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Consider an association which contains a key which must not be prematurely evaluated.

assoc = <| key :> value |>

For example, a value of a[[b]] would throw a Part error until symbols a and b are substituted.

We might wish to obtain this value in its unevaluated form, e.g. to display. Simply calling

assoc[key]

will evaluate value (and in that example, throw a Part error).

One trick is to exploit that matched patterns are unevaluated:

assoc /. KeyValuePattern[ key :> v_ ] -> Hold[v]

but this seems needlessly hacky for what must be a common operation. What's the right way to obtain Hold[value]?

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Ironically, the probably simplest answer is literally Extract, just as in the question's title:

Extract[assoc, {Key[key]}, Hold]

(* Hold[value] *)
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There is a Lookup variant that supports wrapping with a head before evaluation:

Lookup[assoc, key, Missing[], Hold]
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    $\begingroup$ Note this requires v12.1 (incompatible with my 11.1) $\endgroup$
    – Anti Earth
    May 9 at 16:34

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