From c0a483632c3e0193678dd58c994528d88d42ea00 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Mon, 13 Aug 2012 14:19:31 -0400
Subject: [PATCH] =?UTF-8?q?Eliminate=20some=20calls=20to=20=E2=80=98tail?=
 =?UTF-8?q?=E2=80=99?=
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---
 pkgs/lib/lists.nix   | 4 ++--
 pkgs/lib/modules.nix | 4 ++--
 pkgs/lib/options.nix | 8 ++++----
 pkgs/lib/strings.nix | 6 +++---
 pkgs/lib/types.nix   | 2 +-
 5 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/pkgs/lib/lists.nix b/pkgs/lib/lists.nix
index dcb89aeec141..f5845c32ee9c 100644
--- a/pkgs/lib/lists.nix
+++ b/pkgs/lib/lists.nix
@@ -169,11 +169,11 @@ rec {
   # order.  The implementation does a quick-sort.
   sort = strictLess: list:
     let
-      # This implementation only have one element lists on the left hand
+      # This implementation only has one element list on the left hand
       # side of the concatenation operator.
       qs = l: concat:
         if l == [] then concat
-        else if tail l == [] then l ++ concat
+        else if length l == 1 then l ++ concat
         else let
           part = partition (strictLess (head l)) (tail l);
         in
diff --git a/pkgs/lib/modules.nix b/pkgs/lib/modules.nix
index eb528f141fe2..13be0daa2877 100644
--- a/pkgs/lib/modules.nix
+++ b/pkgs/lib/modules.nix
@@ -2,7 +2,7 @@
 
 let lib = import ./default.nix; in
 
-with { inherit (builtins) head tail; };
+with { inherit (builtins) head; };
 with import ./trivial.nix;
 with import ./lists.nix;
 with import ./misc.nix;
@@ -64,7 +64,7 @@ rec {
           config = getConfig;
         } // (
           if getImportedSets != [] then
-            assert tail getImportedSets == [];
+            assert length getImportedSets == 1;
             { options = head getImportedSets; }
           else
             {}
diff --git a/pkgs/lib/options.nix b/pkgs/lib/options.nix
index cd60b8f3b88c..05c9c5f9c0f5 100644
--- a/pkgs/lib/options.nix
+++ b/pkgs/lib/options.nix
@@ -2,7 +2,7 @@
 
 let lib = import ./default.nix; in
 
-with { inherit (builtins) head tail; };
+with { inherit (builtins) head length; };
 with import ./trivial.nix;
 with import ./lists.nix;
 with import ./misc.nix;
@@ -133,7 +133,7 @@ rec {
   # separate the merge & apply fields from the interface.
   mergeOptionDecls = opts:
     if opts == [] then {}
-    else if tail opts == [] then
+    else if length opts == 1 then
       let opt = head opts; in
       if opt ? options then
         opt // { options = toList opt.options; }
@@ -189,7 +189,7 @@ rec {
     ) (attrNames defs));
 
   mergeDefaultOption = list:
-    if list != [] && tail list == [] then head list
+    if length list == 1 then head list
     else if all builtins.isFunction list then x: mergeDefaultOption (map (f: f x) list)
     else if all isList list then concatLists list
     else if all isAttrs list then fold lib.mergeAttrs {} list
@@ -214,7 +214,7 @@ rec {
 
   mergeOneOption = list:
     if list == [] then abort "This case should never happen."
-    else if tail list != [] then throw "Multiple definitions. Only one is allowed for this option."
+    else if length list != 1 then throw "Multiple definitions. Only one is allowed for this option."
     else head list;
 
 
diff --git a/pkgs/lib/strings.nix b/pkgs/lib/strings.nix
index 071bb2c995b3..fb2752d97c19 100644
--- a/pkgs/lib/strings.nix
+++ b/pkgs/lib/strings.nix
@@ -2,12 +2,12 @@
 
 let lib = import ./default.nix;
 
-inherit (builtins) add sub lessThan;
+inherit (builtins) add sub lessThan length;
 
 in
 
 rec {
-  inherit (builtins) stringLength substring head tail lessThan sub;
+  inherit (builtins) stringLength substring head tail;
 
 
   # Concatenate a list of strings.
@@ -22,7 +22,7 @@ rec {
   # Place an element between each element of a list, e.g.,
   # `intersperse "," ["a" "b" "c"]' returns ["a" "," "b" "," "c"].
   intersperse = separator: list:
-    if list == [] || tail list == []
+    if list == [] || length list == 1
     then list
     else [(head list) separator]
          ++ (intersperse separator (tail list));
diff --git a/pkgs/lib/types.nix b/pkgs/lib/types.nix
index 956368138aa2..38ae7b43630a 100644
--- a/pkgs/lib/types.nix
+++ b/pkgs/lib/types.nix
@@ -157,7 +157,7 @@ rec {
     uniq = elemType: mkOptionType {
       inherit (elemType) name check iter fold docPath hasOptions;
       merge = list:
-        if tail list == [] then
+        if length list == 1 then
           head list
         else
           throw "Multiple definitions. Only one is allowed for this option.";