Custom Types are how new data types are defined in Gleam. They can have multiple variants, each with their own name.
pub type Season {
Spring
Summer
Autumn
Winter
}
Each case of a custom type can optionally hold some data, and different cases can have different types of data. When a variant holds data it is called a record.
pub type Number {
SomeInt(Int)
SomeFloat(Float)
Invalid
}
Creating a value for a specific case can be done by referring to its name if it contains no additional data (Spring
), or by calling it as a function if it does (SomeInt(2)
).
let spring = Spring
let integerTwo = SomeInt(2)
Custom types, along with everything in Gleam, have structural equality, which means that two values of the same variant and with the same data are equivalent.
Spring == Spring // -> True
Spring == Autumn // -> False
SomeInt(2) == SomeInt(2) // -> True
SomeInt(2) == SomeFloat(2.0) // -> False
Custom type variants can be pattern matched on using case expressions.
import gleam/int
import gleam/float
pub fn describe(flexible_number: Number) -> String {
case flexible_number {
SomeFloat(f) -> "Float: " <> float.to_string(f)
SomeInt(i) -> "Int: " <> int.to_string(i)
Invalid -> "Neither a float nor an int"
}
}
In this exercise, it's Valentine's day and you and your partner are planning on doing something nice together. Your partner has lots of ideas, and is now asking you to rate the ideas, in order to find the activity to engage in.
The following ideas are proposed by your partner:
You have six tasks to help choose your Valentine's day activity.
For each idea your partner proposes, you respond with one of three options: yes, no or maybe.
Define the Approval
custom type to represent these options as the following three cases: Yes
, No
and Maybe
.
Your partner has selected two possible restaurants: one based on the Korean cuisine and the other based on the Turkish cuisine.
Define the Cuisine
custom type to represent these cuisines as the following two cases: Korean
and Turkish
.
There are tons of movies to choose from, so to narrow things down, your partner also lists their genre.
Define the Genre
custom type to represent the following genres as cases: Crime
, Horror
, Romance
and Thriller
.
As said, your partner has come up with five different activities: playing a board game, chill out, watch a movie, go to a restaurant and taking a walk.
Define the Activity
custom type to represent these activity types:
BoardGame
: no associated data.Chill
: no associated data.Movie
: has its Genre
as associated data.Restaurant
: has its Cuisine
as associated data.Walk
: has an Int
representing the number of kilometers to walk as associated data.Finally, you're ready to rate your partner's ideas. This is how you feel about your partner's idea:
Implement a function named rate_activity
that takes an Activity
value and returns the Approval
based on the above sentiments:
rate_activity(Restaurant(Turkish))
// -> Maybe
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