Failure when exporting dataset with real-valued cells

Question

Consider the following dataset:

dat = Dataset[<| "A" -> 1, "B" -> 2|>];
ExportString[dat, "CSV"]

It exports perfectly, with the expected result:

(* "A",1
   "B",2 *)

The above also works fine if the cells of the dataset are strings:

dat = Dataset[<| "A" -> "1", "B" -> "2"|>];

However, if the cells are reals (only one of them seems to be enough), then

dat = Dataset[<| "A" -> 1.0, "B"-> 2.0 |>];
ExportString[dat, "CSV"]

returns $Failed, with the following message:

Dataset::exportfail: Dataset of the given form could not be exported as format "CSV".

The same issue appears when using Export rather than ExportString. I am currently using ToString to convert all cells to strings before exporting. Is this a bug?

I am using OS X, Mathematica version 11.

Answer 1

In version 11.0.0.0, a detailed examination of the traced execution of ExportString[dat, "CSV"] reveals that the error occurs due to a bug in the internal function Dataset`ImportExport`PackagePrivate`txt.

This function is supposed to convert values of various types to strings. But we find that it has problems with real numbers:

Dataset;

Dataset`ImportExport`PackagePrivate`txt[1]
(* 1 *)

Dataset`ImportExport`PackagePrivate`txt[1.0]
(* GeneralUtilities`TextString`PackagePrivate`fmtReal[1., 10] *)

We can inspect the definitions of txt, looking for the definition applicable for real numbers:

Needs["GeneralUtilities`"]
PrintDefinitions[Dataset`ImportExport`PackagePrivate`txt]

(*
   ...
   txt[r_Real | r_Rational] := fmtReal[r, 10];
   ...
*)

However, if we look at the definitions of fmtReal, we find that the only definition expects one argument, not two:

PrintDefinitions[GeneralUtilities`TextString`PackagePrivate`fmtReal]

(*
   fmtReal[r_] := FromCharacterCode@...
*)

Patch

If we are willing to apply an unsanctioned patch ourselves, we can fix the bug:

Dataset;

Dataset`ImportExport`PackagePrivate`txt[r_Real | r_Rational] :=
  GeneralUtilities`TextString`PackagePrivate`fmtReal[r];

dat = Dataset[<| "A" -> 1.0, "B"-> 2.0 |>];
ExportString[dat, "CSV"]

(*
   "A",1.
   "B",2.
*)

Note that this patch must be applied after the dataset packages have been auto-loaded. The otherwise apparently useless expression Dataset; forces the auto-loading to happen.