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Day 17 - Structured data

Skills: 4​

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

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.
  • 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-disposesssed.pages # evaluates to 387

Class Exercise (40 mins)​

  • Design a function that returns a "summary string" for a book, including the title, author, and pages.
  • Design a function is-long-book that returns a boolean if the book has more than 350 pages.
  • Design a new data type for a Podcast, and figure out the fields that make sense to include in it.
  • Design a podcast-summary that produces a string summarizing a Podcast.
  • Design a Recipe data definition, and then write a row-to-recipe function that consumes a Row from the following table: 
    include csv
    recipes = load-table:
      title :: String,
      servings :: Number,
      prep-time :: Number
      source: csv-table-url("https://pdi.run/f25-2000-recipes.csv", default-options)
    end
  • Test your function using recipes.row-n(0) or some other 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.