Get association element that has a parameter

Question

Say I have an Association of the form:

assoc = <|first->"abc", second[param]->"xyz"|>

Now to access first, all I have to do is call assoc[first]. Now the problem is that I need an easy way to access second, but without knowing the parameter (which is here param).

Is it possible to use something in the form assoc[second] with some minor modification? Or do I have to write my own function that manipulates Keys[assoc] to find second?

In short, the question is: What's the easiest way to access second and get the information in param?

Answer 1

FilterRules[assoc, second[_]][[1, 1, 1]]
(*param*)

Answer 2

You can Select from the Keys of the Association the elements with Head equal to second and then pick out the parameter from second.

Select[Head@# === second &]@Keys[assoc] /. second[p_] -> p
(* {param} *)

Hope this helps.

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Update with version that uses only key functions.

KeySelect[Head@# === second &] /* KeyMap[Replace[_[p_] -> p]] /* Keys@assoc
(* {param} *)