lotsa-ramda.js

const posLens = R.lensProp('positives')
const negLens = R.lensProp('negatives')

const positives = R.view(posLens)
const negatives = R.view(negLens)

const incrementPositiveCount = R.over(posLens, R.inc)
const incrementNegativeCount = R.over(negLens, R.inc)

const seenBoth = c => R.allPass([
  R.compose(R.gt(R.__, 0), positives),
  R.compose(R.gt(R.__, 0), negatives),
])(c)

const updateCounts = (counts, num) => R.cond([
  [R.equals(1), R.always(incrementPositiveCount)],
  [R.equals(-1), R.always(incrementNegativeCount)],
  [R.equals(0), R.always(R.identity)],
  [R.T, R.always(() => { throw new Error(`ERR: Unexpected value (${num})`) })]
])(num)(counts)

const onlyPositives = R.allPass([
  R.compose(R.gt(R.__, 0), positives),
  R.compose(R.equals(0), negatives),
])

const onlyNegatives = R.allPass([
  R.compose(R.gt(R.__, 0), negatives),
  R.compose(R.equals(0), positives),
])

const classifyResults = length => R.cond([
  [R.propEq('positives', length), R.always(Infinity)],
  [R.propEq('negatives', length), R.always(-Infinity)],
  [onlyPositives, positives],
  [onlyNegatives, R.compose(R.negate, negatives)],
  [R.T, R.always(0)]
])

const produceSummary = R.reduce(
  R.compose(R.when(seenBoth, R.reduced), updateCounts),
  { negatives: 0, positives: 0})

const example = arr => R.compose(
  classifyResults(arr.length),
  produceSummary
)(arr)

// Tests
const assert = (expr, to, description) => { const r = expr(); console.log(`${R.equals(r, to)} : ${description}`); };

console.log('== TESTS ==')

assert(() => example([0,0,0,0]), 0, 'if all elements of an array are 0, return 0.')
assert(() => example([1,1,1,1]), Infinity, 'if all elements of an array are 1, return Infinity.')
assert(() => example([-1,-1,-1,-1]), -Infinity, 'if all elements of an array are -1, return -Infinity.')
assert(() => example([1,0,1,1]), 3, 'if all elements of an array are either 0 or 1, return number of 1s.')
assert(() => example([-1,0,-1,-1]), -3, 'if all elements of an array are either 0 or -1, return -1 * number of 1s.')
assert(() => example([1,0,-1]), 0, 'if all the above fail, return 0.')
assert(() => example([1,-1,undefined]), 0, 'short circuits when an incompatible state is reached.')