first-repeated-letter created by anatomic - https://repl.it/@anatomic/first-repeated-letter

first-repeated-letter.js

// Needs to return the first repeated letter
// Points for reduced O complexity

// Challenge: Write a function that given a string, returns the first char that is repeated
// Example, the first letter to be repeated in this sentence is 't'
import {
	safeLift,
	isString,
	head,
	tail,
	pipeK,
	Maybe,
} from 'crocks';

const hasRepeat = 'foo bar baz';
const noRepeat = 'a b c d e f g';
const example = `The first letter to be repeated in this sentence is 't'`;

// split :: a -> Maybe (Array Char)
const split = safeLift(isString, s => [...s]);

//  toLower :: a -> Maybe String
const toLower = safeLift(isString, s => s.toLowerCase()); // can remove this if case sensitive

// isRepeat :: Char -> Array Char -> Bool
const isRepeat = x => xs => xs.some(c => c === x && c !== " ");

// firstRepeatingChar :: Array Char -> Maybe Char
const getRepeat = l =>
	Maybe.of(isRepeat).ap(head(l)).ap(tail(l)).option(false)
		? head(l)
		: tail(l).chain(getRepeat);

const firstRepeatingChar = pipeK(Maybe.of, toLower, split, getRepeat);

firstRepeatingChar(example);
// firstRepeatingChar(hasRepeat);
// firstRepeatingChar(noRepeat);