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Monads for the Rest of Us

by Bruce Pascoe - 1 May, 2019

"A monad is just a monoid in the category of endofunctors. What's the problem?" ~James Iry[^1]

The problem... is that there are several problems.

It's been said that monads bear a dreadful curse. Once you finally understand what they are, you begin to see them everywhere--but somehow become completely incapable of explaining them to anyone else. Many tutorial writers have tried to break the Great Curse--the Web is lousy with bold attempts and half successes that attest to this--and just as many have failed. Well, I'm here to address the elephant in the room[^2] and tell you that I intend to break the Great Curse once and for all.

There are basically two ways a monad tutorial tends to go. One is a paragraph or two of minimal descriptions of one or two common monads (Haskell's Maybe in particular is very popular), followed by a lot of intimidating Haskell syntax trying to explain--precisely--how it all fits together. This is well

	#!/bin/bash
	################################################################################
	# Introduction
	################################################################################
	# This is an attempt at explaining how to vendor git repositories by storing
	# their _entire_ commit history inside of the local repository. By vendoring
	# dependencies using this technique, the remote repository can entirely
	# recreated from the local repository if the original were to be lost.
	# Additionally, clones of the local repository also satisfy that property.
	#

	const Benchmark = require("benchmark");

	console.log("setup");
	var suite = new Benchmark.Suite();

	class TestInheritance extends Set {
	constructor() { super() }
	op(q) { this.add(q) }
	}


	import alias from "@rollup/plugin-alias";
	import commonjs from "@rollup/plugin-commonjs";
	import resolve from "@rollup/plugin-node-resolve";
	import typescript from "@rollup/plugin-typescript";
	import { createFilter } from "@rollup/pluginutils";
	import surplusCompiler from "surplus/compiler";

	function surplus({ include, exclude, sourceMap = false }) {
	const filter = createFilter(include, exclude);

	type UnionToIntersection<
	T,
	U = T extends unknown ? { _: { [_ in T & PropertyKey]: never } } : {},
	V = keyof U[keyof U],

	// We need to remove the primitive we branded T with when we made it a
	// property key. However, if the union T contains a branded type already,
	// we must avoid stripping those brands off.
	X = true extends (T extends PropertyKey ? true : never) ? never : PropertyKey
	> = T extends never ? never : V extends X & infer W ? W : V;

	# To be used with current stable/prometheus-operator helm chart on kops clusters
	# where 4001/4002 ports are blocked from the worker nodes
	#
	# In you helm, disable etcd monitor by setting kubeEtcd.enabled=false
	# Then apply this yaml, and you should see the etcd stats on your main instance
	#
	apiVersion: monitoring.coreos.com/v1
	kind: Prometheus
	metadata:
	name: prometheus-operator-prometheus-master

	import jenkins.model.*;
	import hudson.security.*;

	// JVM did not like 'hypen' in the class name, it will crap out saying it is
	// illegal class name.
	class BuildPermission {
	static buildNewAccessList(userOrGroup, permissions) {
	def newPermissionsMap = [:]
	permissions.each {
	newPermissionsMap.put(Permission.fromId(it), userOrGroup)

	# to generate your dhparam.pem file, run in the terminal
	openssl dhparam -out /etc/nginx/ssl/dhparam.pem 2048

	-- show running queries (pre 9.2)
	SELECT procpid, age(clock_timestamp(), query_start), usename, current_query
	FROM pg_stat_activity
	WHERE current_query != '<IDLE>' AND current_query NOT ILIKE '%pg_stat_activity%'
	ORDER BY query_start desc;

	-- show running queries (9.2)
	SELECT pid, age(clock_timestamp(), query_start), usename, query
	FROM pg_stat_activity
	WHERE query != '<IDLE>' AND query NOT ILIKE '%pg_stat_activity%'