Dispatch not behaving like a normal expression in Mathematica 10

Question

In Mathematica 9, I used to be able to do

rules = Dispatch[{'a' -> 1, 'b' -> 2}]
rules[[1]]

to extract the rules from Dispatchwrapped list of rules. In Mathematica 10 the following error is raised:

Part::partd: Part specification Dispatch[...][[1]] is longer than depth of object. >>

Is this a bug? I am using only the trial version currently. I got aware of it because it breaks the AutomaticUnits package.

score 11 · Accepted Answer · edited Apr 13 '17 at 12:56

11

No, this is not a bug. Dispatch tables are now atomic:

Dispatch[{"a" -> 1, "b" -> 2, "c" -> 3}] // AtomQ

True

As noted by user 18w here we can use Normal to recover the rules.

Dispatch[{"a" -> 1, "b" -> 2, "c" -> 3}] // Normal

{"a" -> 1, "b" -> 2, "c" -> 3}

edited Apr 13 '17 at 12:56

Community

answered Jul 14 '14 at 11:28

Mr.Wizard

1

And (they became atoms) for good reason, Dispatch tables with symbols in them were hard to use properly. See Performance of Dispatch and lists of Rules. Maybe I should see if I can simplify my answer to your "Why is there no PositionFunction in Mathematica?" Q&A. There was another issue with performance I think. – Jacob Akkerboom Jul 14 '14 at 13:37
2

Sweet, Dispatch now works with Verbatim! The example in my answer (below "This may be because Dispatch just does not work well with Verbatim. Example:") now works! – Jacob Akkerboom Jul 14 '14 at 13:40

1 Answers1