms of Slot.

Can anyone reproduce?

Answer 1

Following discussion w/ Jason Grigsby at WRI, rather than edit the Q w/ additional detail, the conclusion is that composite Keys such as lists of strings, interfere with above-mentioned named slot access:

data = <|"a" -> 1, {"b", "c"} -> 2|> // Dataset;

data[#["a"] &]

1

while

data[#"a" &]

Failure[Function, 
Association[
 "MessageTemplate" :> MessageName[Function, "slot1"], 
  "MessageParameters" -> {"a", Slot["a"]& , 
TypeSystem`SkeletonForm[
TypeSystem`Assoc[TypeSystem`AnyType, 
TypeSystem`Atom[Integer], 2]]}]]