From fb692e5f7b34def8cf590298036ab63e40747062 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Fri, 31 May 2019 23:44:42 +0200
Subject: [PATCH] Bindings: Add convenience method for requiring an attribute

---
 src/libexpr/attr-set.hh | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/libexpr/attr-set.hh b/src/libexpr/attr-set.hh
index 6c5fb21ad..d6af99912 100644
--- a/src/libexpr/attr-set.hh
+++ b/src/libexpr/attr-set.hh
@@ -72,6 +72,14 @@ public:
         return {};
     }
 
+    Attr & need(const Symbol & name, const Pos & pos = noPos)
+    {
+        auto a = get(name);
+        if (!a)
+            throw Error("attribute '%s' missing, at %s", name, pos);
+        return **a;
+    }
+
     iterator begin() { return &attrs[0]; }
     iterator end() { return &attrs[size_]; }