Lifting values to an effect


Working with effectful types


I have:

  • A value of type A.

I want to have:

  • A value of type F<A>, where F is my effect type.


We can use the pure function present in the Applicative type class to embed a value into an effect.


We can lift plain type values, like Int or String, into effect types, like Option or ArrayK, buy using pure:

Option.pure(3)^       // Returns Some(3)
ArrayK.pure("Hello")^ // Returns ArrayK(["Hello"])

Notice that in the case of ArrayK, we only have a possibility to create an ArrayK with a single value with pure, but in the case of Option, there could have been two possible implementations: using Option.some and Option.none. As a rule of thumb, the implementation of pure will always lead to the successful case of the effect type.

pure is also available for types that have more than one type parameter, like Either, but we will have to provide help on the type parameters, as the compiler cannot infer the left type argument from a single value:

Either<Int, String>.pure("Hello")^

Note here that the type of the argument passed to pure must be the rightmost type argument in the effect type signature.