Assignment: Make a NullSafe monad in the Promise fashion

Copy the following code into the edit area and fill the ___ gaps.

const NullSafe = x => {
    const isNullSafe = y => y && y.then;
    const maywrap = y => ___; // if y is not NullSafe yet, make it so
    return {
        then: fn => ___ // see (1)
    }
};

💡

(1) if x is not null or undefined, call fn(x); either way, make sure the result is a NullSafe

Fill the gaps such that NullSafe objects can be chained with their then function just like Promises do, incl. auto-promotion of result values to NullSafe objects.

NullSafe(1).then(console.log);                   // will call the log
NullSafe(null).then(console.log);                // will not call the log
NullSafe(2).then( x => null).then(console.log);  // will not call the log

Solution

const NullSafe = x => {
    const isNullSafe = y => y && y.then;
    const maywrap = y => isNullSafe(y) ? y : NullSafe(y);
    return {
        then: fn => maywrap(x !== null && x !== undefined ? fn(x) : null)
    };
};

Your solution will be tested against:

// x_ and y_ are given. do not override.
NullSafe(x_)
    .then(x => x * 2)           // must auto-promote
    .then(x => NullSafe(x))     // must not auto-promote
    .then(x => y_ = x + 1)      // store value, check no double promotion
    .then(x => null)            // jump over rest
    .then(x => x.mustNotBeCalled) !== null && y_ === x_ * 2 + 1

Quiz 24 Quiz 28a