Learn X in Y minutes

//// This comment with four slashes is a module-level.
//// This kind of comments are used to describe the whole module.

import gleam/bool
import gleam/io
import gleam/int
import gleam/float
import gleam/list
import gleam/iterator
import gleam/option.{type Option, None, Some}
import gleam/result
import gleam/string
import gleam/string as text

// A type's name always starts with a capital letter, contrasting to variables
// and functions, which start with a lowercase letter.

// When the pub keyword is used the type alias is public and can be referred to
// by other modules.

pub type UserId =
  Int

pub fn main() {
  io.println("Hello from learnxinmyminutes.com!")
  // io.println("This statement got commented out by a two slashes comment.!")

  // Modules are the units in which all Gleam code gets organized.
  // In a module full will find a bunch of definitions of types, functions, etc.
  // that seem to belong together.
  // For example, the gleam/io module contains a variety of functions for
  // printing, like println.

  // All gleam code is in some module or other, whose name comes from the name
  // of the file it's in.
  // For example, gleam/io is in a file called io.gleam in a directory called
  // gleam.

  // Gleam has a robust static type system that helps you as you write and edit
  // code, catching mistakes and showing you where to make changes.
  // io.println(10)
  // If you uncomment the previous line you'll get a compile time error reported
  // as the io.println function only works with strings, not ints.

  // The compile will output an error that looks like this:
  // error: Type mismatch
  //  ┌─ /home/contributor/learnxinmyminutes/src/learnxinmyminutes.gleam:21:14
  //  │
  // 21 │   io.println(10)
  //  │              ^^
  //
  // Expected type:
  //
  //     String
  //
  // Found type:
  //
  //     Int

  // Working with numbers

  // When running on the Erlang virtual machine ints have no maximum and minimum
  // size.
  // When running on JavaScript runtimes ints are represented using JavaScript's
  // 64 bit floating point numbers.

  // Int arithmetic
  io.debug(1 + 1)
  io.debug(5 - 1)
  io.debug(5 / 2)
  io.debug(3 * 3)
  io.debug(5 % 2)

  // Int comparisons
  io.debug(2 > 1)
  io.debug(2 < 1)
  io.debug(2 >= 1)
  io.debug(2 <= 1)

  // Equality works for any type and is checked structurally, meaning that two
  // values are equal if they have the same structure rather than if they are at
  // the same memory location.
  io.debug(1 == 1)
  // True
  io.debug(2 != 2)
  // False

  // Standard library int functions
  io.debug(int.min(142, 137))
  // 137
  io.debug(int.clamp(-80, min: 0, max: 100))
  // 0
  io.debug(int.base_parse("10", 2))
  // Ok(2)

  // Binary, octal, and hex Int literals
  io.debug(0b00001111)
  io.debug(0o17)
  io.debug(0xF)

  // Use underscores to enhance integer readibility
  io.debug(1_000_000)

  // Gleam's numerical operators are not overloaded, so there are dedicated
  // operators for working with floats.

  // Float arithmetic
  io.debug(1.0 +. 1.5)
  io.debug(5.0 -. 1.5)
  io.debug(5.0 /. 2.5)
  io.debug(3.0 *. 3.5)

  // Float comparisons
  io.debug(2.2 >. 1.3)
  io.debug(2.2 <. 1.3)
  io.debug(2.2 >=. 1.3)
  io.debug(2.2 <=. 1.3)

  // Floats are represented as 64 bit floating point numbers on both the Erlang
  // and JavaScript runtimes.
  // The floating point behaviour is native to their respective runtimes, so
  // their exact behaviour will be slightly different on the two runtimes.

  // Under the JavaScript runtime, exceeding the maximum (or minimum)
  // representable value for a floating point value will result in Infinity
  // (or -Infinity). Should you try to divide two infinities you will get NaN
  // as a result.

  // When running on the BEAM any overflow will raise an error. So there is no
  // NaN or Infinity float value in the Erlang runtime.

  // Division by zero is not an error
  io.debug(3.14 /. 0.0)
  // 0.0

  // Standard library float functions
  io.debug(float.max(2.0, 9.5))
  // 9.5
  io.debug(float.ceiling(5.4))
  // 6.0

  // Underscores for floats are also supported
  io.debug(10_000.01)

  // Division by zero will not overflow, but is instead defined to be zero.

  // Working with strings
  io.debug("⭐ Gleam ⭐ - 별")
  io.debug(
    "this
    is
    a
    multi
    line
    string",
  )
  io.debug("\u{1F600}")
  // Outputs a smiley 😀

  // Double quote can be escaped
  io.println("\"X\" marks the spot")

  // String concatenation
  io.debug("One " <> "Two")

  // String functions
  io.debug(text.reverse("1 2 3 4 5"))
  io.debug(text.append("abc", "def"))

  io.println(text.reverse("!desrever tog gnirts sihT"))
  // Outputs "This string got reversed!"

  // Several escape sequences are supported:

  // \" - double quote
  // \\ - backslash
  // \f - form feed
  // \n - newline
  // \r - carriage return
  // \t - tab

  // Bool operators
  // The || and && operators work by short-circuiting

  io.debug(True && False)
  // False

  io.debug(True && True)
  // True

  io.debug(False || False)
  // False

  io.debug(False || True)
  // True

  // Bool functions
  io.debug(bool.to_string(True))
  // "True"

  io.debug(bool.to_int(False))
  // 0

  // Assignments
  let x = "Original value"
  io.debug(x)

  // Assign `y` to the value of `x`
  let y = x
  io.debug(y)

  // Assign `x` to a new value
  let x = "New value"
  io.debug(x)

  // The `y` still refers to the original value
  io.debug(y)

  // In Gleam variable and function names are written in snake_case.
  let answer_to_the_universe = 42
  io.debug(answer_to_the_universe)

  let and_everything = answer_to_the_universe
  // Now using a variable produces a warning

  // warning: Unused variable
  //     ┌─ /home/contributor/learnxinmyminutes/src/learnxinmyminutes.gleam:199:7
  //     │
  // 199 │   let and_everything = answer_to_the_universe
  //     │       ^^^^^^^^^^^^^^ This variable is never used
  // Hint: You can ignore it with an underscore: `_and_everything`.

  // Type annotations

  let _name: String = "Gleam"

  let _is_cool: Bool = True

  let _version: Int = 1
  // Useful for documentation purposes but they do not change how the compiler
  // type checks the code beyond making sure the annotation matches the type,
  // otherwise you get an error.

  // let _has_wrong_type_annotation: Int = True

  //  error: Type mismatch
  //      ┌─ /home/contributor/learnxinmyminutes/src/learnxinmyminutes.gleam:219:41
  //      │
  //  219 │   let _has_wrong_type_annotation: Int = True
  //      │                                         ^^^^
  //
  //  Expected type:
  //
  //      Int
  //
  //  Found type:
  //
  //      Bool

  // Type aliases
  let one: UserId = 1
  // Refer to the beginning of the file for the definition of the UserId type

  let two: Int = 2

  // Aliases are just for creating more readable code and more precise
  // documentation.
  // Under the hood they are still values of the same type so operations
  // still work
  io.debug(one + two)
  // 3

  // Blocks: scoping and value
  let radius = {
    let value = 100.0
    value
  }
  // io.debug(value) // <- This will not compile because "value" is out of scope

  let area = 3.14159 *. radius *. radius
  io.debug(area)

  // Use blocks to group operations instead of parenthesis
  let n1 = { 3 + 2 } * 5
  let n2 = 3 + { 2 * 5 }
  io.debug(n1 != n2)
  // True

  // Lists

  // Nephews of Scrooge McDuck
  let nephews = ["Huey", "Dewey", "Louie"]
  io.debug(nephews)
  // ["Huey", "Dewey", "Louie"]

  // Immutably prepend so the original list is not changed
  io.debug(["Donald", ..nephews])
  // ["Donald", "Huey", "Dewey", "Louie"]

  // Some standard library functions for lists

  list.each(nephews, io.println)
  // Huey
  // Dewey
  // Louie

  io.debug(list.drop(nephews, 2))
  // ["Louie"]

  more_examples()
  more_function_examples()
  generic_typing_examples()
  beloved_pipelines_demo()
  labels_in_function_calls()
  showcase_flow_control()
  more_on_recursion()
  more_on_pattern_matching()
  showcase_types()
  more_on_types()
  more_on_callbacks()
  showcase_externals()
  showcase_panic()
}

// The fn keyword is used to define new functions.
fn multiply(a: Int, b: Int) -> Int {
  // No explicit return
  // The last expression gets returned
  a * b
}

// The double and multiply functions are defined without the pub keyword.
// This makes them private functions, they can only be used within this module.
// If another module attempted to use them it would result in a compiler error.
fn double(a: Int) -> Int {
  multiply(a, 2)
}

// Only public functions are exported and can be called from outside the module.

// Type annotations are optional for function arguments and return values
// but are considered good practice for clarity and in order to encourage
// intentional and thoughtful design.

pub fn is_leap_year(year: Int) -> Bool {
  { year % 4 == 0 } && { { year % 100 != 0 } || { year % 400 == 0 } }
}

fn more_examples() {
  // Debug also returns a value so its output is the return value of
  // this function
  io.debug(double(10))
  // 20
  io.debug(is_leap_year(2000))
  // True
}

// Gleam supports higher order functions:
// They can be assigned to variables, passed as arguments to other functions
// or even be returned as values from blocks or other functions
fn call_func_on_int(func: fn(Int) -> Int, value: Int) -> Int {
  func(value)
}

fn more_function_examples() -> Int {
  io.debug(call_func_on_int(double, 2))
  // 4

  let square = fn(x: Int) -> Int { x * x }
  io.debug(square(3))
  // 9

  // Calling an anonymous function inmediately after defining it
  io.debug(fn(x: Int) { x + 1 }(1))

  // Closure example
  let make_adder = fn(n: Int) -> fn(Int) -> Int {
    fn(argument: Int) -> Int { argument + n }
  }

  let adder_of_fives = make_adder(5)
  io.debug(adder_of_fives(10))
  // 15

  // Anonymous functions can be used interchangeably with named functions.
  io.debug(call_func_on_int(fn(x: Int) -> Int { x + 100 }, 900))
  // 1000

  // Let's create a function decorator
  let twice = fn(wrapped_func: fn(Int) -> Int) -> fn(Int) -> Int {
    fn(argument: Int) -> Int { wrapped_func(wrapped_func(argument)) }
  }
  let quadruple = twice(double)
  io.debug(quadruple(1))

  let quadruple_2 = fn(a: Int) -> Int { multiply(4, a) }
  io.debug(quadruple_2(2))
  // 8

  // A function capture is a shorthand syntax for creating anonymous functions
  // that take one argument and immediately call another function with that
  // argument
  let quadruple_3 = multiply(4, _)
  io.debug(quadruple_3(4))
  // 16
}

// Generic functions are supported using type variables.
fn generic_twice(func: fn(value) -> value, argument: value) -> value {
  func(func(argument))
}

// In generic_twice value was the type variable.
// In generic_twice_decorator the_type is the type variable.
// As in any other variable you get to choose the name.
fn generic_twice_decorator(
  func: fn(the_type) -> the_type,
) -> fn(the_type) -> the_type {
  fn(argument: the_type) -> the_type { func(func(argument)) }
}

fn generic_typing_examples() {
  let double_integers = fn(a: Int) -> Int { a * 2 }
  let double_floats = fn(a: Float) -> Float { a *. 2.0 }
  io.debug(generic_twice(double_integers, 3))
  io.debug(generic_twice(double_floats, 3.0))

  let quadruple_integers = generic_twice_decorator(double_integers)
  let quadruple_floats = generic_twice_decorator(double_floats)
  io.debug(quadruple_integers(1))
  // 4
  io.debug(quadruple_floats(1.0))
  // 4.0
}

// Gleam's pipe operator |> takes the result of the expression on its left
// and passes it as an argument to the function on its right.
fn beloved_pipelines_demo() {
  // Let's be honest: you want to use Gleam just for this cool operator, right?
  ["hello", "world"]
  |> list.intersperse(" ")
  |> list.append(["!"])
  |> string.concat
  |> string.capitalise
  |> io.debug

  // Match cleaner than this right?
  io.debug(
    string.capitalise(
      string.concat(
        list.append(list.intersperse(["hello", "world"], " "), ["!"]),
      ),
    ),
  )

  // Solution to the first problem of Project Euler:
  // URL: https://projecteuler.net/problem=1
  // Description: Find the sum of all the multiples of 3 and 5 below 1000.
  iterator.iterate(1, fn(n) { n + 1 })
  |> iterator.take(1000 - 1)
  |> iterator.filter(fn(n) { { n % 3 == 0 } || { n % 5 == 0 } })
  |> iterator.fold(from: 0, with: fn(acc, element) { element + acc })
  |> int.to_string
  |> fn(sum_as_text: String) {
    "Solution to Project Euler's problem #1: " <> sum_as_text
  }
  |> io.debug
  // Solution to Project Euler's problem #1: 233168
}

// Labels can be added before each argument
fn call_func_on_int_with_labels(
  func passed_func: fn(Int) -> Int,
  value n: Int,
) -> Int {
  passed_func(n)
}

// The label and the argument can have the same name
fn add_one(number number: Int) -> Int {
  number + 1
}

fn add_two_integers(first n: Int, second m: Int) -> Int {
  n + m
}

fn labels_in_function_calls() -> Int {
  // Since we are labelling the arguments we can switch the order
  // if we want to
  io.debug(call_func_on_int_with_labels(value: 8, func: double))
  io.debug(add_one(number: 1))
  // 2
  io.debug(string.contains(does: "theme", contain: "the"))
  // True
  // Unlabeled arguments must go first
  io.debug(add_two_integers(2, second: 2))
  // 4
}

fn showcase_flow_control() {
  // Use case if you want to use pattern-matching in order to
  // select which code to execute.
  // Gleam will make sure all possible values are covered
  // by performing exhaustiveness checks.
  // Otherwise you get compilation errors.
  let puppies = ["Bear", "Frisco", "Ranger"]
  let count = list.length(of: puppies)
  {
    "We have "
    <> int.to_string(count)
    <> " "
    <> // The underscore matches with any other value
    case count {
      1 -> "puppy"
      _ -> "puppies"
    }
  }
  |> io.debug

  // Gleam allows patterns in case expressions to also assign variables.
  {
    "Puppy count: "
    <> case list.length(puppies) {
      0 -> "None."
      1 -> "Just one."
      other -> "As many as " <> int.to_string(other) <> " puppies."
    }
  }
  |> io.debug

  // Consider BEAM languages are functional in design and Gleam is no exception
  // so there are no if, for or while constructs available.

  // Use pattern-matching for conditionals
  let answer = 42
  case answer == 42 {
    True -> {
      io.debug("This is the answer to the universe.")
    }
    False -> {
      io.debug("This is the answer to something else.")
    }
  }

  // Use recursion instead of looping
  from_one_to_ten(1)
}

// Recursive function
fn from_one_to_ten(n: Int) {
  io.debug(n)
  case n {
    10 -> Nil
    _ -> from_one_to_ten(n + 1)
  }
}

// In order to avoid memory exhaustion due to creating excesive
// stack frames when calling functions recursively, Gleam supports
// "tail call optimisation" which means that the compiler can reuse
// the stack frame for the current function if a function call is
// the last thing the function does.

pub fn fib(x: Int) -> Int {
  // The public function calls the private tail recursive function
  fib_loop(x, 1)
}

fn fib_loop(x: Int, accumulator: Int) -> Int {
  case x {
    1 -> accumulator

    // The last thing this function does is call itself
    // In the previous lesson the last thing it did was multiply two ints
    _ -> fib_loop(x - 1, accumulator + x)
  }
}

// Gleam supports pattern-matching the first element and the remainder
// of a list with the [x, ..y] pattern inside a case expression.
fn reverse_list(the_list: List(value)) -> List(value) {
  case the_list {
    [head, ..tail] -> list.concat([reverse_list(tail), [head]])
    [] -> []
  }
}

fn more_on_recursion() {
  io.debug(fib(10))
  // 55
  io.debug(reverse_list([1, 2, 3]))
}

fn more_on_pattern_matching() {
  // When pattern-matching on strings the <> operator match on strings
  // with a specific prefix and assigns the reminder to a variable
  io.debug(case "Hello, Lucy" {
    "Hello, " <> name -> "Grettings for " <> name
    _ -> "Potentially no greetings"
  })

  // Alternative patters are supported so the same clause is used
  // for multiple values
  let month = 2
  let year = 2024
  let number_of_days = case month {
    2 ->
      case is_leap_year(year) {
        False -> 28
        True -> 29
      }
    4 | 6 | 9 | 11 -> 30
    1 | 3 | 5 | 7 | 8 | 10 | 12 -> 31
    _ -> 0
  }
  io.debug("Number of days: " <> int.to_string(number_of_days))
  // 29

  // Guards in pattern-matching:
  // When using the if keyword an expression must evaluate to True
  // for the pattern to match.
  let list_starts_with = fn(the_list: List(value), the_value: value) -> Bool {
    case the_list {
      [head, ..] if head == the_value -> True
      _ -> False
    }
  }
  io.debug(list_starts_with([10, 20, 30], 10))
  // True
}

pub type Gender {
  Male
  Female
  Other
}

// Records:
// - Support variants
// - Each variant is similar to a struct with fields
pub type Shape {
  Rectangle(base: Float, height: Float)
  Triangle(base: Float, height: Float)
}

// Records with one variant resemble structs
pub type Point {
  Point(x: Float, y: Float)
}

fn showcase_types() {
  // Tuples:
  // - Can mix together elements of different types
  // - Their type is implicit e.g. #{1, "Hello"} is of type #{Int, String}
  // - Their elements can be accessed by numeric indexes
  let tuple_01 = #(1, "Ferris", "rustacean", True)
  let tuple_02 = #(1, "Lucy", "starfish", True)
  io.debug(tuple_01)
  io.debug(tuple_01.0)
  // 1
  io.debug(tuple_02.1)
  // Lucy
  let #(_, name, species, _) = tuple_01
  io.debug(name <> " the " <> species)

  // Pattern-matching with tuples including variable assignment
  case tuple_02 {
    #(_, name, _, True) -> io.debug(name <> " is a mascot.")
    #(_, name, _, False) -> io.debug(name <> " is not a mascot.")
  }

  // Using a custom type with pattern-matching
  let gender = Other
  io.debug(case gender {
    Male -> "Boy"
    Female -> "Girl"
    _ -> "Undetermined"
  })

  // Using records
  let rectangle_1 = Rectangle(base: 10.0, height: 20.0)
  io.debug(rectangle_1.height)
  // 10.3

  let point_1 = Point(x: 3.2, y: 4.3)
  io.debug(point_1)

  // Updating a record
  let point_2 = Point(..point_1, y: 5.7)
  io.debug(point_2)

  // In Gleam, values ar not nullable.
  // Nil is the only value of its type.
  let some_var = Nil
  let result = io.println("Hello!")
  io.debug(some_var == result)
  // True
}

pub type Mineral {
  Gold
  Silver
  Copper
}

// Generic custom types with contained types as parameters
pub type Purity(inner_type) {
  Pure(inner_type)
  Impure(inner_type)
}

pub type Beverage {
  Water
  Juice
}

// Existing custom types from the gleam/option and gleam/result modules
// facilitate working with nullable values and handling potential errors
pub type Person {
  Person(name: String, nickname: Option(String))
}

pub type DiceError {
  DiceValueOutOfRange
}

fn checked_dice_value(value: Int) -> Result(Int, DiceError) {
  case value {
    1 | 2 | 3 | 4 | 5 | 6 -> Ok(value)
    _ -> Error(DiceValueOutOfRange)
  }
}

fn double_dice_value(value: Int) -> Result(Int, DiceError) {
  case value {
    1 | 2 | 3 -> Ok(value * 2)
    _ -> Error(DiceValueOutOfRange)
  }
}

fn more_on_types() {
  let mineral_sample_01: Purity(Mineral) = Pure(Gold)
  let mineral_sample_02 = Impure(Silver)
  io.debug(mineral_sample_01)
  io.debug(mineral_sample_02)

  // A glass can be empty or not
  let glass_01: Option(Beverage) = Some(Water)
  let glass_02 = None
  io.debug(glass_01)
  io.debug(glass_02)

  // A person can have a nickname or not
  let person_01 = Person(name: "John", nickname: Some("The Ripper"))
  let person_02 = Person(name: "Martin", nickname: None)
  io.debug(person_01)
  io.debug(person_02)

  // Working with functions that return values of type Result
  let dice_01 = 5
  case checked_dice_value(dice_01) {
    Ok(checked_value) ->
      io.debug("The value of " <> int.to_string(checked_value) <> " is OK.")
    Error(DiceValueOutOfRange) ->
      io.debug("The value of the dice is out of range")
  }

  // Let's attempt to double the value if the resulting value is still
  // a number in any of the sides of the dice.
  // Otherwise, let's put the max value.
  2
  |> checked_dice_value
  |> result.try(double_dice_value)
  |> result.unwrap(or: 6)
  |> io.debug
}

pub fn throw_dice_as_result() {
  Ok(int.random(6) + 1)
}

pub fn sum_dice_values(a: Int, b: Int) {
  Ok(a + b)
}

// Betting on first-class functions and pattern-matching
// can easily lead to tons of indentation
fn roll_two_dices_without_use() {
  result.try(throw_dice_as_result(), fn(first_dice) {
    result.try(throw_dice_as_result(), fn(second_dice) {
      result.map(sum_dice_values(first_dice, second_dice), fn(sum) { sum })
    })
  })
}

// The use expression still let us write code that uses callbacks
// but cleans up excessive indentation:
// - A call to higher order function go the right side of the <- operator
// - The argument names for the callback function go on the left hand side of
//   the <- operator
// - All the remaining code in the enclosing {} block becomes the body of the
//   callback function.
fn roll_two_dices_with_use() {
  use first_dice <- result.try(throw_dice_as_result())
  use second_dice <- result.try(throw_dice_as_result())
  use sum <- result.map(sum_dice_values(first_dice, second_dice))
  // this is the remaing code in innermost callback function
  sum
}

fn more_on_callbacks() {
  io.debug(roll_two_dices_without_use())
  io.debug(roll_two_dices_with_use())
}

pub type DateTime

// External functions must annotate a return type
@external(erlang, "calendar", "local_time")
pub fn now() -> DateTime

fn showcase_externals() {
  io.debug(now())
  // #(#(2024, 4, 6), #(14, 4, 16))
}

fn showcase_panic() {
  // We can deliberately abort execution by using the panic keyword
  // in order to make our program crash immediately
  case 3 == 2 {
    True -> panic as "The equality operator is broken!"
    False -> "Equality operator works for integers"
  }
  // Calling a function that uses the todo keyword also crashes
  // homework()
}

pub fn homework() {
  todo
}