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02-building-blocks-functions.markdown

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  • With our DATA building blocks, we can construct a new world for the computer to operate in

  • We can teach it about accounts, we can teach it about transactions

  • We can teach it about cars, buildings, businesses, any other concrete object in the world, and even more abstract concepts

  • But this world isn't interesting if nothing moves

  • Twitter wouldn't be interesting unless you could CREATE tweets, or LIKE tweets, or RETWEET tweets

  • Youtube wouldn't be interesting unless you could DOWNLOAD videos and WATCH them

  • Google Calendar wouldn't be interesting unless you could SCHEDULE events, MOVE events, ADD locations, INVITE participants

  • Gmail wouldn't be interesting unless you could DRAFT a new email, ADD recipients, CHANGE the email body, SEND the email, RECEIVE email

  • Note that all the interesting things above involve a data building block (tweet, video, events, locations, participants, email, email addresses)

  • They become interesting when they are given life

  • We need a way to represent action in our world

  • Functions do 3 things

    • Accept some input
    • Do something
    • Return some output

  • Elixir has already written some functions for you. Let's play around with them

    • String.capitalize("california") (takes "california" as input transforms it by capitalizing the first letter returns the transformed string)
    • String.length("how many letters?") (takes "how many letters?" as input counts the number of letters in the string, including spaces and question marks returns that number)
    • List.first(["Nancy", "Charles", "Wilfred"]) (takes a list as input extracts the first item from the list returns that item)
    • List.delete(["my account", "their account", "your account", "my friend's account"], "your account") (takes a list as input removes the specified item from the list returns a shorter list without the specified item)
    • Map.get(account, "owner") (takes a map as input finds the "owner" key and extracts the corresponding value returns the value)
    • Map.put(account, "owner", "Jake") (takes a map as input transforms it by setting the "owner" to "Jake" returns the map)
    • Map.keys(account) (takes a map as input extracts a list of all the keys it can find in the map returns the list)

  • Let's briefly point out the structure of the above functions:

    • Module.function_name(inputs, go, here)
    • What's a module? Just like files have folders, functions have modules. They're a way for you to organize your functions
      • Look at the functions above:
        • All the functions that operate on strings (capitalize, length) are in the String module
        • All the functions that operate on lists (first, delete) are in the List module
        • All the functions that operate on maps (get, put, keys) are in the Map module
      • As a side note, this lets use reuse function names:
        • Map.delete transforms a map by deleting a key
        • List.delete transforms a list by removing an item

  • You might write a function to reschedule a calendar event

    • Inputs: the event to reschedule, when it should be rescheduled to
    • Do something: transform the event by saving the new time
    • Return the new event
    • CalendarEvent.move(event, "Mar 21, 2018")

  • You might write a function to like a tweet

    • Inputs: tweet to be liked
    • Do something: add 1 to the # of likes the tweet has
    • Return the changed tweet
    • Tweet.like(tweet)

  • You might write a function to add a recipient to your email draft

    • Inputs: unsent email, new recipient address
    • Do something: add the recipient address to the recipient list
    • Return the updated email
    • Email.add_recipient(email, "[email protected]")

  • You might write a function to draft a new email

    • Inputs: nothing!
    • Do something: construct a bare bones email map with empty values
    • Return the new email draft
    • Email.new()