lib/lists: document all functions

1 files changed, 217 insertions, 55 deletions

diff --git a/lib/lists.nix b/lib/lists.nix
index 9e4ab9e2d95d..deb7dcfde428 100644
--- a/lib/lists.nix
+++ b/lib/lists.nix
@@ -6,17 +6,26 @@ rec {
 
   inherit (builtins) head tail length isList elemAt concatLists filter elem genList;
 
-
-  # Create a list consisting of a single element.  `singleton x' is
-  # sometimes more convenient with respect to indentation than `[x]'
-  # when x spans multiple lines.
+  /*  Create a list consisting of a single element.  `singleton x' is
+      sometimes more convenient with respect to indentation than `[x]'
+      when x spans multiple lines.
+
+      Example:
+        singleton "foo"
+        => [ "foo" ]
+  */
   singleton = x: [x];
 
+  /* "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" 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).
+     Example:
+       concat = fold (a: b: a + b) "z"
+       concat [ "a" "b" "c" ]
+       => "abcnul"
+  */
   fold = op: nul: list:
     let
       len = length list;
@@ -26,8 +35,14 @@ rec {
         else op (elemAt list n) (fold' (n + 1));
     in fold' 0;
 
-  # Left fold: `fold op nul [x_1 x_2 ... x_n] == op (... (op (op nul
-  # x_1) x_2) ... x_n)'.
+  /* Left fold: `fold op nul [x_1 x_2 ... x_n] == op (... (op (op nul
+     x_1) x_2) ... x_n)'.
+
+     Example:
+       lconcat = foldl (a: b: a + b) "z"
+       lconcat [ "a" "b" "c" ]
+       => "zabc"
+  */
   foldl = op: nul: list:
     let
       len = length list;
@@ -37,13 +52,22 @@ rec {
         else op (foldl' (n - 1)) (elemAt list n);
     in foldl' (length list - 1);
 
+  /* Strict version of foldl.
 
-  # Strict version of foldl.
+     The difference is that evaluation is forced upon access. Usually used
+     with small whole results (in contract with lazily-generated list or large
+     lists where only a part is consumed.)
+  */
   foldl' = builtins.foldl' or foldl;
 
+  /* Map with index
+
+     FIXME(zimbatm): why does this start to count at 1?
 
-  # Map with index: `imap (i: v: "${v}-${toString i}") ["a" "b"] ==
-  # ["a-1" "b-2"]'. FIXME: why does this start to count at 1?
+     Example:
+       imap (i: v: "${v}-${toString i}") ["a" "b"]
+       => [ "a-1" "b-2" ]
+  */
   imap =
     if builtins ? genList then
       f: list: genList (n: f (n + 1) (elemAt list n)) (length list)
@@ -57,73 +81,141 @@ rec {
             else [ (f (n + 1) (elemAt list n)) ] ++ imap' (n + 1);
       in imap' 0;
 
+  /* Map and concatenate the result.
 
-  # Map and concatenate the result.
+     Example:
+       concatMap (x: [x] ++ ["z"]) ["a" "b"]
+       => [ "a" "z" "b" "z" ]
+  */
   concatMap = f: list: concatLists (map f list);
 
+  /* Flatten the argument into a single list; that is, nested lists are
+     spliced into the top-level lists.
 
-  # 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]'.
+     Example:
+       flatten [1 [2 [3] 4] 5]
+       => [1 2 3 4 5]
+       flatten 1
+       => [1]
+  */
   flatten = x:
     if isList x
     then foldl' (x: y: x ++ (flatten y)) [] x
     else [x];
 
+  /* Remove elements equal to 'e' from a list.  Useful for buildInputs.
 
-  # Remove elements equal to 'e' from a list.  Useful for buildInputs.
+     Example:
+       remove 3 [ 1 3 4 3 ]
+       => [ 1 4 ]
+  */
   remove = e: filter (x: x != e);
 
-
-  # Find the sole element in the list matching the specified
-  # predicate, returns `default' if no such element exists, or
-  # `multiple' if there are multiple matching elements.
+  /* Find the sole element in the list matching the specified
+     predicate, returns `default' if no such element exists, or
+     `multiple' if there are multiple matching elements.
+
+     Example:
+       findSingle (x: x == 3) "none" "multiple" [ 1 3 3 ]
+       => "multiple"
+       findSingle (x: x == 3) "none" "multiple" [ 1 3 ]
+       => 3
+       findSingle (x: x == 3) "none" "multiple" [ 1 9 ]
+       => "none"
+  */
   findSingle = pred: default: multiple: list:
     let found = filter pred list; len = length found;
     in if len == 0 then default
       else if len != 1 then multiple
       else head found;
 
+  /* Find the first element in the list matching the specified
+     predicate or returns `default' if no such element exists.
 
-  # Find the first element in the list matching the specified
-  # predicate or returns `default' if no such element exists.
+     Example:
+       findFirst (x: x > 3) 7 [ 1 6 4 ]
+       => 6
+       findFirst (x: x > 9) 7 [ 1 6 4 ]
+       => 7
+  */
   findFirst = pred: default: list:
     let found = filter pred list;
     in if found == [] then default else head found;
 
+  /* Return true iff function `pred' returns true for at least element
+     of `list'.
 
-  # Return true iff function `pred' returns true for at least element
-  # of `list'.
+     Example:
+       any isString [ 1 "a" { } ]
+       => true
+       any isString [ 1 { } ]
+       => false
+  */
   any = builtins.any or (pred: fold (x: y: if pred x then true else y) false);
 
+  /* Return true iff function `pred' returns true for all elements of
+     `list'.
 
-  # Return true iff function `pred' returns true for all elements of
-  # `list'.
+     Example:
+       all (x: x < 3) [ 1 2 ]
+       => true
+       all (x: x < 3) [ 1 2 3 ]
+       => false
+  */
   all = builtins.all or (pred: fold (x: y: if pred x then y else false) true);
 
+  /* Count how many times function `pred' returns true for the elements
+     of `list'.
 
-  # Count how many times function `pred' returns true for the elements
-  # of `list'.
+     Example:
+       count (x: x == 3) [ 3 2 3 4 6 ]
+       => 2
+  */
   count = pred: foldl' (c: x: if pred x then c + 1 else c) 0;
 
+  /* Return a singleton list or an empty list, depending on a boolean
+     value.  Useful when building lists with optional elements
+     (e.g. `++ optional (system == "i686-linux") flashplayer').
 
-  # Return a singleton list or an empty list, depending on a boolean
-  # value.  Useful when building lists with optional elements
-  # (e.g. `++ optional (system == "i686-linux") flashplayer').
+     Example:
+       optional true "foo"
+       => [ "foo" ]
+       optional false "foo"
+       => [ ]
+  */
   optional = cond: elem: if cond then [elem] else [];
 
+  /* Return a list or an empty list, dependening on a boolean value.
 
-  # Return a list or an empty list, dependening on a boolean value.
+     Example:
+       optionals true [ 2 3 ]
+       => [ 2 3 ]
+       optionals false [ 2 3 ]
+       => [ ]
+  */
   optionals = cond: elems: if cond then elems else [];
 
 
-  # If argument is a list, return it; else, wrap it in a singleton
-  # list.  If you're using this, you should almost certainly
-  # reconsider if there isn't a more "well-typed" approach.
+  /* If argument is a list, return it; else, wrap it in a singleton
+     list.  If you're using this, you should almost certainly
+     reconsider if there isn't a more "well-typed" approach.
+
+     Example:
+       toList [ 1 2 ]
+       => [ 1 2 ]
+       toList "hi"
+       => [ "hi "]
+  */
   toList = x: if isList x then x else [x];
 
+  /* Return a list of integers from `first' up to and including `last'.
 
-  # Return a list of integers from `first' up to and including `last'.
+     Example:
+       range 2 4
+       => [ 2 3 4 ]
+       range 3 2
+       => [ ]
+  */
   range =
     if builtins ? genList then
       first: last:
@@ -136,9 +228,13 @@ rec {
         then []
         else [first] ++ range (first + 1) last;
 
+  /* Splits the elements of a list in two lists, `right' and
+     `wrong', depending on the evaluation of a predicate.
 
-  # Partition the elements of a list in two lists, `right' and
-  # `wrong', depending on the evaluation of a predicate.
+     Example:
+       partition (x: x > 2) [ 5 1 2 3 4 ]
+       => { right = [ 5 3 4 ]; wrong = [ 1 2 ]; }
+  */
   partition = pred:
     fold (h: t:
       if pred h
@@ -146,7 +242,14 @@ rec {
       else { right = t.right; wrong = [h] ++ t.wrong; }
     ) { right = []; wrong = []; };
 
+  /* Merges two lists of the same size together. If the sizes aren't the same
+     the merging stops at the shortest. How both lists are merged is defined
+     by the first argument.
 
+     Example:
+       zipListsWith (a: b: a + b) ["h" "l"] ["e" "o"]
+       => ["he" "lo"]
+  */
   zipListsWith =
     if builtins ? genList then
       f: fst: snd: genList (n: f (elemAt fst n) (elemAt snd n)) (min (length fst) (length snd))
@@ -161,21 +264,37 @@ rec {
           else [];
       in zipListsWith' 0;
 
+  /* Merges two lists of the same size together. If the sizes aren't the same
+     the merging stops at the shortest.
+
+     Example:
+       zipLists [ 1 2 ] [ "a" "b" ]
+       => [ { fst = 1; snd = "a"; } { fst = 2; snd = "b"; } ]
+  */
   zipLists = zipListsWith (fst: snd: { inherit fst snd; });
 
+  /* Reverse the order of the elements of a list.
 
-  # Reverse the order of the elements of a list.
+     Example:
+
+       reverseList [ "b" "o" "j" ]
+       => [ "j" "o" "b" ]
+  */
   reverseList =
     if builtins ? genList then
       xs: let l = length xs; in genList (n: elemAt xs (l - n - 1)) l
     else
       fold (e: acc: acc ++ [ e ]) [];
 
+  /* Sort a list based on a comparator function which compares two
+     elements and returns true if the first argument is strictly below
+     the second argument.  The returned list is sorted in an increasing
+     order.  The implementation does a quick-sort.
 
-  # Sort a list based on a comparator function which compares two
-  # elements and returns true if the first argument is strictly below
-  # the second argument.  The returned list is sorted in an increasing
-  # order.  The implementation does a quick-sort.
+     Example:
+       sort (a: b: a < b) [ 5 3 7 ]
+       => [ 3 5 7 ]
+  */
   sort = builtins.sort or (
     strictLess: list:
     let
@@ -193,8 +312,14 @@ rec {
       if len < 2 then list
       else (sort strictLess pivot.left) ++  [ first ] ++  (sort strictLess pivot.right));
 
+  /* Return the first (at most) N elements of a list.
 
-  # Return the first (at most) N elements of a list.
+     Example:
+       take 2 [ "a" "b" "c" "d" ]
+       => [ "a" "b" ]
+       take 2 [ ]
+       => [ ]
+  */
   take =
     if builtins ? genList then
       count: sublist 0 count
@@ -209,8 +334,14 @@ rec {
               [ (elemAt list n) ] ++ take' (n + 1);
         in take' 0;
 
+  /* Remove the first (at most) N elements of a list.
 
-  # Remove the first (at most) N elements of a list.
+     Example:
+       drop 2 [ "a" "b" "c" "d" ]
+       => [ "c" "d" ]
+       drop 2 [ ]
+       => [ ]
+  */
   drop =
     if builtins ? genList then
       count: list: sublist count (length list) list
@@ -225,9 +356,15 @@ rec {
               drop' (n - 1) ++ [ (elemAt list n) ];
         in drop' (len - 1);
 
+  /* Return a list consisting of at most ‘count’ elements of ‘list’,
+     starting at index ‘start’.
 
-  # Return a list consisting of at most ‘count’ elements of ‘list’,
-  # starting at index ‘start’.
+     Example:
+       sublist 1 3 [ "a" "b" "c" "d" "e" ]
+       => [ "b" "c" "d" ]
+       sublist 1 3 [ ]
+       => [ ]
+  */
   sublist = start: count: list:
     let len = length list; in
     genList
@@ -236,20 +373,36 @@ rec {
        else if start + count > len then len - start
        else count);
 
+  /* Return the last element of a list.
 
-  # Return the last element of a list.
+     Example:
+       last [ 1 2 3 ]
+       => 3
+  */
   last = list:
     assert list != []; elemAt list (length list - 1);
 
+  /* Return all elements but the last
 
-  # Return all elements but the last
+     Example:
+       init [ 1 2 3 ]
+       => [ 1 2 ]
+  */
   init = list: assert list != []; take (length list - 1) list;
 
 
+  /* FIXME(zimbatm) Not used anywhere
+   */
   crossLists = f: foldl (fs: args: concatMap (f: map f args) fs) [f];
 
 
-  # Remove duplicate elements from the list. O(n^2) complexity.
+  /* Remove duplicate elements from the list. O(n^2) complexity.
+
+     Example:
+
+       unique [ 3 2 3 4 ]
+       => [ 3 2 4 ]
+   */
   unique = list:
     if list == [] then
       []
@@ -259,15 +412,24 @@ rec {
         xs = unique (drop 1 list);
       in [x] ++ remove x xs;
 
+  /* Intersects list 'e' and another list. O(nm) complexity.
 
-  # Intersects list 'e' and another list. O(nm) complexity.
+     Example:
+       intersectLists [ 1 2 3 ] [ 6 3 2 ]
+       => [ 3 2 ]
+  */
   intersectLists = e: filter (x: elem x e);
 
+  /* Subtracts list 'e' from another list. O(nm) complexity.
 
-  # Subtracts list 'e' from another list. O(nm) complexity.
+     Example:
+       subtractLists [ 3 2 ] [ 1 2 3 4 5 3 ]
+       => [ 1 4 5 ]
+  */
   subtractLists = e: filter (x: !(elem x e));
 
-  deepSeqList = throw "removed 2016-02-29 because unused and broken";
+  /*** deprecated stuff ***/
 
+  deepSeqList = throw "removed 2016-02-29 because unused and broken";
 
 }