lib/attrsets: document all the functions

1 files changed, 109 insertions, 28 deletions

diff --git a/lib/attrsets.nix b/lib/attrsets.nix
index f5184af42309..4161fa546c8f 100644
--- a/lib/attrsets.nix
+++ b/lib/attrsets.nix
@@ -12,9 +12,15 @@ rec {
   inherit (builtins) attrNames listToAttrs hasAttr isAttrs getAttr;
 
 
-  /* 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. */
+  /* Return an attribute from nested attribute sets.
+
+     Example:
+       x = { a = { b = 3; }; }
+       attrByPath ["a" "b"] 6 x
+       => 3
+       attrByPath ["z" "z"] 6 x
+       => 6
+  */
   attrByPath = attrPath: default: e:
     let attr = head attrPath;
     in
@@ -24,8 +30,15 @@ rec {
       else default;
 
   /* Return if an attribute from nested attribute set exists.
-     For instance ["x" "y"] applied to some set e returns true, if e.x.y exists. False
-     is returned otherwise. */
+
+     Example:
+       x = { a = { b = 3; }; }
+       hasAttrByPath ["a" "b"] x
+       => true
+       hasAttrByPath ["z" "z"] x
+       => false
+
+  */
   hasAttrByPath = attrPath: e:
     let attr = head attrPath;
     in
@@ -35,14 +48,28 @@ rec {
       else false;
 
 
-  /* Return nested attribute set in which an attribute is set.  For instance
-     ["x" "y"] applied with some value v returns `x.y = v;' */
+  /* Return nested attribute set in which an attribute is set.
+
+     Example:
+       setAttrByPath ["a" "b"] 3
+       => { a = { b = 3; }; }
+  */
   setAttrByPath = attrPath: value:
     if attrPath == [] then value
     else listToAttrs
       [ { name = head attrPath; value = setAttrByPath (tail attrPath) value; } ];
 
 
+  /* Like `getAttrPath' without a default value. If it doesn't find the
+     path it will throw.
+
+     Example:
+       x = { a = { b = 3; }; }
+       getAttrFromPath ["a" "b"] x
+       => 3
+       getAttrFromPath ["z" "z"] x
+       => error: cannot find attribute `z.z'
+  */
   getAttrFromPath = attrPath: set:
     let errorMsg = "cannot find attribute `" + concatStringsSep "." attrPath + "'";
     in attrByPath attrPath (abort errorMsg) set;
@@ -109,9 +136,11 @@ rec {
       ) (attrNames set)
     );
 
-  /* foldAttrs: apply fold functions to values grouped by key. Eg accumulate values as list:
-     foldAttrs (n: a: [n] ++ a) [] [{ a = 2; } { a = 3; }]
-     => { a = [ 2 3 ]; }
+  /* Apply fold functions to values grouped by key.
+
+     Example:
+       foldAttrs (n: a: [n] ++ a) [] [{ a = 2; } { a = 3; }]
+       => { a = [ 2 3 ]; }
   */
   foldAttrs = op: nul: list_of_attrs:
     fold (n: a:
@@ -147,7 +176,12 @@ rec {
 
 
   /* Utility function that creates a {name, value} pair as expected by
-     builtins.listToAttrs. */
+     builtins.listToAttrs.
+
+     Example:
+       nameValuePair "some" 6
+       => { name = "some"; value = 6; }
+  */
   nameValuePair = name: value: { inherit name value; };
 
 
@@ -248,11 +282,19 @@ rec {
     listToAttrs (map (n: nameValuePair n (f n)) names);
 
 
-  /* Check whether the argument is a derivation. */
-  isDerivation = x: isAttrs x && x ? type && x.type == "derivation";
+  /* Check whether the argument is a derivation. Any set with
+     { type = "derivation"; } counts as a derivation.
 
+     Example:
+       nixpkgs = import <nixpkgs> {}
+       isDerivation nixpkgs.ruby
+       => true
+       isDerivation "foobar"
+       => false
+  */
+  isDerivation = x: isAttrs x && x ? type && x.type == "derivation";
 
-  /* Convert a store path to a fake derivation. */
+  /* Converts a store path to a fake derivation. */
   toDerivation = path:
     let path' = builtins.storePath path; in
     { type = "derivation";
@@ -262,32 +304,49 @@ rec {
     };
 
 
-  /* If the Boolean `cond' is true, return the attribute set `as',
-     otherwise an empty attribute set. */
+  /* If `cond' is true, return the attribute set `as',
+     otherwise an empty attribute set.
+
+     Example:
+       optionalAttrs (true) { my = "set"; }
+       => { my = "set"; }
+       optionalAttrs (false) { my = "set"; }
+       => { }
+  */
   optionalAttrs = cond: as: if cond then as else {};
 
 
   /* Merge sets of attributes and use the function f to merge attributes
-     values. */
+     values.
+
+     Example:
+       zipAttrsWithNames ["a"] (name: vs: vs) [{a = "x";} {a = "y"; b = "z";}]
+       => { a = ["x" "y"]; }
+  */
   zipAttrsWithNames = names: f: sets:
     listToAttrs (map (name: {
       inherit name;
       value = f name (catAttrs name sets);
     }) names);
 
-  # implentation note: Common names  appear multiple times in the list of
-  # names, hopefully this does not affect the system because the maximal
-  # laziness avoid computing twice the same expression and listToAttrs does
-  # not care about duplicated attribute names.
+  /* Implentation note: Common names  appear multiple times in the list of
+     names, hopefully this does not affect the system because the maximal
+     laziness avoid computing twice the same expression and listToAttrs does
+     not care about duplicated attribute names.
+
+     Example:
+       zipAttrsWith (name: values: values) [{a = "x";} {a = "y"; b = "z";}]
+       => { a = ["x" "y"]; b = ["z"] }
+  */
   zipAttrsWith = f: sets: zipAttrsWithNames (concatMap attrNames sets) f sets;
+  /* Like `zipAttrsWith' with `(name: values: value)' as the function.
 
+    Example:
+      zipAttrs [{a = "x";} {a = "y"; b = "z";}]
+      => { a = ["x" "y"]; b = ["z"] }
+  */
   zipAttrs = zipAttrsWith (name: values: values);
 
-  /* backward compatibility */
-  zipWithNames = zipAttrsWithNames;
-  zip = builtins.trace "lib.zip is deprecated, use lib.zipAttrsWith instead" zipAttrsWith;
-
-
   /* Does the same as the update operator '//' except that attributes are
      merged until the given pedicate is verified.  The predicate should
      accept 3 arguments which are the path to reach the attribute, a part of
@@ -351,6 +410,15 @@ rec {
       !(isAttrs lhs && isAttrs rhs)
     ) lhs rhs;
 
+  /* Returns true if the pattern is contained in the set. False otherwise.
+
+     FIXME(zimbatm): this example doesn't work !!!
+
+     Example:
+       sys = mkSystem { }
+       matchAttrs { cpu = { bits = 64; }; } sys
+       => true
+   */
   matchAttrs = pattern: attrs:
     fold or false (attrValues (zipAttrsWithNames (attrNames pattern) (n: values:
       let pat = head values; val = head (tail values); in
@@ -359,10 +427,23 @@ rec {
       else pat == val
     ) [pattern attrs]));
 
-  # override only the attributes that are already present in the old set
-  # useful for deep-overriding
+  /* Override only the attributes that are already present in the old set
+    useful for deep-overriding.
+
+    Example:
+      x = { a = { b = 4; c = 3; }; }
+      overrideExisting x { a = { b = 6; d = 2; }; }
+      => { a = { b = 6; d = 2; }; }
+  */
   overrideExisting = old: new:
     old // listToAttrs (map (attr: nameValuePair attr (attrByPath [attr] old.${attr} new)) (attrNames old));
 
+
+  /*** deprecated stuff ***/
+
   deepSeqAttrs = throw "removed 2016-02-29 because unused and broken";
+  zipWithNames = zipAttrsWithNames;
+  zip = builtins.trace
+    "lib.zip is deprecated, use lib.zipAttrsWith instead" zipAttrsWith;
+
 }