Ep 019: Dazed by Weak Weeks

Functional Design in Clojure

Nate wants to see more than data structures in a REPL.

• Goal: see a text-based calendar of hours worked by day and week.
• "With the power of the predicate, I summon thee from the bag of data!"
• If data items share structure, you only need one predicate function, not separate functions per "type".
• Common pattern: filter then map, then reduce
• Output:
  • Looks like a calendar with one line per week
  • Show daily total for each day
  • Show weekly total
• Problem: how do we split the days into weeks?
• "I don't remember split-weeks in the Clojure cheat sheet."
• "What day does your week start on? My week starts on Tuesday."
• "I always assume that when a programmer says 'most', they mean 'most of the people around me'." "I actually think it just means 'me'."
• "Anecdata: when you have two anecdotes, it makes data."
• Create higher-level summaries: entry → day → week
• Sift through data at the level of your question.
• Problem: A date identifies a "day", but what identifies a week? A "week ID", so to speak.
• It would be nice if that week ID was ordered, so we could sort on it.
• Idea: Use the date of the first day of the week
  • uniq, sortable, spans years
  • weeks that start on Tuesday will never have the same ID as weeks that start on Sunday
• Might as well have the starting day of the week in the map too: :sunday, :tuesday, etc.
• Convenient to have two different views for the same data in the same data structure
  • immutable data won't change and get inconsistent.
  • it's pre-calculated if you need to use it a lot vs just using a predicate "on the fly".
• Want to group-by the week:
  • need a predicate from "date ID" → "week ID": (week-id starting-day-of-week day)
  • need to take a "date" and produce "date of first day in week"
  • Eg. (week-id :sunday {:date "2019-03-08" ...}) => "2019-03-03"
• "You're teaching Clojure core how to describe your data by giving it vocabulary."
• Write something that converts data to data, not data to println.
• Principle: move the I/O to the edges
  • make the println a trivial step at the end
  • even making the strings is a data transform
  • everything prior to the println could be tested, even the formatting!
• "Test it to the level of detail you can tolerate."
• Much easier to reason about pure functions.

Related episodes:

• 015: Finding the Time
• 016: When 8 - 1 = 6
• 017: Data, at Your Service
• 018: Did I Work Late on Tuesday?

Clojure in this episode:

• filter, map, reduce
• group-by
• sort-by with partition-by
• take-while, drop-while, and split-with

Code sample from this episode:

(ns time.week-05  (:require  [time.week-04 :as week-04]  [java-time :as jt]  ))   ; Date helpers  (defn start-of-week  "Get the the starting date of the week containing local-date. The week will  start on the named day. Eg. :sunday"  [starting-day-of-week local-date]  (jt/adjust local-date :previous-or-same-day-of-week (jt/day-of-week starting-day-of-week)))  (defn week-dates  "Create a week's worth of dates starting from the given date."  [starting-date]  (->> (range 0 7)  (map #(jt/plus starting-date (jt/days %)))))   ; Aggregates  (defn sum-minutes  "Sums the minutes for all kinds: entry, day, week"  [entries]  (->> entries  (map :minutes)  (reduce +)))   ; Conversions  (defn entries->days  "Convert a seq of entries to days."  [entries]  (->> (group-by :date entries)  (map (fn [[date xs]] {:date date :minutes (sum-minutes xs)}))))  (defn days->week  "Convert a seq of days into a week. Week is picked by first date."  [starting-day-of-week days]  (let [lookup (into {} (map (juxt :date identity) days))  starting-date (start-of-week starting-day-of-week (:date (first days)))  all-days (->> (week-dates starting-date)  (map #(or (get lookup %)  {:date %  :minutes 0})))]  {:starting-day-of-week starting-day-of-week  :date starting-date  :days (vec all-days)  :minutes (sum-minutes all-days)}))  (defn partition-weeks  [starting-day-of-week days]  (->> days  (sort-by :date)  (partition-by #(start-of-week starting-day-of-week (:date %)))))  (defn days->weeks  "Convert a seq of days into an ordered seq of weeks."  [starting-day-of-week days]  (->> (partition-weeks starting-day-of-week days)  (map (partial days->week starting-day-of-week))))   (comment  (->> (week-04/log-times "time-log.txt")  (entries->days))   (->> (week-04/log-times "time-log.txt")  (entries->days)  (days->weeks :sunday))  )