Skip to main content

Day 18 - Structured data

Date: October 22, 2026

⚠️Homework
Homework 6 due October 25, 2026
‼️Ending Skills
Last week for skills: 2

Skills: 4​

Pre-reading: 6.1.1.1, 6.1.2.1, 6.1.2.2, 6.1.3.1​

Supplementary Videos​

Structured Data

Intro (10 mins)​

  • Many pieces of information consist of multiple parts that should be kept together.

  • e.g., in tables, the row consists of several values. But, this same idea is useful outside of tables.

  • Consider, for example, a record of a book used by a library program -- which has a title, author, and number of pages.

  • We can define a new type of data for a BookRecord using a language feature called data:

    data BookRecord:
      | book(title :: String, author :: String, pages :: Number)
    end

  • This defines both the type of data (BookRecord), for use in type annotations, and a way of constructing it -- book, which is a function of three arguments (title, author, and pages) that returns a new piece of data of type BookRecord. Functions like book are known as constructors.

  • For example, we can create three BookRecords as:

    the-dispossessed = book("The Dispossessed", "Ursula K. Le Guin", 387)
    to-the-lighthouse = book("To the Lighthouse", "Virginia Woolf", 209)
    brave-new-world = book("Brave New World", "Aldous Huxley", 268)

  • The field names are used to access the parts of the BookRecord, using the dotted "field access" notation:

    the-dispossessed.pages # evaluates to 387

Class Exercise (45 mins)​

  1. Design a function that returns a "summary string" for a BookRecord, including the title, author, and pages.

  2. Design a function is-long-book that takes in a Podcast and returns whether or not the book has more than 350 pages.

  3. Design a new data type for a Podcast, and based on your own knowledge figure out the fields that make sense to include in it.

  4. Design a function podcast-summary that takes in a Podcast and produces a string summarizing a Podcast.

  5. Design a Recipe data definition, and then write a row-to-recipe function that consumes a Row from the following table (convert numeric columns after loading):

    include csv
    recipes-raw = load-table:
      title :: String,
      servings :: String,
      prep-time :: String
      source: csv-table-url("https://raw.githubusercontent.com/neu-pdi/cs2000-public-resources/refs/heads/main/static/support/5-recipes.csv", default-options)
    end
    recipes-with-servings = transform-column(recipes-raw, "servings", string-to-number-default(0))
    recipes = transform-column(recipes-with-servings, "prep-time", string-to-number-default(0))

  6. Test your function using get-row(recipes, 0) or some other row number, and then use it to add a new column with build-column.

Wrap-up (5 mins)​

  • Structured data groups multiple fields together.
  • We define structured types using data and create instances by calling the constructor.
  • Fields are accessed with the dot notation.