Ramda Js: Setting property on an object using a value from the same object

Question

Using Ramda Js, I need to create a function that can set one object property using the value of a different property on the same object. My attempt so far is as follows:

var foo = R.set(R.lensProp('bar'), 'foo' + R.prop('foo'));
var result = foo({foo:"bar"});

Desired result:

{foo:"bar", bar:"foobar"}

Actual result:

{foo:"bar", bar: "foofunction f1(a) {... etc"}

Clearly I'm misunderstanding something here, and any insights into how to approach this would be appreciated.

score 13 · Accepted Answer · answered Jan 07 '16 at 19:54

13

Lenses are not a good fit when the value of one property depends on the value of another property. A lambda is probably best here:

const foo = o => R.assoc('bar', 'foo' + o.foo, o);

foo({foo: 'bar'});
// => {foo: 'bar', bar: 'foobar'}

answered Jan 07 '16 at 19:54

davidchambers

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score 11 · Answer 2 · edited Oct 28 '22 at 17:40

11

I had just coded something like the answer from @davidchambers and then made a points-free version, only to show how much simpler the lambda actually is. Rather than throw it out, here's how bad it looks in comparison:

var foo = (obj) => R.assoc('bar', 'foo' + obj.foo, obj);
var foo = R.converge(R.assoc('bar'), [R.pipe(R.prop('foo'), R.concat('foo')), R.identity]);

These two, with an intermediate version are available on the Ramda REPL

edited Oct 28 '22 at 17:40

General Grievance

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answered Jan 07 '16 at 20:10

Scott Sauyet

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Cheers Scott! Interesting how long winded the points-free version needs to be. – James Flight Jan 07 '16 at 21:18
I'm not guaranteeing that there isn't a more concise points-free version. But the lambda expression is already fairly clean. – Scott Sauyet Jan 07 '16 at 21:53

score 0 · Answer 3 · answered Apr 29 '19 at 13:06

I would slice the issue into 2 parts: first you need to copy the foo property of an object to bar, then change bar's value. There is no out of the box solution in ramda for the 1st, but you can use evolve for the second:

import { curry, assoc, compose, evolve } from 'ramda'

// String -> String -> {k: v}
const copyPropAs = curry((from, to, obj) => assoc(to, obj[from], obj))

// String -> String -> String
const prefix = curry((value, string) => value + string)

const fn = compose(
  evolve({
    foo: prefix('foo')
  }),
  copyPropAs('foo', 'bar')
)

fn({foo: 'bar'})

I know, that it's not all point free, but with this way the problematic part is isolated to a point where it can no longer be broken to smaller parts, and we can always come back to those to find better implementations.

Ramda Js: Setting property on an object using a value from the same object

3 Answers3