Function signature of Tap (K-combinator)

问题:

I've read in a book that the function signature of tap function (also called K-Combinator) is below:

tap :: (a -> *) -> a -> a

"This function takes an input object a and a function that performs some action on a. It runs the given function with the supplied object and then returns the object."

1. Can someone help me to explain what is the meaning of star (*) in the function signature?
2. Are below implementation correct?
3. If all the three implementation are correct, which one should be used when? Any examples?

Implementation 1:

const tap = fn => a => { fn(a); return a; };

tap((it) => console.log(it))(10); //10

Implementation 2:

const tap = a => fn => { fn(a); return a; }; 

tap(10)((it) => console.log(it)); //10

Implementation 3:

const tap = (a, fn) => {fn(a); return a; };

tap(10, (it) => console.log(it)); //10

回答1:

This looks much like the Ramda definition. The * in there is probably a mistake. (Disclaimer: I'm one of the Ramda authors.) It should probably read

// tap :: (a -> b) -> a -> a

An implementation like your first one:

const tap = fn => a => { fn(a); return a; };

or Ramda's version:

const tap = curry((fn, a) => { fn(a); return a; });

match that signature and are useful IMHO mostly in debugging contexts. I use it to temporarily introduce logging statements into functional pipelines¹:

// :: Map String User
const users = fetchUsersFromSomewhere();

// :: [Comment] -> [Number]  
const userRatingForComments = R.pipe(
    R.pluck('username'),     // [Comment] -> [String]
    R.tap(console.log),      // for debugging, need to include `bind` in browser envs
//  ^^^^^^^^^^^^^^^^^^      
    R.map(R.propOf(users)),  // [String] -> [User]
    R.pluck('rating')        // [User] -> [Number]
);

This is really not the K combinator, though.

---

¹ This code sample is from an old article of mine on Ramda.