https://github.com/brandonbloom/dispatch-map

Persistent map with Clojure's multimethod dispatch semantics
https://github.com/brandonbloom/dispatch-map

Last synced: about 1 month ago
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Persistent map with Clojure's multimethod dispatch semantics

• Host: GitHub
• URL: https://github.com/brandonbloom/dispatch-map
• Owner: brandonbloom
• Created: 2012-12-10T07:10:09.000Z (about 12 years ago)
• Default Branch: master
• Last Pushed: 2024-06-05T05:00:04.000Z (7 months ago)
• Last Synced: 2024-11-14T17:51:16.275Z (about 1 month ago)
• Language: Clojure
• Size: 20.5 KB
• Stars: 29
• Watchers: 3
• Forks: 3
• Open Issues: 1
• Metadata Files:
  • Readme: README.md

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README

        
# dispatch-map

A Clojure library which provides the internals of Clojure's multimethods as a persistent, immutible map implementation.

Also includes an implementation of the multimethod macros and functions in terms of a dispatch-map and an atom.

## Motivation

See https://www.brandonbloom.name/blog/2012/12/10/clojures-multimethod-dispatch-as-library/

## Example

```clojure

(require '[dispatch-map.core :refer (dispatch-map)])

(derive ::rect ::shape)

(let [m (-> (dispatch-map identity

              [::rect ::shape] :rect-shape

              [::shape ::rect] :shape-rect)

            (prefer [::rect ::shape] [::shape ::rect]))]

  (println (m [::rect ::rect]))

  (println (m [::shape ::rect]))

  (println (m [::rect ::circle] :not-found)))

```

Prints:

```

:rect-shape

:shape-rect

:not-found

```

## IHierarchy Abstraction

Java's type hierarchy gets special treatment in Clojure's native multimethods via the `isa?` function.

You can now get similarly special treatment by implementing the  `-isa` method on the `dispatch-map.hierarchy/IHierarchy` protocol.

This is useful if you have a hiearchy defined by some other means that you don't wish to manually reproduce by successive calls to `derive`.

See [the tests](https://github.com/brandonbloom/dispatch-map/blob/93631c/test/dispatch_map/core_test.clj#L76-86) for an example.

Note that your `-isa` method must produce a result that is safe to memoize.

## DispatchMapFn

This is how you use the custom multimethod implementation. The only compelling reason to prefer this over

Clojure's native MultiFn or a raw dispatch-map is if you need to provide a custom IHierarchy implementation.

```clojure

(ns your-ns

  (:refer-clojure :exclude [defmulti defmethod remove-all-methods remove-method

                            prefer-method methods get-method prefers])

  (:use [dispatch-map.multi-fn])))

```

## License

Copyright © 2012 Brandon Bloom

Distributed under the Eclipse Public License, the same as Clojure.