From 512b3f99bc34b8bc17dd8612ab034b4b173ac0ca Mon Sep 17 00:00:00 2001
From: Robbert Krebbers <mail@robbertkrebbers.nl>
Date: Fri, 11 Jan 2019 14:39:20 +0100
Subject: [PATCH] `tc_to_bool P` to turn a type class into a Boolean that
 represents if there is an instance.

---
 theories/base.v | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/theories/base.v b/theories/base.v
index 3043b473..403948dd 100644
--- a/theories/base.v
+++ b/theories/base.v
@@ -145,6 +145,12 @@ Inductive TCDiag {A} (C : A → Prop) : A → A → Prop :=
 Existing Class TCDiag.
 Existing Instance TCDiag_diag.
 
+(** Given a proposition [P] that is a type class, [tc_to_bool P] will return
+[true] iff there is an instance of [P]. It is often useful in Ltac programming,
+where one can do [lazymatch tc_to_bool P with true => .. | false => .. end]. *)
+Definition tc_to_bool (P : Prop)
+  {p : bool} `{TCIf P (TCEq p true) (TCEq p false)} : bool := p.
+
 (** Throughout this development we use [stdpp_scope] for all general purpose
 notations that do not belong to a more specific scope. *)
 Delimit Scope stdpp_scope with stdpp.
-- 
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