From e465c840f58e60df51f2d9608dccc1966babf0c8 Mon Sep 17 00:00:00 2001
From: Steve Purcell <steve@sanityinc.com>
Date: Wed, 8 Apr 2020 16:24:35 +1200
Subject: [PATCH] Avoid calling builtins.head on potentially-empty list

When no matching wheels are found, builtins.head causes this to fail
with a very low-level error. Better to return an empty list and
trigger the assertion in the code which calls this.
---
 pep425.nix | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pep425.nix b/pep425.nix
index cda4e8c..6b63146 100644
--- a/pep425.nix
+++ b/pep425.nix
@@ -87,8 +87,8 @@ let
         let
           osxMatches = [ "10_12" "10_11" "10_10" "10_9" "any" ];
           linuxMatches = [ "manylinux1_" "manylinux2010_" "manylinux2014_" "any" ];
-          chooseLinux = x: lib.singleton (builtins.head (findBestMatches linuxMatches x));
-          chooseOSX = x: lib.singleton (builtins.head (findBestMatches osxMatches x));
+          chooseLinux = x: lib.take 1 (findBestMatches linuxMatches x);
+          chooseOSX = x: lib.take 1 (findBestMatches osxMatches x);
         in
           if isLinux
           then chooseLinux files