lesson4.affix

-- # Beginners Tutorial: Lesson 4
-- ### Data Variant Aliases
-- In this lesson you'll learn how to display data variants in user-friendly ways.

-- Say you wanted to write a story about a character.

data Character = val | sam | penny

rule story = name.Character "went to the store."

rule name.Character =
    .val -> "val"
    .sam -> "sam"
    .penny -> "penny"
    
-- This will work, but it's kinda annoying that we have to write a whole separate rule just to be able to show a character's name.

-- Here's a more concise way to do it:

rule story2 = @Character "went to the store."

-- A data variant variable can be displayed by putting an `@` sign in front. The `story2` rule produces sentences like:
-- > "penny went to the store."
-- 
-- > "val went to the store."
-- 
-- > "sam went to the store."

-- By default, `@`-displaying a data variant displays the variants programmatic name, which must be all lower-case alphanumeric characters (A through Z). This means if one of your characters is called Björk, you would have to enter a data variant name like `bjork` (dropping the umlaut). So when `bjork` is `@`-displayed,
-- 1. the name wouldn't be capitalized, and
-- 2. the umlaut wouln't be displayed.
-- 
-- That's no good.

-- ## The Solution
-- You can tell the system how a data variant should be `@`-displayed by putting it's human-readable form in parentheses after the data variant declaration:

rule AnotherCharacter
    = penny ("Penny")
    | sam ("Sam")
    | bjork ("Björk")
    | val ("Val" | "Valarie" | "Val," val_title)

rule val_title = "Destroyer of Worlds" | "Ruler of this Hamlet" | "(may they live forever)"

-- Also notice that `val` can be given nicknames as well! You can even reference rules inside these custom display sentences!

-- Finally, note that you can print out a specific data variant just by putting an `@` in front of it's name:

rule specific_data_variant = "The artist formerly known as" @penny "(who now goes by Persephone)."

rule start
    = "story:" story
    | "story2:" story2
    | "AnotherCharacter:" @AnotherCharacter
    | "specific_data_variant:" specific_data_variant
    
-- # Next Lesson
-- [Click here to move on to Lesson 5](#)