From cf3b953228c0f61c439d9c1100b49bafcedc366c Mon Sep 17 00:00:00 2001
From: David Kemsley <david@livebranchstudios.com.au>
Date: Mon, 15 Aug 2022 23:03:05 +1000
Subject: [PATCH] Avoid exception for immutable list from folded

* Added check to avoid error thrown when returning a immutable list from a function called by a comprehension expression
---
 interpreter/interpretable.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/interpreter/interpretable.go b/interpreter/interpretable.go
index 324edcca..4fdd1202 100644
--- a/interpreter/interpretable.go
+++ b/interpreter/interpretable.go
@@ -835,7 +835,9 @@ func (fold *evalFold) Eval(ctx Activation) ref.Val {
 	varActivationPool.Put(accuCtx)
 	// Convert a mutable list to an immutable one, if the comprehension has generated a list as a result.
 	if !types.IsUnknownOrError(res) && buildingList {
-		res = res.(traits.MutableLister).ToImmutableList()
+		if _, ok := res.(traits.MutableLister); ok {
+			res = res.(traits.MutableLister).ToImmutableList()
+		}
 	}
 	return res
 }