bea

Chapter 5: Collections

Wine cellar

  • labelled arguments
  • guards on fields

color color: Color gives the parameter an outside label (color) and an inside name. Callers write wines_of_color(wines, color: White), so the call reads clearly.

Clause guards can look into the matched value: if head.color == color. Types can also be declared after the functions that use them.

Open in playground
import gleam/list

def filter_by_color(wines: List(Wine), color: Color, acc: List(Wine)) -> List(Wine)

def filter_by_color([], _, acc) {
  acc |> list.reverse()
}

def filter_by_color([head | tail], color, acc) if head.color == color {
  filter_by_color(tail, color, [head | acc])
}

def filter_by_color([_head | tail], color, acc) {
  filter_by_color(tail, color, acc)
}

pub def wines_of_color(wines: List(Wine), color color: Color) -> List(Wine) {
  filter_by_color(wines, color, [])
}

def filter_by_country(wines: List(Wine), country: String, acc: List(Wine)) -> List(Wine)

def filter_by_country([], _, acc) {
  acc |> list.reverse()
}

def filter_by_country([head | tail], country, acc) if head.country == country {
  filter_by_country(tail, country, [head | acc])
}

def filter_by_country([_head | tail], country, acc) {
  filter_by_country(tail, country, acc)
}

pub def wines_from_country(wines: List(Wine), country country: String) -> List(Wine) {
  filter_by_country(wines, country, [])
}

pub def filter(wines: List(Wine), color color: Color, country country: String) -> List(Wine) {
  wines
  |> wines_of_color(color)
  |> wines_from_country(country)
}

pub type Wine {
  Wine(name: String, year: Int, country: String, color: Color)
}

pub type Color {
  Red
  Rose
  White
}

pub def main() {
  echo wines_of_color(
      [
        Wine("Chardonnay", 2015, "Italy", White),
        Wine("Pinot grigio", 2017, "Germany", White),
        Wine("Pinot noir", 2016, "France", Red),
        Wine("Dornfelder", 2018, "Germany", Rose)
      ],
      color: White
    )

  echo wines_from_country(
      [
        Wine("Chardonnay", 2015, "Italy", White),
        Wine("Pinot grigio", 2017, "Germany", White),
        Wine("Pinot noir", 2016, "France", Red),
        Wine("Dornfelder", 2018, "Germany", Rose)
      ],
      country: "Germany"
    )

  echo filter(
      [
        Wine("Chardonnay", 2015, "Italy", White),
        Wine("Pinot grigio", 2017, "Germany", White),
        Wine("Pinot noir", 2016, "France", Red),
        Wine("Dornfelder", 2018, "Germany", Rose)
      ],
      color: White,
      country: "Italy"
    )
}

An exercism exercise from the Gleam track, solved in Bea: beacode/src/e19_wine_cellar.bea