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@thi.ng/seq

seq

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This project is part of the @thi.ng/umbrella monorepo.

About

Various implementations of the @thi.ng/api ISeq interface / sequence abstraction and related tooling (inspired by Clojure). Think of ISeqs as readonly sequential views & cursors of an underlying (not necessarily sequential) collection...

Unlike with ES6 iterators, ISeq.next() is decoupled from an iteration step and merely produces a new view / sequence head of the remaining sequence values. This allows forking & sharing the sequence head(s) among multiple consumers, each able to read the remaining values at their own pace.

Status

ALPHA - bleeding edge / work-in-progress

Installation

yarn add @thi.ng/seq
// ES module
<script type="module" src="https://unpkg.com/@thi.ng/seq?module" crossorigin></script>

// UMD
<script src="https://unpkg.com/@thi.ng/seq/lib/index.umd.js" crossorigin></script>

Package sizes (gzipped, pre-treeshake): ESM: 537 bytes / CJS: 623 bytes / UMD: 683 bytes

Dependencies

API

Generated API docs

import { aseq, rseq, concat, iterator } from "@thi.ng/seq";

// create a sequence abstraction of an array
const a = aseq([1,2,3]);

// aseq returns `undefined` for nullish or empty array arguments
// this is in accordance w/ the `ISeqable` interface
// see further below...
aseq([])
// undefined

// first() returns first value of seq (or undefined if empty)
a.first()
// 1

// next() returns new seq of remaining items
a.next()
// { first: [Function: first], next: [Function: next] }

a.next().first();
// 2
a.next().next().first();
// 3

// if the resulting sub-sequence is empty, next() returns undefined
a.next().next().next();
// undefined

// aseq() can take optional index range args
aseq([10, 20, 30, 40], 2).first()
// 30
aseq([10, 20, 30, 40], 2).next().first()
// 40

// the iterator here is only used for demo purposes
// (i.e. to convert the sequence to a standard ES6 iterable & show the result)
[...iterator(aseq([10, 20, 30, 40], 2))]
// [30, 40]

// rseq() produces a reverse sequence of the given array
rseq([1, 2, 3]).first()
// 3
rseq([1, 2, 3]).next().first()
// 2

// index ranges only (start MUST be > end)
[...iterator(rseq([0, 1, 2, 3, 4], 2))]
// [2, 1, 0]
[...iterator(rseq([0, 1, 2, 3, 4], 3, 1))]
// [3, 2]

// values can be prepended via cons()
[...iterator(cons(42, aseq([1, 2, 3])))]
// [ 42, 1, 2, 3 ]

// create new (or single value) seqs
cons(42).first()
// 42
cons(42).next()
// undefined

// zero-copy concat (supporting nullable parts/sub-sequences)
[...iterator(concat(null, aseq([]), aseq([1, 2]), undefined, cons(3)))]
// [ 1, 2, 3 ]

// if only arrays are used as sources, can also use concatA()
[...iterator(concatA(null, [], [1, 2], undefined, [3]))]
// [ 1, 2, 3 ]

ISeqable support

Since the entire approach is interface based, sequences can be defined for any custom datatype (preferably via the ISeqable interface), for example here using @thi.ng/dcons:

import { dcons } from "@thi.ng/dcons";

// concat reversed array with doubly-linked list
[...iterator(concat(rseq([1, 2, 3]), dcons([4, 5, 6])))]
// [ 3, 2, 1, 4, 5, 6 ]

Lazy sequences

Lazily instantiated (possibly infinite) sequences can be created via lazyseq(). This function returns an ISeq which only realizes its values when they're requested.

import { defmultiN } from "@thi.ng/defmulti";

// defmultiN only used here to define multiple arities for `fib`
const fib = defmultiN({
    0: () => fib(0, 1),
    2: (a, b) => lazyseq(() => {
        console.log(`realize: ${a}`);
        return cons(a, fib(b, a + b));
    })
});

fib()
// { first: [Function: first], next: [Function: next] }

fib().first()
// realize: 0
// 0

fib().next().first()
// realize: 0
// realize: 1
// 1

fib().next().next().first()
// realize: 0
// realize: 1
// realize: 1
// 1

fib().next().next().next().first()
// realize: 0
// realize: 1
// realize: 1
// realize: 2
// 2

fib().next().next().next().next().first()
// realize: 0
// realize: 1
// realize: 1
// realize: 2
// realize: 3
// 3

Authors

Karsten Schmidt

License

© 2019 - 2020 Karsten Schmidt // Apache Software License 2.0

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