I'm trying to figure out what is a quick way to add error reporting to my user defined functions. I was experimenting with Success and Failure until I stumbled upon what seems to me an oddity of sorts. The examples are taken from the Documentation Center:

f = Failure["ExternalOperation", <|
  "MessageTemplate" -> "External operation `1` failed.", 
  "MessageParameters" -> {"file upload"}, 
  "File" -> "MyFile.wl"|>];


s = Success["MailSent", <|
  "MessageTemplate" :> "Mail sent to `recipient`.", 
  "MessageParameters" -> <|"recipient" -> "[email protected]"|>, 
  "Sender" -> "[email protected]", "Recipient" -> "[email protected]", 
  "MessageID" -> "4ac4dsdasd483k28d4"|>];

In my code I found really useful the fact that it was possible to use something like s["Sender"] and obtain the desired result "[email protected]". That way, you could pass results and other information inside the Success wrapper.

Now, the odd thing is that such is not the case with Failure. Trying to evaluate f["File"] will not return "MyFile.wl" as I'd expect, but will return the Failure wrapper with the supplied key, like in Failure[...]["File"].

I understand it is possible to obtain the desired result using something like Extract[f, {2, Key["File"]}] but I'd rather use the f["File"] approach, if that were possible.

My question is, why do Failure and Success behave differently despite the fact they appear to implement the same underlying interface in their definitions?

If I am wrong to anticipate that they should behave similarly, what am I missing? Is there something inherent in the way Failure is implemented that makes it different?

-- Edit --

It appears that it's no longer an issue in v.12. That's very thoughtful of them.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Kuba
    Feb 16, 2019 at 15:12

1 Answer 1


It does not work because it was not implemented, imo by omission. What was implemented is an overloaded Part:



It won't work for Success though :)

What works for both of them is:

f[[2, "File"]]
s[[2, "Sender"]]


"[email protected]"


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