[SOLVED] Bracket syntax in Ramda replace function

Bracket syntax in Ramda replace function

I've used JavaScript for quite a long time, yet am struggling to understand how this example from the Ramda docs works:

const greet = R.replace('{name}', R.__, 'Hello, {name}!');
greet('Alice'); //=> 'Hello, Alice!'

How is the {name} variable/match recognized and then later interpolated into the Hello, {name} string? Is this a Ramda feature? A JavaScript trick? I've never seen this pattern before.

I checked the Ramda replace source code and couldn't find anything.

Solution

There's nothing special going on with {name} or replace. R.__ creates a partially applied function where the argument in that position is supplied later.

It might help to see it written this way-

const greet = R.replace('{name}', R.__, 'Hello, {name}!');

const greet = function(replacement) {
  return R.replace('{name}', replacement, 'Hello, {name}!')
}

You can see it working here without the R.replace function -

const stringReplace = R.curry((pattern, replacement, s) =>
  s.replace(pattern, replacement) // String.prototype.replace
)

const greet = stringReplace("***", R.__, "hello ***")
console.log(greet("world")) // hello world
console.log(greet("earth")) // hello earth

The "magic" of R.__ is made possible because every function in the Ramda library is curried with its curry helper. A curried function waits for all of its arguments before it is finally applied and R.__ allows you to apply arguments out of position.