Ep 016: When 8 - 1 = 6

Functional Design in Clojure

Christoph discovers that time creates its own alternate universe.

• Continuing our exploration of "literate" time logs
• We want a function to turn the timestamps into minutes.
• Format: Fri Feb 08 2019 11:30-13:45
• Keep it simple: extract times with a regex and use math
  • minutes in day = hours * 60 + minutes
  • total minutes = end minutes - starting minutes
• Problem: What happens when we work past midnight? Negative minutes!
• We decided to have only one date, so a time before the starting time must span midnight.
• Format only allows for an activity to be <24 hours.
• "If we end up doing any activity in our life longer than 24 hours, I think we should that we might have other problems."
• Easy Solution: If the end time is before start time, add 24 hours.
• "When I get past any sort of simpler math, I just type it into my connected editor REPL because I know Clojure can do it faster than I can."
• Now we have a function to get minutes, want to add them all up.
• Use loop and recur to iterate through the array and track the sum.
• Oh wait, what about Daylight Savings Time?
• "We all pretend that time is actually in the future."
• If it involves dates and times, we can't just do simple math.
• "If I do this the right way, I now have to open a whole new can of worms."
• Easy way out: write "doesn't support DST" in the release notes and call it "user error"!
• "Any time you have to be careful about something, you're probably doing it wrong."
• Use a time API.
• "The Clojure library is just a wrapper because nobody wants to reinvent this thing."
• Java time is immutable, so that works nicely with functional languages. No cloneing!
• Lots of functions in the time API. Which ones do we need?
• Our workflow: try out different expressions in a comment block in our connected editor to figure out the specific time functions we need.
• "Local" dates and times don't have time zones, but "zoned" dates and times do have them.
• Need to create timestamps for accurate math: date + time + zone
• In practice, when we use dates and times, they are implicitly connected to a time zone.
• Your time zone is your alternate universe: it affects the meaning of your dates and times.
• We added support for DST without changing the function signature.
• But, how do we add up other totals? Looks like we're going to need to change even more.

Related episodes:

• 015: Finding the Time

Clojure in this episode:

• nil
• re-matches
• loop, recur

Related projects:

• Java Time
• Joda Time

Code sample from this episode:

(ns app.time (:require [clojure.java.io :as io] [java-time :as jt])) (def timestamp-re #"(\w+\s\w+\s\d+\s\d+)\s+(\d{2}:\d{2})-(\d{2}:\d{2})") (defn localize [dt tm] (jt/zoned-date-time dt tm (jt/zone-id "America/Los_Angeles"))) (defn parse-time [time-str] (jt/local-time "HH:mm" time-str)) (defn parse-date [date-str] (jt/local-date "EEE MMM dd yyyy" date-str)) (defn parse-for-minutes [line] (if-let [[whole dt start end] (re-matches timestamp-re line)] (let [date (parse-date dt) start (localize date (parse-time start)) end (localize date (parse-time end))] (if (jt/before? start end) (jt/time-between start end :minutes) (jt/time-between start (jt/plus end (jt/days 1)) :minutes))) 0)) (defn total-time [filename] (with-open [rdr (io/reader filename)] (loop [total 0 lines (line-seq rdr)] (if lines (recur (+ total (or (some-> (first lines) parse-for-minutes) 0)) (next lines)) total))))