How to make Association reflect new assignments

Question

How do I get Association to act like List in the following?

list = {a};
assoc = <|x -> a|>;
a = 1;
{list, assoc}

{{1}, <|x -> a|>}

The List picks up the newly assigned value of a, but the Association does not. And why does Association behave like this?

Answer 1

As halirutan notes the original value of list is retained; only its evaluation is changed by the new assignment a = 1.

Likewise the evaluation of the Association is also changed:

assoc = <|x -> a|>;
a = 1;
assoc[x]

1

In Mathematica 10.0.2 assoc is also printed with the evaluated form of a:

assoc

<|x -> 1|>

Its definition remains unchanged however:

?assoc

Global`assoc

assoc=<|x->a|>

To actually update the definition of list one could use list = list because the RHS fully evaluates. However this is not the case with the Association. You can force an evaluation of the values with a Map operation:

assoc = Identity /@ assoc;

?assoc

Global`assoc

assoc=<|x->1|>

The behaviors illustrated may change as Association is further developed. See for example:

• Held keys in associations