Transforming data

beginner

Using a transforming function

Problem

I have:

• A value of type F<A>.
• A function (A) -> B.

I want to have:

• A value of type F<B>.

Solution

You can transform the inner type of an effect with the map function. Instead of unwrapping the value/s potentially wrapped in your effect type, you pass the function to map and it will be able to apply the transformation.

map is available in the Functor type class and most of the types in Bow provide an instance for Functor.

Example

Consider we have a function to convert an Int to a String. We can use it to map over many different types, like Option or ArrayK:

let toString: (Int) -> String = { n in "\(n)" }

Option.some(2).map(toString)^    // Returns Option.some("2")
ArrayK([1, 2, 3]).map(toString)^ // Returns ArrayK(["1", "2", "3"]

If we map over an Option, it will transform the value contained in the some case; if we do it on ArrayK, it will transform each of the values in the array.

We can map over an empty Option or Array:

Option<Int>.none().map(toString)^ // Returns Option<String>.none()
ArrayK<Int>([]).map(toString)^    // Returns ArrayK<String>([])

Despite not having values, the function map still changes the return type.

map also works on types that have more than one type parameter, but only transforms the last one. For instance, we could apply it to Either:

Either<Bool, Int>.right(2).map(toString)^    // Returns Either<Bool, String>.right("2")
Either<Bool, Int>.left(false).map(toString)^ // Returns Either<Bool, String>.left(false)

Using a constant value

Problem

I have:

• A value of type F<A>.
• A value of type B.

I want:

• A value of type F<B>.

Solution

You can still use map and pass a function that returns the value of B, ignoring the input; however, the Functor type class provides the function as, that behaves like map but takes constant values instead of a transforming function.

Clearing the type in the transformation

Problem

I have:

• A value of type F<A>.

I want to have:

• A value of type F<Void>.

Solution

Likewise, you could use map with a function that returns Void, or as with the constant value of (), but the Functor type class provides the void function that does this for you.