Introduction
Maybe
The Maybe type is the solution in the Elm language for optional values.
It is thus present in type signatures of a wide number of core Elm functions and understanding it is crucial.
The Maybe type is defined as follows:
type Maybe a = Nothing | Just a
This is known as a "custom type" definition in Elm terminology.
It indicates that a value of this type can either be Nothing OR be Just something of type a.
Creating a Maybe value is done via one of its two constructors Nothing and Just.
Reading the content of a Maybe is done via pattern matching.
matthieu : Maybe String
matthieu = Just "Matthieu"
anon : Maybe String
anon = Nothing
sayHello : Maybe String -> String
sayHello maybeName =
case maybeName of
Nothing -> "Hello, World!"
Just someName -> "Hello, " ++ someName ++ "!"
sayHello matthieu
--> "Hello, Matthieu!"
sayHello anon
--> "Hello, World!"
There are also a number of useful functions in the Maybe module to manipulate Maybe types.
sayHelloAgain : Maybe String -> String
sayHelloAgain name = "Hello, " ++ Maybe.withDefault "World" name ++ "!"
capitalizeName : Maybe String -> Maybe String
capitalizeName name = Maybe.map String.toUpper name
capitalizeName matthieu
--> Just "MATTHIEU"
capitalizeName anon
--> Nothing
Instructions
In this exercise, you'll be implementing mechanics of a role-playing game.
A player's character is represented by the following type:
type alias Player =
{ name : Maybe String
, level : Int
, health : Int
, mana : Maybe Int
}
Players in this game must reach level 10 before they unlock a mana pool so that they can start casting spells.
You're working on two pieces of functionality in this game, the revive mechanic and the spell casting mechanic.
1. Introduce yourself
Write the content of the introduce function.
Stealthy players may be hiding their name and will be introduced as "Mighty Magician".
Otherwise, just use your name to introduce yourself.
introduce { name = Nothing, level = 2, health = 8, mana = Nothing }
--> "Mighty Magician"
introduce { name = Just "Merlin", level = 2, health = 8, mana = Nothing }
--> "Merlin"
2. Implement the revive mechanic
The revive function should check that the player's character is indeed dead (their health has reached 0).
If they are, it should return a new Player instance with 100 health.
Otherwise, if the player's character isn't dead, the revive function returns Nothing.
If the player's level is 10 or above, they should also be revived with 100 mana.
If they player's level is below 10, their mana should be Nothing.
The revive function should preserve the player's level.
deadPlayer = { name = Nothing, level = 2, health = 0, mana = Nothing }
revive deadPlayer
--> Just { name = Nothing, level = 2, health = 100, mana = Nothing }
If the revive method is called on a player whose health is 1 or above, then the function should return Nothing.
alivePlayer = { name = Nothing, level = 2, health = 42, mana = Nothing }
revive alivePlayer
--> Nothing
3. Implement the spell casting mechanic
The castSpell function takes as arguments an Int indicating how much mana the spell costs as well as a Player.
It returns the updated player, as well as the amount of damage that the cast spell performs.
A successful spell cast does damage equal to two times the mana cost of the spell.
However, if the player has insufficient mana, nothing happens, the player is unchanged and no damage is done.
If the player does not even have a mana pool, attempting to cast the spell must decrease their health by the mana cost of the spell and does no damage.
wizard = { name = Nothing, level = 18, health = 123, mana = Just 30 }
( updatedWizard, damage ) = castSpell 14 wizard
updatedWizard.mana --> Just 16
damage --> 28
Learning Exercise
