# ht.el

The missing hash table library for Emacs.

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**Table of Contents**

- [ht.el](#htel)
    - [Functions](#functions)
        - [Return a hash table](#return-a-hash-table)
        - [Accessing the hash table](#accessing-the-hash-table)
        - [Mutating the hash table](#mutating-the-hash-table)
        - [Iterating over the hash table](#iterating-over-the-hash-table)
        - [Predicates](#predicates)
        - [Converting from a hash table](#converting-from-a-hash-table)
        - [Converting to a hash table](#converting-to-a-hash-table)
    - [Macros](#macros)
        - [Returning a hash table](#returning-a-hash-table)
        - [Iterating over the hash table (anaphoric)](#iterating-over-the-hash-table-anaphoric)
    - [Examples](#examples)
    - [Why?](#why)
        - [Similar libraries](#similar-libraries)
    - [Installation](#installation)
    - [Changelog](#changelog)
    - [Running tests](#running-tests)
    - [What's an alist/plist?](#whats-an-alistplist)

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## Functions

### Return a hash table

* `ht-create` `(test?)`
* `ht-merge` `(&rest tables)`
* `ht-copy` `(table)`
* `ht-select` `(function table)`
* `ht-reject` `(function table)`
* `ht-select-keys` `(table keys)`

### Accessing the hash table

* `ht-get` `(table key default?)`
* `ht-get*` `(table &rest keys)`
* `ht-keys` `(table)`
* `ht-values` `(table)`
* `ht-items` `(table)`
* `ht-find` `(function table)`
* `ht-size` `(table)`

### Mutating the hash table

* `ht-set!` `(table key value)`
* `ht-update!` `(table table)`
* `ht-remove!` `(table key)`
* `ht-clear!` `(table)`
* `ht-reject!` `(function table)`

### Iterating over the hash table

* `ht-map` `(function table)`
* `ht-each` `(function table)`

### Predicates

* `ht?` `(table-or-object)`
* `ht-contains?` `(table key)`
* `ht-equal?` `(table1 table2)`
* `ht-empty?` `(table)`

### Converting from a hash table

* `ht->alist` `(table)`
* `ht->plist` `(table)`

### Converting to a hash table

* `ht<-alist` `(alist test?)`
* `ht<-plist` `(plist test?)`

## Macros

### Returning a hash table

* `ht` `(&rest pairs)`

### Iterating over the hash table (anaphoric)

* `ht-amap` `(form table)`
* `ht-aeach` `(form table)`

## Examples

Creating a hash table and accessing it:

``` emacs-lisp
(require 'ht)

(defun say-hello (name)
  (let ((greetings (ht ("Bob" "Hey bob!")
                       ("Chris" "Hi Chris!"))))
    (ht-get greetings name "Hello stranger!")))
```

This could be alternatively written as:

``` emacs-lisp
(require 'ht)

(defun say-hello (name)
  (let ((greetings (ht-create)))
    (ht-set! greetings "Bob" "Hey Bob!")
    (ht-set! greetings "Chris" "Hi Chris!")
    (ht-get greetings name "Hello stranger!")))
```

Accessing nested hash tables:

``` emacs-lisp
(let ((alphabets (ht ("Greek" (ht (1 (ht ('letter "α")
                                         ('name "alpha")))
                                  (2 (ht ('letter "β")
                                         ('name "beta")))))
                     ("English" (ht (1 (ht ('letter "a")
                                           ('name "A")))
                                    (2 (ht ('letter "b")
                                           ('name "B"))))))))
  (ht-get* alphabets "Greek" 1 'letter))  ; => "α"
```

`ht-get` and `ht-get*` have gv-setters and so will work with `setf`:

``` emacs-lisp
(let ((table (ht-create)))
  (ht-set! table 1 "A"))
```

is equivalent to

``` emacs-lisp
(let ((table (ht-create)))
  (setf (ht-get table 1) "A"))
```

and

``` emacs-lisp
(let ((table (ht (1 (ht (2 (ht (3 "three"))))))))
  (ht-set! (ht-get (ht-get table 1) 2) 3 :three))
```

is equivalent to

``` emacs-lisp
(let ((table (ht (1 (ht (2 (ht (3 "three"))))))))
  (setf (ht-get* table 1 2 3) :three))
```

## Why?

Libraries like [s.el](https://github.com/magnars/s.el) (strings) and
[dash.el](https://github.com/magnars/dash.el) (lists) have shown how
much nicer Emacs lisp programming can be with good libraries. ht.el
aims to similarly simplify working with hash tables.

Common operations with hash tables (e.g. enumerate the keys) are too
difficult in Emacs lisp.

ht.el offers:

* A consistent naming scheme (contrast `make-hash-table` with `puthash`)
* A more natural argument ordering
* Mutation functions always return `nil`
* A more comprehensive range of hash table operations, including a
  conventional map (`ht-map` returns a list, elisp's `maphash` returns
  nil).

### Similar libraries

* [kv.el](https://github.com/nicferrier/emacs-kv) (focuses more on
  alists)
* [mon-hash-utils](http://www.emacswiki.org/emacs/mon-hash-utils.el)

## Installation

ht.el is available on [MELPA](https://melpa.org/) (recommended) and
[Marmalade](http://marmalade-repo.org/).

Add MELPA to your .emacs.d/init.el:

``` emacs-lisp
(require 'package)
(add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/") t)
```

then run `M-x package-install <RET> ht <RET>`.

## Changelog

ht.el uses semantic versioning, so an incompatible API change will
result in the major version increasing. See
[CHANGELOG.md](CHANGELOG.md) for a history of all changes.

## Running tests

`M-x ht-run-tests`

## What's an alist/plist?

An alist is an association list, which is a list of pairs. It looks like this:

    ((key1 . value1)
     (key2 . value2)
     (key3 . value3))

An alist can also look like this:

    ((key1 . value1)
     (key2 . value2)
     (key1 . oldvalue))

A plist is a property list, which is a flat list with an even number
of items. It looks like this:

    (key1 value1
     key2 value2
     key3 value3)

Both of these are slow. ht.el provides `ht<-alist` and
`ht<-plist` to help you convert to hash tables. If you need to
work with an alist or plist, use the functions `ht->alist` and
`ht->plist` to convert an hash table to those formats.
