Logic functions behave unpredictably with `undefined` as predicate

[michsa]

Our logic functions (ifElse, when, unless, etc) accept booleans and lodash iteratees as predicates, so you can do

F.when('foo', _.merge({ bar: 1 }), { foo: 1 })  //-> { foo: 1, bar: 1 }

or

let shouldDoExtraStuff = true
F.when(shouldDoExtraStuff, doExtraStuff, value)  //-> doExtraStuff(value)

Futil's behavior is to return the predicate's value if it is a boolean and wrap it in _.iteratee otherwise, which causes odd behavior when the value is undefined.

One would most likely expect F.when(undefined, fn, x) to produce the same result as F.when(false, fn, x). However, since apparently _.iteratee(undefined)(x) behaves equivalently to _.identity(x), F.when(undefined, fn, x) instead acts like F.whenTruthy(fn, x).

https://runkit.com/michsa/f-when-undefined

[daedalus28]

What about when it's null? If we only care about undefined, we can solve this elegantly with a default value on condition.

@chris110408 if we care about null, use _.isNil to grab them both - otherwise use a default value

[daedalus28]

We went the route of treating null and undefined the same