From 486a26eb7cea0c99818fde2c3fd933f7aca40b52 Mon Sep 17 00:00:00 2001
From: Nadrieril
Date: Thu, 18 Apr 2019 22:08:25 +0200
Subject: Use a context for substitution in normalization

---
 dhall_core/src/context.rs | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

(limited to 'dhall_core')

diff --git a/dhall_core/src/context.rs b/dhall_core/src/context.rs
index b500226..91eec69 100644
--- a/dhall_core/src/context.rs
+++ b/dhall_core/src/context.rs
@@ -32,7 +32,13 @@ impl<K: Hash + Eq + Clone, T> Context<K, T> {
     /// lookup k n (insert j v c) = lookup k  n      c  -- k /= j
     /// ```
     pub fn lookup<'a>(&'a self, k: &K, n: usize) -> Option<&'a T> {
-        self.0.get(k).and_then(|v| v.get(v.len() - 1 - n))
+        self.0.get(k).and_then(|v| {
+            if n < v.len() {
+                v.get(v.len() - 1 - n)
+            } else {
+                None
+            }
+        })
     }
 
     pub fn map<U, F: Fn(&T) -> U>(&self, f: F) -> Context<K, U> {
-- 
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