Shrove Tuesday is over. After eating my weight in pancakes, I had a longer look at the recipe.
100g plain flour
2 eggs
300ml milk
1tbsp sunflower oil
pinch of salt
Recipes hold a lot of information: ingredients, quantities, and units of measure. Like the mars climate orbiter, types would make cooking safer.
case class IngredientMeasurement(
ingredient: String,
amount: Double,
unit: String) {
def +(that: IngredientMeasurement) =
this.copy(quantity = this.quantity + that.quantity)
}
val crepeRecipe = Iterable(
new IngredientMeasurement("plain flour", 100, "gram"),
new IngredientMeasurement("egg", 2, "whole"),
new IngredientMeasurement("milk", 300, "milliliters"),
new IngredientMeasurement("sunflower oil", 1, "tablespoon"),
new IngredientMeasurement("salt", 1, "pinch")
)
IngredientMeasurement is a step in the right direction, but it doesn’t make it safe. Any ingredients can be combined with another. Type parameters are required to differentiate apples from oranges.
trait Ingredient
case object PlainFlour extends Ingredient
case object Egg extends Ingredient
case object Milk extends Ingredient
case object SunflowerOil extends Ingredient
case object Salt extends Ingredient
case class IngredientMeasurement[I <: Ingredient](
ingredient: I,
amount: Double,
unit: String) {
def +(that: IngredientMeasurement[I]) =
this.copy(amount = this.amount + that.amount)
}
val crepeRecipe = Iterable(
new IngredientMeasurement(PlainFlour, 100, "gram"),
new IngredientMeasurement(Egg, 2, "whole"),
new IngredientMeasurement(Milk, 300, "millilitre"),
new IngredientMeasurement(SunflowerOil, 1, "tablespoon"),
new IngredientMeasurement(Salt, 1, "pinch")
)
While this representation fixes the issue with ingredients, the units one remain.
Units are split in quantities like mass and volume. Within the same quantity, converting one unit to another often only takes a multiplication. Cross quantity operations aren’t allowed.
trait Quantity
trait Mass extends Quantity
trait Volume extends Quantity
case class QuantityUnit[Q <: Quantity](
value: Double,
multiplier: Double) {
def +(that: QuantityUnit[Q]) = {
val thatValue = that.value * that.multiplier / this.multiplier
this.copy(value = this.value + thatValue)
}
}
object Mass {
def gram(value: Double) = QuantityUnit[Mass](value, 1)
}
object Volume {
def millilitre(value: Double) = QuantityUnit[Volume](value, 0.001)
def tablespoon(value: Double) = QuantityUnit[Volume](value, 0.017)
def pinch(value: Double) = QuantityUnit[Volume](value, 0.0074)
}
Writing a conversion library isn’t hard, but why reinvent the wheel. Squants and Libra offer better solutions. Furthermore, this allows me to focus on my pancakes.
import squants._
import squants.mass._
import squants.space._
trait Ingredient
case object PlainFlour extends Ingredient
case object Egg extends Ingredient
case object Milk extends Ingredient
case object SunflowerOil extends Ingredient
case object Salt extends Ingredient
case class IngredientMeasurement[
I <: Ingredient,
Q <: Quantity[Q]](
ingredient: I,
amount: Q
) {
def +(that: IngredientMeasurement[I, Q]) =
this.copy(amount = this.amount + that.amount)
}
case class WholeQuantity(count: Int) extends
Quantity[WholeQuantity] {
def value: Double = count
def dimension: Dimension[WholeQuantity] = ???
def unit: UnitOfMeasure[WholeQuantity] = ???
}
object Pinch extends VolumeUnit {
val conversionFactor = 0.0074
val symbol = "pinch"
}
val crepeRecipe = Iterable(
new IngredientMeasurement(PlainFlour, Grams(100)),
new IngredientMeasurement(Egg, WholeQuantity(2)),
new IngredientMeasurement(Milk, Millilitres(300)),
new IngredientMeasurement(SunflowerOil, Tablespoons(1)),
new IngredientMeasurement(Salt, Pinch(1))
)
My recipe is now type-safe. It doesn’t make my crepes any healthier, but it should help keep them tasty.
Cooking aside. The main selling point of statically typed languages is their pre-runtime checks. Having specific types, instead of generic ones, increase the effectiveness of those tests. Instead of using strings everywhere, don’t be afraid of creating your own types.