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lib/lists: rename fold to foldr & improve fold docs

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In order to better distinguish foldr from foldl the default name is changed to
foldr, but fold is still a synonym.

Additionally the docs are improved and examples added.
This commit is contained in:
Profpatsch 2017-03-15 19:05:01 +01:00
parent b052a36294
commit cb9ff8bfa7
1 changed files with 29 additions and 12 deletions
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41
lib/lists.nix
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@ -16,17 +16,22 @@ rec {
*/
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).
/* “right fold” a binary function `op' between successive elements of
`list' with `nul' as the starting value, i.e.,
`foldr op nul [x_1 x_2 ... x_n] == op x_1 (op x_2 ... (op x_n nul))'.
Type:
foldr :: (a -> b -> b) -> b -> [a] -> b
Example:
concat = fold (a: b: a + b) "z"
concat = foldr (a: b: a + b) "z"
concat [ "a" "b" "c" ]
=> "abcz"
# different types
strange = foldr (int: str: toString (int + 1) + str) "a"
strange [ 1 2 3 4 ]
=> "2345a"
*/
fold = op: nul: list:
foldr = op: nul: list:
let
len = length list;
fold' = n:
@ -35,13 +40,25 @@ 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)'.
/* `fold' is an alias of `foldr' for historic reasons */
# FIXME(Profpatsch): deprecate?
fold = foldr;
/* “left fold”, like `foldr', but from the left:
`foldl op nul [x_1 x_2 ... x_n] == op (... (op (op nul x_1) x_2) ... x_n)`.
Type:
foldl :: (b -> a -> b) -> b -> [a] -> b
Example:
lconcat = foldl (a: b: a + b) "z"
lconcat [ "a" "b" "c" ]
=> "zabc"
# different types
lstrange = foldl (str: int: str + toString (int + 1)) ""
strange [ 1 2 3 4 ]
=> "a2345"
*/
foldl = op: nul: list:
let
@ -52,7 +69,7 @@ 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
@ -140,7 +157,7 @@ rec {
any isString [ 1 { } ]
=> false
*/
any = builtins.any or (pred: fold (x: y: if pred x then true else y) false);
any = builtins.any or (pred: foldr (x: y: if pred x then true else y) false);
/* Return true iff function `pred' returns true for all elements of
`list'.
@ -151,7 +168,7 @@ rec {
all (x: x < 3) [ 1 2 3 ]
=> false
*/
all = builtins.all or (pred: fold (x: y: if pred x then y else false) true);
all = builtins.all or (pred: foldr (x: y: if pred x then y else false) true);
/* Count how many times function `pred' returns true for the elements
of `list'.
@ -219,7 +236,7 @@ rec {
=> { right = [ 5 3 4 ]; wrong = [ 1 2 ]; }
*/
partition = builtins.partition or (pred:
fold (h: t:
foldr (h: t:
if pred h
then { right = [h] ++ t.right; wrong = t.wrong; }
else { right = t.right; wrong = [h] ++ t.wrong; }