could some explain and show how to implement the fromPredicate function?

[deleite]

Could some help me understand the fromPredicate example in the:
https://github.com/MostlyAdequate/mostly-adequate-guide/blob/master/ch12.md#effect-assortment

I get that the first one would keep the lefts and rights and the second won't but I would like to see it running so it is easier to get the idea.

Thanks

[deleite]

here is what I came up. Please feel free to correct or confirm if this is right :)

const fromPredicate = f => v => { const predicate = f(v); return predicate?Either.of(predicate):left(Error('false!')); }

[KtorZ]

@deleite this doesn't quite work, if you look at the type signature, the end result should be a Either e a. There's little we know about e but we can settle on an Error or String here. However, we know that a must be the same type of the a also provided in input for the predicate. So instead of Either.of(predicate), you should return Either.of(v).

You could write it the following way:

// fromPredicate :: (a -> Bool) -> a -> Either e a
const fromPredicate = curry((predicate, a) => predicate(a)
  ? new Right(a)
  : new Left("predicate failed"));

// --- example

const predicate = s => s.startsWith("a", s);

const fruitsA = new List([ "apple", "pear", "banana", "apricot", "orange" ]);
const fruitsB = new List([ "apple", "apricot" ]);

validate(predicate)(fruitsA);
// Left('predicate failed')

validate(predicate)(fruitsB);
// Right({$value: ['apple', 'apricot']})