From 683a2da055ca223098298fae27b729dbcf13f69e Mon Sep 17 00:00:00 2001
From: Patrick Thomson <patrickt@github.com>
Date: Wed, 30 Sep 2020 16:19:34 -0400
Subject: [PATCH] Fix crash when extras function doesn't return an array.

Fixes #745, which failed due to attempting to call `map` on a
non-array. This bails out at the same spot, but with a more
illuminating error message.
---
 cli/src/generate/dsl.js | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/cli/src/generate/dsl.js b/cli/src/generate/dsl.js
index 55594871..62fb1d70 100644
--- a/cli/src/generate/dsl.js
+++ b/cli/src/generate/dsl.js
@@ -292,7 +292,12 @@ function grammar(baseGrammar, options) {
 
     extras = options.extras
       .call(ruleBuilder, ruleBuilder, baseGrammar.extras)
-      .map(normalize);
+
+    if (!Array.isArray(extras)) {
+      throw new Error("Grammar's 'extras' function must return an array.")
+    }
+
+    extras = extras.map(normalize);
   }
 
   let word = baseGrammar.word;