Psi combinator

[guillaumearm]

Please see #250 and http://hackage.haskell.org/package/base-4.10.1.0/docs/Data-Function.html#v:on

implementation idea :

// Psi
const on = const psi = curry((f, g, x, y) => f(g(x))(g(y)));

// usage
RA.on(R.prop('type'), R.comparator(myComparator))

[char0n]

legitimate.

[drupol]

The Haskell implementation of On is const on = curry((f, g, x, y) => g(f(x), f(y)));.

However, I would not use the comma to separate the terms, I would rather write it this way:

const on = curry((f, g, x, y) => f(g(x))(g(y));

See a PHP implementation here.

[char0n]

Thanks for your comment. fantasy-land combinators defines psi combinator as following:

const psi = curry((f, g, x, y) => f(g(x))(g(y)));

It's almost the same as your suggested signature, but the order of execution of f and g is reversed. Any idea why that is?

[drupol]

You're right, my comment was wrong. I updated it.

The correct implementation is: const on = curry((f, g, x, y) => f(g(x))(g(y));