Category Archives: Lisp Flavored Erlang

Ibid.

During my interview with Gene Kim on for Functional Geekery, Episode 128, Gene talked about how he had a problem he was asking different people for how they would solve it nicely with a functional approach, to see how to improve his Clojure solution to be more idiomatic.

His problem was on “rewriting” Ibid. entries in citation references, to get the authors names instead of the Ibid. value, as Ibid. is a shorthand that stands for “the authors listed in the entry before this”.

As he was describing this problem, I was picturing the general pseudo-code with a pattern match in my head. To be fair, this has come from a number of years of getting used to thinking in a functional style as well as thinking in a pattern matching style.

The following Erlang code is a close representation to the pseudo-code that was in my head.

-module(ibid).

-export([ibid/1]).

ibid(Authors) ->
    ibid(Authors, []).

ibid([], UpdatedAuthors) ->
    {ok, lists:reverse(UpdatedAuthors)};
ibid(["Ibid." | _], []) ->
    {error, "No Previous Author for 'Ibid.' citation"};
ibid(["Ibid." | T], UpdatedAuthors=[H | _]) ->
    ibid(T, [H | UpdatedAuthors]);
ibid([H | T], UpdatedAuthors) ->
    ibid(T, [H | UpdatedAuthors]).

Running this in the Erlang shell using erl results in the following

> ibid:ibid(["Mike Nygard", "Gene Kim", "Ibid.", "Ibid.", "Nicole Forsgren", "Ibid.", "Jez Humble", "Gene Kim", "Ibid."]).
{ok,["Mike Nygard","Gene Kim","Gene Kim","Gene Kim",
     "Nicole Forsgren","Nicole Forsgren","Jez Humble","Gene Kim",
     "Gene Kim"]}
> ibid:ibid(["Ibid."]).
{error,"No Previous Author for 'Ibid.' citation"}

Throughout the editing of the podcast, I continued to think about his problem, and how I would approach it in Clojure without built-in pattern matching, and came up with the following using a cond instead of a pure pattern matching solution:

(defn
  update_ibids
  ([authors] (update_ibids authors []))
  ([[citation_author & rest_authors :as original_authors] [last_author & _ :as new_authors]]
    (let [ibid? (fn [author] (= "Ibid." author))]
      (cond
        (empty? original_authors) (reverse new_authors)
        (and (ibid? citation_author) (not last_author))
          (throw (Exception. "Found `Ibid.` with no previous author"))
        :else (recur
          rest_authors
          (cons
            (if (ibid? citation_author)
                last_author
                citation_author)
            new_authors))))))

And if we run this in the Clojure REPL we get the following:

user=> (def references ["Gene Kim", "Jez Humble", "Ibid.", "Gene Kim", "Ibid.", "Ibid.", "Nicole Forsgren", "Micheal Nygard", "Ibid."])

user=> (update_ibids [])
()
user=> (update_ibids ["Ibid."])
Execution error at user/update-ibids (REPL:8).
Found `Ibid.` with no previous author
user=> (update_ibids references)
("Gene Kim" "Jez Humble" "Jez Humble" "Gene Kim" "Gene Kim" "Gene Kim" "Nicole Forsgren" "Micheal Nygard" "Micheal Nygard")

That solution didn’t sit well with me (and if there is a more idiomatic way to write it I would love some of your solutions as well), and because of that, I wanted to see what could be done using the core.match library, which moves towards the psuedo-code I was picturing.

(ns ibid
  (:require [clojure.core.match :refer [match]]))


(defn
  update_ibids
  ([authors] (update_ibids authors []))
  ([orig updated]
    (match [orig updated]
      [[] new_authors] (reverse new_authors)
      [["Ibid." & _] []] (throw (Exception. "Found `Ibid.` with no previous author"))
      [["Ibid." & r] ([last_author & _] :seq) :as new_authors] (recur r (cons last_author new_authors))
      [[author & r] new_authors] (recur r (cons author new_authors)) )))

And if you are trying this yourself, don’t forget to add to your deps.edn file:

{:deps
  {org.clojure/core.match {:mvn/version "0.3.0"}}

After the first couple of itches were scratched, Gene shared on Twitter Stephen Mcgill’s solution and his solution inspired by Stephen’s.

And then, just for fun (or “just for defun” if you prefer the pun intended version), I did a version in LFE (Lisp Flavored Erlang) due to it being a Lisp with built in pattern matching from being on the Erlang runtime.

(defmodule ibid
  (export (ibid 1)))


(defun ibid [authors]
  (ibid authors '[]))


(defun ibid
  ([[] updated]
    (tuple 'ok (: lists reverse updated)))
  (((cons "Ibid." _) '[])
    (tuple 'error "No Previous Author for 'Ibid.' citation"))
  ([(cons "Ibid." authors) (= (cons h _) updated)]
    (ibid authors (cons h updated)))
  ([(cons h rest) updated]
    (ibid rest (cons h updated))))

Which if we call it in LFE’s REPL gives us the following:

lfe> (: ibid ibid '["Mike Nygard" "Gene Kim" "Ibid." "Ibid." "Nicole Forsgren" "Ibid." "Jez Humble" "Gene Kim" "Ibid."])
#(ok
  ("Mike Nygard"
   "Gene Kim"
   "Gene Kim"
   "Gene Kim"
   "Nicole Forsgren"
   "Nicole Forsgren"
   "Jez Humble"
   "Gene Kim"
   "Gene Kim"))
lfe> (: ibid ibid '["Ibid."])
#(error "No Previous Author for 'Ibid.' citation")

If you have different solutions shoot them my way as I would love to see them, and if there looks to be interest, and some responses, I can create a catalog of different solutions similar to what Eric Normand does on his weekly challenges with his PurelyFunctional.tv Newsletter.

DFW Erlang – Erlang, Elixir, and LFE on a Raspberry Pi

For those of you who are, or will be, in the Dallas/Fort Worth area, this is your friendly reminder that next Wednesday, July 1st, will be our monthly DFW Erlang User Group meeting.

We have recently moved to be hosted by Improving Enterprises, and now meet on the first Wednesday of the month.

This upcoming meeting, I will be going over different Erlang VM languages running on a Raspberry Pi. We will show Erlang, Elixir, and LFE running and see how many processes we can manage to kick off on a Raspberry Pi with 435MB of memory.

To RSVP and join us you can find the meeting announcement on Meetup.com here.

If you can’t join us at this meeting, we would love to see you at a future meeting.

And lastly, I want to give a great big Thank You to Duncan McGreggor and Robert Virding for all of their help in getting LFE setup on a Raspberry Pi. And another Thank You to Duncan for coming up with a demo for LFE after asking him if he knew of any good LFE projects that show off a number of processes being spawned up and interacting to push the bounds of a Raspberry Pi. If you can’t make it to our user group, make sure to check out his project at https://github.com/oubiwann/laotzi-demo.

–Proctor