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nixpkgs/lib/lists.nix
Johannes Kirschbauer c8885b86b2
lib.foldl': document eta expansion
2024-03-16 22:58:14 +01:00

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/**
General list operations.
*/
{ lib }:
let
inherit (lib.strings) toInt;
inherit (lib.trivial) compare min id warn;
inherit (lib.attrsets) mapAttrs;
in
rec {
inherit (builtins) head tail length isList elemAt concatLists filter elem genList map;
/**
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.
# Inputs
`x`
: 1\. Function argument
# Type
```
singleton :: a -> [a]
```
# Examples
:::{.example}
## `lib.lists.singleton` usage example
```nix
singleton "foo"
=> [ "foo" ]
```
:::
*/
singleton = x: [x];
/**
Apply the function to each element in the list.
Same as `map`, but arguments flipped.
# Inputs
`xs`
: 1\. Function argument
`f`
: 2\. Function argument
# Type
```
forEach :: [a] -> (a -> b) -> [b]
```
# Examples
:::{.example}
## `lib.lists.forEach` usage example
```nix
forEach [ 1 2 ] (x:
toString x
)
=> [ "1" "2" ]
```
:::
*/
forEach = xs: f: map f xs;
/**
“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))`.
# Inputs
`op`
: 1\. Function argument
`nul`
: 2\. Function argument
`list`
: 3\. Function argument
# Type
```
foldr :: (a -> b -> b) -> b -> [a] -> b
```
# Examples
:::{.example}
## `lib.lists.foldr` usage example
```nix
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"
```
:::
*/
foldr = op: nul: list:
let
len = length list;
fold' = n:
if n == len
then nul
else op (elemAt list n) (fold' (n + 1));
in fold' 0;
/**
`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)`.
# Inputs
`op`
: 1\. Function argument
`nul`
: 2\. Function argument
`list`
: 3\. Function argument
# Type
```
foldl :: (b -> a -> b) -> b -> [a] -> b
```
# Examples
:::{.example}
## `lib.lists.foldl` usage example
```nix
lconcat = foldl (a: b: a + b) "z"
lconcat [ "a" "b" "c" ]
=> "zabc"
# different types
lstrange = foldl (str: int: str + toString (int + 1)) "a"
lstrange [ 1 2 3 4 ]
=> "a2345"
```
:::
*/
foldl = op: nul: list:
let
foldl' = n:
if n == -1
then nul
else op (foldl' (n - 1)) (elemAt list n);
in foldl' (length list - 1);
/**
Reduce a list by applying a binary operator from left to right,
starting with an initial accumulator.
Before each application of the operator, the accumulator value is evaluated.
This behavior makes this function stricter than [`foldl`](#function-library-lib.lists.foldl).
Unlike [`builtins.foldl'`](https://nixos.org/manual/nix/unstable/language/builtins.html#builtins-foldl'),
the initial accumulator argument is evaluated before the first iteration.
A call like
```nix
foldl' op acc₀ [ x₀ x₁ x₂ ... xₙ₋₁ xₙ ]
```
is (denotationally) equivalent to the following,
but with the added benefit that `foldl'` itself will never overflow the stack.
```nix
let
acc₁ = builtins.seq acc₀ (op acc₀ x₀ );
acc₂ = builtins.seq acc₁ (op acc₁ x₁ );
acc₃ = builtins.seq acc₂ (op acc₂ x₂ );
...
accₙ = builtins.seq accₙ₋₁ (op accₙ₋₁ xₙ₋₁);
accₙ₊₁ = builtins.seq accₙ (op accₙ xₙ );
in
accₙ₊₁
# Or ignoring builtins.seq
op (op (... (op (op (op acc₀ x₀) x₁) x₂) ...) xₙ₋₁) xₙ
```
# Inputs
`op`
: The binary operation to run, where the two arguments are:
1. `acc`: The current accumulator value: Either the initial one for the first iteration, or the result of the previous iteration
2. `x`: The corresponding list element for this iteration
`acc`
: The initial accumulator value.
The accumulator value is evaluated in any case before the first iteration starts.
To avoid evaluation even before the `list` argument is given an eta expansion can be used:
```nix
list: lib.foldl' op acc list
```
`list`
: The list to fold
# Type
```
foldl' :: (acc -> x -> acc) -> acc -> [x] -> acc
```
# Examples
:::{.example}
## `lib.lists.foldl'` usage example
```nix
foldl' (acc: x: acc + x) 0 [1 2 3]
=> 6
```
:::
*/
foldl' =
op:
acc:
# The builtin `foldl'` is a bit lazier than one might expect.
# See https://github.com/NixOS/nix/pull/7158.
# In particular, the initial accumulator value is not forced before the first iteration starts.
builtins.seq acc
(builtins.foldl' op acc);
/**
Map with index starting from 0
# Inputs
`f`
: 1\. Function argument
`list`
: 2\. Function argument
# Type
```
imap0 :: (int -> a -> b) -> [a] -> [b]
```
# Examples
:::{.example}
## `lib.lists.imap0` usage example
```nix
imap0 (i: v: "${v}-${toString i}") ["a" "b"]
=> [ "a-0" "b-1" ]
```
:::
*/
imap0 = f: list: genList (n: f n (elemAt list n)) (length list);
/**
Map with index starting from 1
# Inputs
`f`
: 1\. Function argument
`list`
: 2\. Function argument
# Type
```
imap1 :: (int -> a -> b) -> [a] -> [b]
```
# Examples
:::{.example}
## `lib.lists.imap1` usage example
```nix
imap1 (i: v: "${v}-${toString i}") ["a" "b"]
=> [ "a-1" "b-2" ]
```
:::
*/
imap1 = f: list: genList (n: f (n + 1) (elemAt list n)) (length list);
/**
Map and concatenate the result.
# Type
```
concatMap :: (a -> [b]) -> [a] -> [b]
```
# Examples
:::{.example}
## `lib.lists.concatMap` usage example
```nix
concatMap (x: [x] ++ ["z"]) ["a" "b"]
=> [ "a" "z" "b" "z" ]
```
:::
*/
concatMap = builtins.concatMap;
/**
Flatten the argument into a single list; that is, nested lists are
spliced into the top-level lists.
# Inputs
`x`
: 1\. Function argument
# Examples
:::{.example}
## `lib.lists.flatten` usage example
```nix
flatten [1 [2 [3] 4] 5]
=> [1 2 3 4 5]
flatten 1
=> [1]
```
:::
*/
flatten = x:
if isList x
then concatMap (y: flatten y) x
else [x];
/**
Remove elements equal to 'e' from a list. Useful for buildInputs.
# Inputs
`e`
: Element to remove from `list`
`list`
: The list
# Type
```
remove :: a -> [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.remove` usage example
```nix
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.
# Inputs
`pred`
: Predicate
`default`
: Default value to return if element was not found.
`multiple`
: Default value to return if more than one element was found
`list`
: Input list
# Type
```
findSingle :: (a -> bool) -> a -> a -> [a] -> a
```
# Examples
:::{.example}
## `lib.lists.findSingle` usage example
```nix
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 index in the list matching the specified
predicate or return `default` if no such element exists.
# Inputs
`pred`
: Predicate
`default`
: Default value to return
`list`
: Input list
# Type
```
findFirstIndex :: (a -> Bool) -> b -> [a] -> (Int | b)
```
# Examples
:::{.example}
## `lib.lists.findFirstIndex` usage example
```nix
findFirstIndex (x: x > 3) null [ 0 6 4 ]
=> 1
findFirstIndex (x: x > 9) null [ 0 6 4 ]
=> null
```
:::
*/
findFirstIndex =
pred:
default:
list:
let
# A naive recursive implementation would be much simpler, but
# would also overflow the evaluator stack. We use `foldl'` as a workaround
# because it reuses the same stack space, evaluating the function for one
# element after another. We can't return early, so this means that we
# sacrifice early cutoff, but that appears to be an acceptable cost. A
# clever scheme with "exponential search" is possible, but appears over-
# engineered for now. See https://github.com/NixOS/nixpkgs/pull/235267
# Invariant:
# - if index < 0 then el == elemAt list (- index - 1) and all elements before el didn't satisfy pred
# - if index >= 0 then pred (elemAt list index) and all elements before (elemAt list index) didn't satisfy pred
#
# We start with index -1 and the 0'th element of the list, which satisfies the invariant
resultIndex = foldl' (index: el:
if index < 0 then
# No match yet before the current index, we need to check the element
if pred el then
# We have a match! Turn it into the actual index to prevent future iterations from modifying it
- index - 1
else
# Still no match, update the index to the next element (we're counting down, so minus one)
index - 1
else
# There's already a match, propagate the index without evaluating anything
index
) (-1) list;
in
if resultIndex < 0 then
default
else
resultIndex;
/**
Find the first element in the list matching the specified
predicate or return `default` if no such element exists.
# Inputs
`pred`
: Predicate
`default`
: Default value to return
`list`
: Input list
# Type
```
findFirst :: (a -> bool) -> a -> [a] -> a
```
# Examples
:::{.example}
## `lib.lists.findFirst` usage example
```nix
findFirst (x: x > 3) 7 [ 1 6 4 ]
=> 6
findFirst (x: x > 9) 7 [ 1 6 4 ]
=> 7
```
:::
*/
findFirst =
pred:
default:
list:
let
index = findFirstIndex pred null list;
in
if index == null then
default
else
elemAt list index;
/**
Return true if function `pred` returns true for at least one
element of `list`.
# Inputs
`pred`
: Predicate
`list`
: Input list
# Type
```
any :: (a -> bool) -> [a] -> bool
```
# Examples
:::{.example}
## `lib.lists.any` usage example
```nix
any isString [ 1 "a" { } ]
=> true
any isString [ 1 { } ]
=> false
```
:::
*/
any = builtins.any;
/**
Return true if function `pred` returns true for all elements of
`list`.
# Inputs
`pred`
: Predicate
`list`
: Input list
# Type
```
all :: (a -> bool) -> [a] -> bool
```
# Examples
:::{.example}
## `lib.lists.all` usage example
```nix
all (x: x < 3) [ 1 2 ]
=> true
all (x: x < 3) [ 1 2 3 ]
=> false
```
:::
*/
all = builtins.all;
/**
Count how many elements of `list` match the supplied predicate
function.
# Inputs
`pred`
: Predicate
# Type
```
count :: (a -> bool) -> [a] -> int
```
# Examples
:::{.example}
## `lib.lists.count` usage example
```nix
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") firefox`).
# Inputs
`cond`
: 1\. Function argument
`elem`
: 2\. Function argument
# Type
```
optional :: bool -> a -> [a]
```
# Examples
:::{.example}
## `lib.lists.optional` usage example
```nix
optional true "foo"
=> [ "foo" ]
optional false "foo"
=> [ ]
```
:::
*/
optional = cond: elem: if cond then [elem] else [];
/**
Return a list or an empty list, depending on a boolean value.
# Inputs
`cond`
: Condition
`elems`
: List to return if condition is true
# Type
```
optionals :: bool -> [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.optionals` usage example
```nix
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.
# Inputs
`x`
: 1\. Function argument
# Examples
:::{.example}
## `lib.lists.toList` usage example
```nix
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`.
# Inputs
`first`
: First integer in the range
`last`
: Last integer in the range
# Type
```
range :: int -> int -> [int]
```
# Examples
:::{.example}
## `lib.lists.range` usage example
```nix
range 2 4
=> [ 2 3 4 ]
range 3 2
=> [ ]
```
:::
*/
range =
first:
last:
if first > last then
[]
else
genList (n: first + n) (last - first + 1);
/**
Return a list with `n` copies of an element.
# Inputs
`n`
: 1\. Function argument
`elem`
: 2\. Function argument
# Type
```
replicate :: int -> a -> [a]
```
# Examples
:::{.example}
## `lib.lists.replicate` usage example
```nix
replicate 3 "a"
=> [ "a" "a" "a" ]
replicate 2 true
=> [ true true ]
```
:::
*/
replicate = n: elem: genList (_: elem) n;
/**
Splits the elements of a list in two lists, `right` and
`wrong`, depending on the evaluation of a predicate.
# Inputs
`pred`
: Predicate
`list`
: Input list
# Type
```
(a -> bool) -> [a] -> { right :: [a]; wrong :: [a]; }
```
# Examples
:::{.example}
## `lib.lists.partition` usage example
```nix
partition (x: x > 2) [ 5 1 2 3 4 ]
=> { right = [ 5 3 4 ]; wrong = [ 1 2 ]; }
```
:::
*/
partition = builtins.partition;
/**
Splits the elements of a list into many lists, using the return value of a predicate.
Predicate should return a string which becomes keys of attrset `groupBy` returns.
`groupBy'` allows to customise the combining function and initial value
# Inputs
`op`
: 1\. Function argument
`nul`
: 2\. Function argument
`pred`
: 3\. Function argument
`lst`
: 4\. Function argument
# Examples
:::{.example}
## `lib.lists.groupBy'` usage example
```nix
groupBy (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
=> { true = [ 5 3 4 ]; false = [ 1 2 ]; }
groupBy (x: x.name) [ {name = "icewm"; script = "icewm &";}
{name = "xfce"; script = "xfce4-session &";}
{name = "icewm"; script = "icewmbg &";}
{name = "mate"; script = "gnome-session &";}
]
=> { icewm = [ { name = "icewm"; script = "icewm &"; }
{ name = "icewm"; script = "icewmbg &"; } ];
mate = [ { name = "mate"; script = "gnome-session &"; } ];
xfce = [ { name = "xfce"; script = "xfce4-session &"; } ];
}
groupBy' builtins.add 0 (x: boolToString (x > 2)) [ 5 1 2 3 4 ]
=> { true = 12; false = 3; }
```
:::
*/
groupBy' = op: nul: pred: lst: mapAttrs (name: foldl op nul) (groupBy pred lst);
groupBy = builtins.groupBy or (
pred: foldl' (r: e:
let
key = pred e;
in
r // { ${key} = (r.${key} or []) ++ [e]; }
) {});
/**
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.
# Inputs
`f`
: Function to zip elements of both lists
`fst`
: First list
`snd`
: Second list
# Type
```
zipListsWith :: (a -> b -> c) -> [a] -> [b] -> [c]
```
# Examples
:::{.example}
## `lib.lists.zipListsWith` usage example
```nix
zipListsWith (a: b: a + b) ["h" "l"] ["e" "o"]
=> ["he" "lo"]
```
:::
*/
zipListsWith =
f:
fst:
snd:
genList
(n: f (elemAt fst n) (elemAt snd n)) (min (length fst) (length snd));
/**
Merges two lists of the same size together. If the sizes aren't the same
the merging stops at the shortest.
# Inputs
`fst`
: First list
`snd`
: Second list
# Type
```
zipLists :: [a] -> [b] -> [{ fst :: a; snd :: b; }]
```
# Examples
:::{.example}
## `lib.lists.zipLists` usage example
```nix
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.
# Inputs
`xs`
: 1\. Function argument
# Type
```
reverseList :: [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.reverseList` usage example
```nix
reverseList [ "b" "o" "j" ]
=> [ "j" "o" "b" ]
```
:::
*/
reverseList = xs:
let l = length xs; in genList (n: elemAt xs (l - n - 1)) l;
/**
Depth-First Search (DFS) for lists `list != []`.
`before a b == true` means that `b` depends on `a` (there's an
edge from `b` to `a`).
# Inputs
`stopOnCycles`
: 1\. Function argument
`before`
: 2\. Function argument
`list`
: 3\. Function argument
# Examples
:::{.example}
## `lib.lists.listDfs` usage example
```nix
listDfs true hasPrefix [ "/home/user" "other" "/" "/home" ]
== { minimal = "/"; # minimal element
visited = [ "/home/user" ]; # seen elements (in reverse order)
rest = [ "/home" "other" ]; # everything else
}
listDfs true hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
== { cycle = "/"; # cycle encountered at this element
loops = [ "/" ]; # and continues to these elements
visited = [ "/" "/home/user" ]; # elements leading to the cycle (in reverse order)
rest = [ "/home" "other" ]; # everything else
```
:::
*/
listDfs = stopOnCycles: before: list:
let
dfs' = us: visited: rest:
let
c = filter (x: before x us) visited;
b = partition (x: before x us) rest;
in if stopOnCycles && (length c > 0)
then { cycle = us; loops = c; inherit visited rest; }
else if length b.right == 0
then # nothing is before us
{ minimal = us; inherit visited rest; }
else # grab the first one before us and continue
dfs' (head b.right)
([ us ] ++ visited)
(tail b.right ++ b.wrong);
in dfs' (head list) [] (tail list);
/**
Sort a list based on a partial ordering using DFS. This
implementation is O(N^2), if your ordering is linear, use `sort`
instead.
`before a b == true` means that `b` should be after `a`
in the result.
# Inputs
`before`
: 1\. Function argument
`list`
: 2\. Function argument
# Examples
:::{.example}
## `lib.lists.toposort` usage example
```nix
toposort hasPrefix [ "/home/user" "other" "/" "/home" ]
== { result = [ "/" "/home" "/home/user" "other" ]; }
toposort hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
== { cycle = [ "/home/user" "/" "/" ]; # path leading to a cycle
loops = [ "/" ]; } # loops back to these elements
toposort hasPrefix [ "other" "/home/user" "/home" "/" ]
== { result = [ "other" "/" "/home" "/home/user" ]; }
toposort (a: b: a < b) [ 3 2 1 ] == { result = [ 1 2 3 ]; }
```
:::
*/
toposort = before: list:
let
dfsthis = listDfs true before list;
toporest = toposort before (dfsthis.visited ++ dfsthis.rest);
in
if length list < 2
then # finish
{ result = list; }
else if dfsthis ? cycle
then # there's a cycle, starting from the current vertex, return it
{ cycle = reverseList ([ dfsthis.cycle ] ++ dfsthis.visited);
inherit (dfsthis) loops; }
else if toporest ? cycle
then # there's a cycle somewhere else in the graph, return it
toporest
# Slow, but short. Can be made a bit faster with an explicit stack.
else # there are no cycles
{ result = [ dfsthis.minimal ] ++ toporest.result; };
/**
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.
See also [`sortOn`](#function-library-lib.lists.sortOn), which applies the
default comparison on a function-derived property, and may be more efficient.
# Inputs
`comparator`
: 1\. Function argument
`list`
: 2\. Function argument
# Type
```
sort :: (a -> a -> Bool) -> [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.sort` usage example
```nix
sort (p: q: p < q) [ 5 3 7 ]
=> [ 3 5 7 ]
```
:::
*/
sort = builtins.sort;
/**
Sort a list based on the default comparison of a derived property `b`.
The items are returned in `b`-increasing order.
**Performance**:
The passed function `f` is only evaluated once per item,
unlike an unprepared [`sort`](#function-library-lib.lists.sort) using
`f p < f q`.
**Laws**:
```nix
sortOn f == sort (p: q: f p < f q)
```
# Inputs
`f`
: 1\. Function argument
`list`
: 2\. Function argument
# Type
```
sortOn :: (a -> b) -> [a] -> [a], for comparable b
```
# Examples
:::{.example}
## `lib.lists.sortOn` usage example
```nix
sortOn stringLength [ "aa" "b" "cccc" ]
=> [ "b" "aa" "cccc" ]
```
:::
*/
sortOn = f: list:
let
# Heterogenous list as pair may be ugly, but requires minimal allocations.
pairs = map (x: [(f x) x]) list;
in
map
(x: builtins.elemAt x 1)
(sort
# Compare the first element of the pairs
# Do not factor out the `<`, to avoid calls in hot code; duplicate instead.
(a: b: head a < head b)
pairs);
/**
Compare two lists element-by-element.
# Inputs
`cmp`
: 1\. Function argument
`a`
: 2\. Function argument
`b`
: 3\. Function argument
# Examples
:::{.example}
## `lib.lists.compareLists` usage example
```nix
compareLists compare [] []
=> 0
compareLists compare [] [ "a" ]
=> -1
compareLists compare [ "a" ] []
=> 1
compareLists compare [ "a" "b" ] [ "a" "c" ]
=> -1
```
:::
*/
compareLists = cmp: a: b:
if a == []
then if b == []
then 0
else -1
else if b == []
then 1
else let rel = cmp (head a) (head b); in
if rel == 0
then compareLists cmp (tail a) (tail b)
else rel;
/**
Sort list using "Natural sorting".
Numeric portions of strings are sorted in numeric order.
# Inputs
`lst`
: 1\. Function argument
# Examples
:::{.example}
## `lib.lists.naturalSort` usage example
```nix
naturalSort ["disk11" "disk8" "disk100" "disk9"]
=> ["disk8" "disk9" "disk11" "disk100"]
naturalSort ["10.46.133.149" "10.5.16.62" "10.54.16.25"]
=> ["10.5.16.62" "10.46.133.149" "10.54.16.25"]
naturalSort ["v0.2" "v0.15" "v0.0.9"]
=> [ "v0.0.9" "v0.2" "v0.15" ]
```
:::
*/
naturalSort = lst:
let
vectorise = s: map (x: if isList x then toInt (head x) else x) (builtins.split "(0|[1-9][0-9]*)" s);
prepared = map (x: [ (vectorise x) x ]) lst; # remember vectorised version for O(n) regex splits
less = a: b: (compareLists compare (head a) (head b)) < 0;
in
map (x: elemAt x 1) (sort less prepared);
/**
Return the first (at most) N elements of a list.
# Inputs
`count`
: Number of elements to take
`list`
: Input list
# Type
```
take :: int -> [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.take` usage example
```nix
take 2 [ "a" "b" "c" "d" ]
=> [ "a" "b" ]
take 2 [ ]
=> [ ]
```
:::
*/
take =
count: sublist 0 count;
/**
Remove the first (at most) N elements of a list.
# Inputs
`count`
: Number of elements to drop
`list`
: Input list
# Type
```
drop :: int -> [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.drop` usage example
```nix
drop 2 [ "a" "b" "c" "d" ]
=> [ "c" "d" ]
drop 2 [ ]
=> [ ]
```
:::
*/
drop =
count:
list: sublist count (length list) list;
/**
Whether the first list is a prefix of the second list.
# Inputs
`list1`
: 1\. Function argument
`list2`
: 2\. Function argument
# Type
```
hasPrefix :: [a] -> [a] -> bool
```
# Examples
:::{.example}
## `lib.lists.hasPrefix` usage example
```nix
hasPrefix [ 1 2 ] [ 1 2 3 4 ]
=> true
hasPrefix [ 0 1 ] [ 1 2 3 4 ]
=> false
```
:::
*/
hasPrefix =
list1:
list2:
take (length list1) list2 == list1;
/**
Remove the first list as a prefix from the second list.
Error if the first list isn't a prefix of the second list.
# Inputs
`list1`
: 1\. Function argument
`list2`
: 2\. Function argument
# Type
```
removePrefix :: [a] -> [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.removePrefix` usage example
```nix
removePrefix [ 1 2 ] [ 1 2 3 4 ]
=> [ 3 4 ]
removePrefix [ 0 1 ] [ 1 2 3 4 ]
=> <error>
```
:::
*/
removePrefix =
list1:
list2:
if hasPrefix list1 list2 then
drop (length list1) list2
else
throw "lib.lists.removePrefix: First argument is not a list prefix of the second argument";
/**
Return a list consisting of at most `count` elements of `list`,
starting at index `start`.
# Inputs
`start`
: Index at which to start the sublist
`count`
: Number of elements to take
`list`
: Input list
# Type
```
sublist :: int -> int -> [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.sublist` usage example
```nix
sublist 1 3 [ "a" "b" "c" "d" "e" ]
=> [ "b" "c" "d" ]
sublist 1 3 [ ]
=> [ ]
```
:::
*/
sublist =
start:
count:
list:
let len = length list; in
genList
(n: elemAt list (n + start))
(if start >= len then 0
else if start + count > len then len - start
else count);
/**
The common prefix of two lists.
# Inputs
`list1`
: 1\. Function argument
`list2`
: 2\. Function argument
# Type
```
commonPrefix :: [a] -> [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.commonPrefix` usage example
```nix
commonPrefix [ 1 2 3 4 5 6 ] [ 1 2 4 8 ]
=> [ 1 2 ]
commonPrefix [ 1 2 3 ] [ 1 2 3 4 5 ]
=> [ 1 2 3 ]
commonPrefix [ 1 2 3 ] [ 4 5 6 ]
=> [ ]
```
:::
*/
commonPrefix =
list1:
list2:
let
# Zip the lists together into a list of booleans whether each element matches
matchings = zipListsWith (fst: snd: fst != snd) list1 list2;
# Find the first index where the elements don't match,
# which will then also be the length of the common prefix.
# If all elements match, we fall back to the length of the zipped list,
# which is the same as the length of the smaller list.
commonPrefixLength = findFirstIndex id (length matchings) matchings;
in
take commonPrefixLength list1;
/**
Return the last element of a list.
This function throws an error if the list is empty.
# Inputs
`list`
: 1\. Function argument
# Type
```
last :: [a] -> a
```
# Examples
:::{.example}
## `lib.lists.last` usage example
```nix
last [ 1 2 3 ]
=> 3
```
:::
*/
last = list:
assert lib.assertMsg (list != []) "lists.last: list must not be empty!";
elemAt list (length list - 1);
/**
Return all elements but the last.
This function throws an error if the list is empty.
# Inputs
`list`
: 1\. Function argument
# Type
```
init :: [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.init` usage example
```nix
init [ 1 2 3 ]
=> [ 1 2 ]
```
:::
*/
init = list:
assert lib.assertMsg (list != []) "lists.init: list must not be empty!";
take (length list - 1) list;
/**
Return the image of the cross product of some lists by a function.
# Examples
:::{.example}
## `lib.lists.crossLists` usage example
```nix
crossLists (x:y: "${toString x}${toString y}") [[1 2] [3 4]]
=> [ "13" "14" "23" "24" ]
```
:::
*/
crossLists = warn
"lib.crossLists is deprecated, use lib.cartesianProductOfSets instead."
(f: foldl (fs: args: concatMap (f: map f args) fs) [f]);
/**
Remove duplicate elements from the `list`. O(n^2) complexity.
# Inputs
`list`
: Input list
# Type
```
unique :: [a] -> [a]
```
# Examples
:::{.example}
## `lib.lists.unique` usage example
```nix
unique [ 3 2 3 4 ]
=> [ 3 2 4 ]
```
:::
*/
unique = foldl' (acc: e: if elem e acc then acc else acc ++ [ e ]) [];
/**
Check if list contains only unique elements. O(n^2) complexity.
# Inputs
`list`
: 1\. Function argument
# Type
```
allUnique :: [a] -> bool
```
# Examples
:::{.example}
## `lib.lists.allUnique` usage example
```nix
allUnique [ 3 2 3 4 ]
=> false
allUnique [ 3 2 4 1 ]
=> true
```
:::
*/
allUnique = list: (length (unique list) == length list);
/**
Intersects list 'list1' and another list (`list2`).
O(nm) complexity.
# Inputs
`list1`
: First list
`list2`
: Second list
# Examples
:::{.example}
## `lib.lists.intersectLists` usage example
```nix
intersectLists [ 1 2 3 ] [ 6 3 2 ]
=> [ 3 2 ]
```
:::
*/
intersectLists = e: filter (x: elem x e);
/**
Subtracts list 'e' from another list (`list2`).
O(nm) complexity.
# Inputs
`e`
: First list
`list2`
: Second list
# Examples
:::{.example}
## `lib.lists.subtractLists` usage example
```nix
subtractLists [ 3 2 ] [ 1 2 3 4 5 3 ]
=> [ 1 4 5 ]
```
:::
*/
subtractLists = e: filter (x: !(elem x e));
/**
Test if two lists have no common element.
It should be slightly more efficient than (intersectLists a b == [])
# Inputs
`a`
: 1\. Function argument
`b`
: 2\. Function argument
*/
mutuallyExclusive = a: b: length a == 0 || !(any (x: elem x a) b);
}
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