Require Import Coq.Strings.String. Open Scope string_scope. Definition hello:string := "Hello, World!". (* Unit test *) Lemma HelloTest: hello = "Hello, World!" . Proof. reflexivity. Qed.
Exercism is fantastic in learning new languages but that is not the extent of it. If you are a "more experienced" programmer you may have encountered impostor syndrome: the idea you don't really know what you think you know. Exercism lets you solve problems and put them in the space of open feedback which is a tremendous learning opportunity to explore the depth of your own knowledge. Even if you have been programming in a language for awhile it is worth checking into Exercism to see where you stand with current implementation practices.
These are a few of the 3 exercises on the Coq track. You can see all the exercises here.