Error with simply Maybe funtion

Question

Background

I am trying to help someone in StackOverflow with a question regarding Futures and Maybes and Eithers. My first approach is to have a simple function that takes in a Maybe and computes something.

Code

I am using Sanctuary, but this is equivalent to Ramda or any other library.

const S = require("sanctuary");

const transform = S.map(
  S.pipe( [ S.trim, S.toUpper ] )
);

S.Maybe.Just( ["  heello", "  world!"] )
  .map( transform )

Problem

The problem is that this simple function is failing with the error:

TypeError: S.Maybe.Just(...).map is not a function

Question

I am confused to say the least. I know that Maybe is a Monad, and that Monads are Functors. Each Functor must have a map method, but somehow Maybe.Just ( which is a Maybe type ) doesn't?

What am I doing wrong?

Answer 1

Maybe is a functor and has a fantasy-land/map attached, and it's consumed using S.map. Also, you build a Just using S.Just or S.of (S.Maybe):

const S = require("sanctuary");

const transform = S.pipe ([ S.trim, S.toUpper ])

const maybeTransformed = S.map (transform) (S.Just (['hello', ' world!']))