lazy-seq

Lazy sequences

Lazy?

The list structure could be defined as

data Seq a = Nil | Cons a (Seq a)

The Cons constuctor takes two arguments, so there are four different laziness variants:

Cons (Strict a) (Strict (Seq a)) -- 1. fully strict
Cons (Lazy a)   (Strict (Seq a)) -- 2. lazy values
Cons (Strict a) (Lazy (Seq a))   -- 3. lazy structure
Cons (Lazy   a) (Lazy (Seq a))   -- 4. fully lazy

This module implements the third variant: lazy structure, but strict values.

Example

var ones = lazyseq.cons(1, function () { return ones; });
console.log(ones === ones.tail()); // true!

Why?

This package is originally made to optimise shrink operations in jsverify, a property-based testing library.

API

• nil : Seq a — Empty sequence.

• cons : (head : a, tail : Array a | Seq a | () → Array a | () → Seq a) → Seq a : Cons a value to the front of a sequence (list or thunk).

• .isNil : Boolean — Constant time check, whether the sequence is empty.

• .toString : () → String — String representation. Doesn't force the tail.

• .length : () → Nat — Return the length of the sequene. Forces the structure.

• .toArray : () → Array a — Convert the sequence to JavaScript array.

• .fold : (z : b, f : (a, () → b) → b) → b — Fold from right.

  fold nil x f        = x
  fold (cons h t) x f = f x (fold t x f)

• .head : () → a — Extract the first element of a sequence, which must be non-empty.

• .tail : () → Seq a — Return the tail of the sequence.

  tail nil        = nil
  tail (cons h t) = t

• .nth : (n : Nat) → a — Return nth value of the sequence.

• .take : (n : Nat) → Seq a — Take n first elements of the sequence.

• .drop : (n : Nat) → Seq a — Drop n first elements of the sequence.

• .map : (f : a → b) : Seq b — The sequence obtained by applying f to each element of the original sequence.

• .append : (ys : Seq a | Array a) : Seq a — Append ys sequence.

• .filter : (p : a -> bool) : Seq a — filter using p predicate.

• *.every : (p = identity: a -> b) : b | true — return first falsy value in the sequence, true otherwise. N.B. behaves slightly differently from Array::every.

• *.some : (p = identity: a -> b) : b | false — return first truthy value in the sequence, false otherwise. N.B. behaves slightly differently from Array::some.

• *.contains : (x : a) : bool — Returns true if x is in the sequence.

• *.containsNot : (x : a) : bool — Returns true if x is not in the sequence.

• fromArray: (arr : Array a) → Seq a — Convert a JavaScript array into lazy sequence.

• singleton: (x : a) → Seq a — Create a singleton sequence.

• append : (xs... : Array a | Seq a | () → Array a | () → Seq a) → Seq a : Append one sequence-like to another.

• iterate : (x : a, f : a → a) → Seq a — Create an infinite sequence of repeated applications of f to x: x, f(x), f(f(x))….

• fold : (seq : Seq a | Array a, z : b, f : (a, () → b) → b) : b — polymorphic version of fold. Works with arrays too.

Release History

• 1.0.0 — 2015-07-28 — Stable
  • Consider stable
  • singleton constructure
  • .contains, .containsNot, .every and .some methods
• 0.2.0 — 2015-04-21 — filter
• 0.1.0 — 2015-03-21 — append
• 0.0.2 — 2014-12-20 — Fixed fold
• 0.0.1 — 2014-12-20 — Initial release

Contributing

• README.md is generated from the source with ljs
• Before creating a pull request run make test, yet travis will do it for you.