# package ppx_deriving_yojson

Library

Module

Module type

Parameter

Class

Class type

List operations.

Some functions are flagged as not tail-recursive. A tail-recursive function uses constant stack space, while a non-tail-recursive function uses stack space proportional to the length of its list argument, which can be a problem with very long lists. When the function takes several list arguments, an approximate formula giving stack usage (in some unspecified constant unit) is shown in parentheses.

The above considerations can usually be ignored if your lists are not longer than about 10000 elements.

The labeled version of this module can be used as described in the `StdLabels`

module.

Compare the lengths of two lists. `compare_lengths l1 l2`

is equivalent to `compare (length l1) (length l2)`

, except that the computation stops after reaching the end of the shortest list.

Compare the length of a list to an integer. `compare_length_with l len`

is equivalent to `compare (length l) len`

, except that the computation stops after at most `len`

iterations on the list.

Return the `n`

-th element of the given list. The first element (head of the list) is at position 0.

Return the `n`

-th element of the given list. The first element (head of the list) is at position 0. Return `None`

if the list is too short.

`init len f`

is `f 0; f 1; ...; f (len-1)`

, evaluated left to right.

Concatenate two lists. Same function as the infix operator `@`

. Not tail-recursive (length of the first argument). The `@`

operator is not tail-recursive either.

`rev_append l1 l2`

reverses `l1`

and concatenates it with `l2`

. This is equivalent to `(`

`rev`

` l1) @ l2`

, but `rev_append`

is tail-recursive and more efficient.

Concatenate a list of lists. The elements of the argument are all concatenated together (in the same order) to give the result. Not tail-recursive (length of the argument + length of the longest sub-list).

Same as `concat`

. Not tail-recursive (length of the argument + length of the longest sub-list).

## Comparison

`equal eq [a1; ...; an] [b1; ..; bm]`

holds when the two input lists have the same length, and for each pair of elements `ai`

, `bi`

at the same position we have `eq ai bi`

.

Note: the `eq`

function may be called even if the lists have different length. If you know your equality function is costly, you may want to check `compare_lengths`

first.

`compare cmp [a1; ...; an] [b1; ...; bm]`

performs a lexicographic comparison of the two input lists, using the same `'a -> 'a -> int`

interface as `Stdlib.compare`

:

`a1 :: l1`

is smaller than`a2 :: l2`

(negative result) if`a1`

is smaller than`a2`

, or if they are equal (0 result) and`l1`

is smaller than`l2`

- the empty list
`[]`

is strictly smaller than non-empty lists

Note: the `cmp`

function will be called even if the lists have different lengths.

## Iterators

`iter f [a1; ...; an]`

applies function `f`

in turn to `a1; ...; an`

. It is equivalent to `begin f a1; f a2; ...; f an; () end`

.

Same as `iter`

, but the function is applied to the index of the element as first argument (counting from 0), and the element itself as second argument.

`map f [a1; ...; an]`

applies function `f`

to `a1, ..., an`

, and builds the list `[f a1; ...; f an]`

with the results returned by `f`

. Not tail-recursive.

Same as `map`

, but the function is applied to the index of the element as first argument (counting from 0), and the element itself as second argument. Not tail-recursive.

`filter_map f l`

applies `f`

to every element of `l`

, filters out the `None`

elements and returns the list of the arguments of the `Some`

elements.

`fold_left_map`

is a combination of `fold_left`

and `map`

that threads an accumulator through calls to `f`

.

`fold_left f init [b1; ...; bn]`

is `f (... (f (f init b1) b2) ...) bn`

.

`fold_right f [a1; ...; an] init`

is `f a1 (f a2 (... (f an init) ...))`

. Not tail-recursive.

## Iterators on two lists

`iter2 f [a1; ...; an] [b1; ...; bn]`

calls in turn `f a1 b1; ...; f an bn`

.

`map2 f [a1; ...; an] [b1; ...; bn]`

is `[f a1 b1; ...; f an bn]`

.

`fold_left2 f init [a1; ...; an] [b1; ...; bn]`

is `f (... (f (f init a1 b1) a2 b2) ...) an bn`

.

`fold_right2 f [a1; ...; an] [b1; ...; bn] init`

is `f a1 b1 (f a2 b2 (... (f an bn init) ...))`

.

## List scanning

`for_all f [a1; ...; an]`

checks if all elements of the list satisfy the predicate `f`

. That is, it returns `(f a1) && (f a2) && ... && (f an)`

for a non-empty list and `true`

if the list is empty.

`exists f [a1; ...; an]`

checks if at least one element of the list satisfies the predicate `f`

. That is, it returns `(f a1) || (f a2) || ... || (f an)`

for a non-empty list and `false`

if the list is empty.

Same as `for_all`

, but for a two-argument predicate.

Same as `exists`

, but for a two-argument predicate.

Same as `mem`

, but uses physical equality instead of structural equality to compare list elements.

## List searching

`find f l`

returns the first element of the list `l`

that satisfies the predicate `f`

.

`find f l`

returns the first element of the list `l`

that satisfies the predicate `f`

. Returns `None`

if there is no value that satisfies `f`

in the list `l`

.

`find_map f l`

applies `f`

to the elements of `l`

in order, and returns the first result of the form `Some v`

, or `None`

if none exist.

`filter f l`

returns all the elements of the list `l`

that satisfy the predicate `f`

. The order of the elements in the input list is preserved.

`find_all`

is another name for `filter`

.

Same as `filter`

, but the predicate is applied to the index of the element as first argument (counting from 0), and the element itself as second argument.

`partition f l`

returns a pair of lists `(l1, l2)`

, where `l1`

is the list of all the elements of `l`

that satisfy the predicate `f`

, and `l2`

is the list of all the elements of `l`

that do not satisfy `f`

. The order of the elements in the input list is preserved.

`val partition_map : ('a -> ('b, 'c) Either.t) -> 'a list -> 'b list * 'c list`

`partition_map f l`

returns a pair of lists `(l1, l2)`

such that, for each element `x`

of the input list `l`

:

- if
`f x`

is`Left y1`

, then`y1`

is in`l1`

, and - if
`f x`

is`Right y2`

, then`y2`

is in`l2`

.

The output elements are included in `l1`

and `l2`

in the same relative order as the corresponding input elements in `l`

.

In particular, `partition_map (fun x -> if f x then Left x else Right x) l`

is equivalent to `partition f l`

.

## Association lists

`assoc a l`

returns the value associated with key `a`

in the list of pairs `l`

. That is, `assoc a [ ...; (a,b); ...] = b`

if `(a,b)`

is the leftmost binding of `a`

in list `l`

.

`assoc_opt a l`

returns the value associated with key `a`

in the list of pairs `l`

. That is, `assoc_opt a [ ...; (a,b); ...] = Some b`

if `(a,b)`

is the leftmost binding of `a`

in list `l`

. Returns `None`

if there is no value associated with `a`

in the list `l`

.

Same as `assoc`

, but uses physical equality instead of structural equality to compare keys.

Same as `assoc_opt`

, but uses physical equality instead of structural equality to compare keys.

Same as `assoc`

, but simply return `true`

if a binding exists, and `false`

if no bindings exist for the given key.

Same as `mem_assoc`

, but uses physical equality instead of structural equality to compare keys.

`remove_assoc a l`

returns the list of pairs `l`

without the first pair with key `a`

, if any. Not tail-recursive.

Same as `remove_assoc`

, but uses physical equality instead of structural equality to compare keys. Not tail-recursive.

## Lists of pairs

Transform a list of pairs into a pair of lists: `split [(a1,b1); ...; (an,bn)]`

is `([a1; ...; an], [b1; ...; bn])`

. Not tail-recursive.

Transform a pair of lists into a list of pairs: `combine [a1; ...; an] [b1; ...; bn]`

is `[(a1,b1); ...; (an,bn)]`

.

## Sorting

Sort a list in increasing order according to a comparison function. The comparison function must return 0 if its arguments compare as equal, a positive integer if the first is greater, and a negative integer if the first is smaller (see Array.sort for a complete specification). For example, `Stdlib.compare`

is a suitable comparison function. The resulting list is sorted in increasing order. `sort`

is guaranteed to run in constant heap space (in addition to the size of the result list) and logarithmic stack space.

The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.

Same as `sort`

, but the sorting algorithm is guaranteed to be stable (i.e. elements that compare equal are kept in their original order).

The current implementation uses Merge Sort. It runs in constant heap space and logarithmic stack space.

Same as `sort`

or `stable_sort`

, whichever is faster on typical input.

Same as `sort`

, but also remove duplicates.

Merge two lists: Assuming that `l1`

and `l2`

are sorted according to the comparison function `cmp`

, `merge cmp l1 l2`

will return a sorted list containing all the elements of `l1`

and `l2`

. If several elements compare equal, the elements of `l1`

will be before the elements of `l2`

. Not tail-recursive (sum of the lengths of the arguments).

## Lists and Sequences

`val to_seq : 'a list -> 'a Seq.t`

Iterate on the list.

`val of_seq : 'a Seq.t -> 'a list`

Create a list from a sequence.