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Bug introduced in 10.0 and fixed in 10.1

I am trying to use the Dataset functionality which was introduced in Mathematica 10.0 :

   dataset = Dataset[{
      <|"a" -> 1, "b" -> "x", "c" -> {1}|>,
      <|"a" -> 2, "b" -> "y", "c" -> {2, 3}|>,
      <|"a" -> 3, "b" -> "z", "c" -> {3}|>,
      <|"a" -> 4, "b" -> "x", "c" -> {4, 5}|>,
      <|"a" -> 5, "b" -> "y", "c" -> {5, 6, 7}|>,
      <|"a" -> 6, "b" -> "z", "c" -> {}|>}]

But this gives me an error as show in the image below:

enter image description here

I tried the same syntax in combination with


but with the same result. How can I resolve this issue?

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I cannot reproduce it. This dataset works well for me, no additional package is needed. – Yi Wang Jul 18 '14 at 9:02
I can reproduce this on OS X. The error message is "An unknown box name (TypeSystem`TypeMap) was sent as the BoxForm for the expression. Check the format rules for the expression." After restarting the kernel it works again. But why did it stop working? That's a good question. – C. E. Jul 18 '14 at 9:21
Thanks, you are right, after quit the kernel, the function works. SAnd I have the same issue with the assocation-function – Michiel van Mens Jul 18 '14 at 9:31
I don't seem to have the problem in v10.0.0 under Windows. There seem to be a lot of localized bugs in v10.0.0 -- hopefully a lot of them will be fixed in the next point release. – Mr.Wizard Jul 18 '14 at 9:31
I ran this on OS X 10.6.8 and it worked perfectly. I had a fresh kernel. – m_goldberg Jul 18 '14 at 9:43

This bug has been fixed as of Mathematica 10.1.0.

The problem had to do with a conflict between the Suggestions Bar and the Dataset paclet code. The same issue has affected other functionality using autoloading in a similar way, for example AssociationMap as mentioned here or here.

It is very likely, as WReach points out in a comment, that this is a manifestation of the same bug.

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