* Put utility functions like fold in lib/default.nix.

svn path=/nixpkgs/trunk/; revision=6605

pkgs/lib/default.nix

@@ -0,0 +1,42 @@
+# Utility functions.
+
+rec {
+
+  # "Fold" a binary function `op' between successive elements of
+  # `list' with `nul' as the starting value, i.e., `fold op nul [x_1
+  # x_2 ... x_n] == op x_1 (op x_2 ... (op x_n nul))'.  (This is
+  # Haskell's foldr).
+  fold = op: nul: list:
+    if list == []
+    then nul
+    else op (builtins.head list) (fold op nul (builtins.tail list));
+
+    
+  # Concatenate a list of strings.
+  concatStrings =
+    fold (x: y: x + y) "";
+
+
+  # Flatten the argument into a single list; that is, nested lists are
+  # spliced into the top-level lists.  E.g., `flatten [1 [2 [3] 4] 5]
+  # == [1 2 3 4 5]' and `flatten 1 == [1]'.
+  flatten = x:
+    if builtins.isList x
+    then fold (x: y: (flatten x) ++ y) [] x
+    else [x];
+
+
+  # Return an attribute from nested attribute sets.  For instance ["x"
+  # "y"] applied to some set e returns e.x.y, if it exists.  The
+  # default value is returned otherwise.
+  getAttr = attrPath: default: e:
+    let {
+      attr = builtins.head attrPath;
+      body =
+        if attrPath == [] then e
+        else if builtins ? hasAttr && builtins.hasAttr attr e
+        then getAttr (builtins.tail attrPath) default (builtins.getAttr attr e)
+        else default;
+    };
+  
+}

pkgs/top-level/all-packages.nix

@@ -77,18 +77,7 @@ rec {
 
   useFromStdenv = hasIt: it: alternative: if hasIt then it else alternative;
 
-  # Return an attribute from nested attribute sets.  For instance ["x"
+  inherit (import ../lib) getAttr;
-  # "y"] applied to some set e returns e.x.y, if it exists.  The
-  # default value is returned otherwise.
-  getAttr = attrPath: default: e:
-    let {
-      attr = builtins.head attrPath;
-      body =
-        if attrPath == [] then e
-        else if builtins ? hasAttr && builtins.hasAttr attr e
-        then getAttr (builtins.tail attrPath) default (builtins.getAttr attr e)
-        else default;
-    };
 
   # Return an attribute from the Nixpkgs configuration file, or
   # a default value if the attribute doesn't exist.