pattern matching - Built in way to de-list singletons?

Is there a built in method to de-list singleton expressions, like this rule-based solution:

expr_ :>  If[Length[expr] == 1, First@expr, expr]

In data analysis, this comes in handy to normalize the output of Cases, since a single exact match will be wrapped in a list, but wrapping that in First would break no-match cases and non-singleton lists.

Answer

You can use a simple replacement rule like in the following example:

list = {a, {b}, {c, d}, {e, {f, g}}, {{h}}};

list /. _[x_] :> x
(* {a, b, {c, d}, {e, {f, g}}, {h}} *)

This will strip a layer from expressions with length 1, for all heads. You can restrict it further to act only on List, if that's what you desire.