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Phantom types in ReasonML

Introduction

"A phantom type is a parametrised type whose parameters do not all appear on the right-hand side of its definition..." Haskell Wiki, PhantomType

The following write-up is intended as an introduction into using phantom types in ReasonML.

Taking a look at the above definition from the Haskell wiki, it states that phantom types are parametrised types where not all parameters appear on the right-hand side. Let's try to see if we can implement a similar example as in said wiki.

	#!/bin/bash
	# herein we backup our indexes! this script should run at like 6pm or something, after logstash
	# rotates to a new ES index and theres no new data coming in to the old one. we grab metadatas,
	# compress the data files, create a restore script, and push it all up to S3.
	TODAY=`date +"%Y.%m.%d"`
	INDEXNAME="logstash-$TODAY" # this had better match the index name in ES
	INDEXDIR="/usr/local/elasticsearch/data/logstash/nodes/0/indices/"
	BACKUPCMD="/usr/local/backupTools/s3cmd --config=/usr/local/backupTools/s3cfg put"
	BACKUPDIR="/mnt/es-backups/"
	YEARMONTH=`date +"%Y-%m"`

	(ns com.neilmock.enduro-redis
	(:require [alandipert.enduro :as enduro]
	[clojure.string :as str])
	(:import (alandipert.enduro EnduroAtom)
	java.net.URI
	(redis.clients.jedis Jedis JedisPool JedisPoolConfig)))

	(defn- jedis-pool [url]
	(let [uri (URI. url)
	host (.getHost uri)

	clojure.lang.ExceptionInfo: Error in component :figwheel-system in system com.stuartsierra.component.SystemMap calling #'com.stuartsierra.component/start {:reason :com.stuartsierra.component/component-function-threw-exception, :function #'com.stuartsierra.component/start, :system-key :figwheel-system, :component #figwheel_sidecar.system.FigwheelSystem{:system #object[clojure.lang.Atom 0x48eb05e9 {:status :ready, :val #<SystemMap>}]}, :system #<SystemMap>}
	at clojure.core$ex_info.invokeStatic(core.clj:4617)
	at clojure.core$ex_info.invoke(core.clj:4617)
	at com.stuartsierra.component$try_action.invokeStatic(component.cljc:119)
	at com.stuartsierra.component$try_action.invoke(component.cljc:116)
	at com.stuartsierra.component$update_system$fn__17933.invoke(component.cljc:139)
	at clojure.lang.ArraySeq.reduce(ArraySeq.java:109)
	at clojure.core$reduce.invokeStatic(core.clj:6544)
	at clojure.core$reduce.invoke(core.clj:6527)
	at com.stuartsierra.comp

	let rec merge x y =>
	switch (x, y) {
	\| ([], l) => l
	\| (l, []) => l
	\| ([hx, ...tx], [hy, ...ty]) =>
	if (hx < hy) {
	[hx, ...merge tx [hy, ...ty]]
	} else {
	[hy, ...merge [hx, ...tx] ty]
	}

	// A small SSH daemon providing bash sessions
	//
	// Server:
	// cd my/new/dir/
	// #generate server keypair
	// ssh-keygen -t rsa
	// go get -v .
	// go run sshd.go
	//
	// Client:

	user> (list 1 2 3)
	(1 2 3)

	user> (cons 1 (list 1 2 3))
	(1 1 2 3)