Code as data is the mantra. Functions and closures as data. So why not
objects as data? What I propose is nothing new, but perhaps a new style.
Making objects from map structures is simple enough in Clojure. And easy
enough to put functions in a map. So why not closures? A closure in a map
is a lot like an object method, hmm?
I found clojure components to be inspirational. But as light-weight as they
are, components are still too heavy-weight to be objects. But a simple map
seems ideal. And with only a map instead of a record or deftype,
composition is simplicity itself. But the key idea here comes from clojure
components: contents of the map should be configuration parameters or
architecture, but not state. Put state in an atom and then (optionally) put
the atom in the map. But once an object is formed, the contents of the map
should not change. There should be no need to update a reference to this
map.
Below is what I am calling a Clojure Object. Like a Java object, the method
holds both data and methods (closures). Note that, because we are using
closures, local data can be accessed without having to be put in the map.
For example, the file-channel variable is not accessed via the map and need
not have been added to the map.
Bill
(ns aatree.db-file
(:require [clojure.tools.logging :as log])
(:import (java.nio.channels FileChannel)
(java.nio.file OpenOption StandardOpenOption)))
(defn db-file-open
([file opts]
(db-file-open (assoc opts :db-file file)))
([opts]
(if (not (:db-file opts))
(throw (Exception. "missing :db-file option")))
(let [file (:db-file opts)
^FileChannel file-channel
(FileChannel/open (.toPath file)
(into-array OpenOption
[StandardOpenOption/CREATE
StandardOpenOption/READ
StandardOpenOption/WRITE]))
opts (assoc opts :db-file-channel file-channel)
opts (assoc opts
:db-close
(fn []
(try
(.close file-channel)
(catch Exception e
(log/warn e "exception on close of db-file")))
(dissoc opts :db-file-channel)))
opts (assoc opts
:db-file-empty?
(fn []
(= 0 (.size file-channel))))
opts (assoc opts
:db-file-read
(fn [byte-buffer position]
(.read file-channel byte-buffer position)))
opts (assoc opts
:db-file-write
(fn [byte-buffer position]
(.write file-channel byte-buffer position)))
opts (assoc opts
:db-file-write-root
(fn [byte-buffer position]
(.force file-channel true)
(.write file-channel byte-buffer position)
(.force file-channel true)))
opts (assoc opts
:db-file-force
(fn []
(.force file-channel true)))]
opts)))
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