Ep 015: Finding the Time

Functional Design in Clojure

Nate spends some time figuring out how to track his time.

• New problem: Want to track time using a text file.
• Text files are great:
  • Keyboard oriented: in terminal, in editor
  • Something you can put under revision control
• Need: date and time range
• Format of the time stamp should be convenient for us to read and edit.
• Let the computer do the work to convert the convenient notation to something usable!
• Format: Fri Feb 08 2019 11:30-13:45
• One timestamp per line. Oh wait, we need a description. Six minutes in and we're already changing the requirements!
• What are "literate" text files? "You can mix human words in amongst data the computer will use to make it more understandable for the humans."
• How to find the times? Attempt to match the time format within each line.
• "It's kind of like an inverse comment. Instead of every line being valid and you have to comment out lines you don't want, if it's in a known format, those are the lines we want and everything else is a comment."
• Can use line-seq with clojure.java.io/reader. That uses lazy I/O.
• Potential Issue: lazy I/O can defer I/O errors to when you're in the middle of processing the data.
• Lazy I/O is less of an issue with files and more of an issue with network sockets.
• Another approach: slurp in all the data at once and call split-lines
• "Trade a little memory for some safety."
• Clojure uses the seq abstraction whichever way you choose. It's Clojure's unifying abstraction.
• "Even maps get squeezed into a seq if you push them hard enough."
• Next step: figure out which lines are time entry lines and which lines are commentary
• Use a regex to match against each string using Clojure's built-in #"..." form.
• In other situations, you can use instaparse for grammar-based parsing.
• Put timestamps on their own line
  • Easier to work with
  • Allows for more error detection (in the future)
• Use capture groups and re-matches detect the match and extract the parts.
• Use when-let to destructure and do something only if it matches
• Better yet, make a function time-info that does the match and returns either nil or the string that matched.
• For now, just start by using doseq to just go through all the lines and test our matching.
• Making sense of the timestamp string is a whole new problem for next time.

Clojure in this episode:

• nil
• seq
• line-seq
• clojure.java.io/reader
• slurp
• clojure.string/split-lines
• #"", re-matches
• let, when-let
• doseq

Related projects:

• Instaparse

Code sample from this episode:

(def timestamp-re #"(\w+)\s(\w+)\s(\d+)\s(\d+)\s+(\d{2}):(\d{2})-(\d{2}):(\d{2})") (with-open [rdr (clojure.java.io/reader "time-log.txt")] (doseq [line (line-seq rdr)] (when-let [[whole d m dt yr hs ms he me] (re-matches timestamp-re line)] (println whole))))