Eliminate some calls to ‘tail’

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

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
             {}

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;
 
 

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));

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.";