How can I make ramda's omit in fp-ts?

[ammarriq]

A friend of mine made a function for deleting multiples values from object but types are not working. I need a help to make types work.
function is;

import { pipe } from 'fp-ts/lib/function';
import { map } from 'fp-ts/Array';
import { deleteAt } from 'fp-ts/Record';

const person = { name: 'Ammar', email: '[email protected]', dob: '12/12/2000' };

const surround = map(deleteAt);
const omit = (arr: string[], obj: object) => pipe(obj, ...surround(arr));
console.log(omit(['name', 'dob'], person));

and the current error while passing the surround function in pipe and using spread operator is;

A spread argument must either have a tuple type or be passed to a rest

[ankrut]

Your code does not work because pipe does not support rest parameter for arrays. It is not "passed to a rest". In other words pipe is unfortunately not generic for an arbitrary number of parameters. It supports only max 20 parameters and that's why it is saying "must [...] have a tuple type".

To reflect a loop in functional programming recursion is preferred. In your example you would need to use reduce. For example

import { pipe } from "fp-ts/lib/function";
import { map, reduce } from "fp-ts/Array";
import { deleteAt } from "fp-ts/Record";

// ...

const omit = (arr: string[], obj: object) =>
  pipe(
      arr,
      surround,
      reduce(obj, (reducedObj, f) => f(reducedObj))
  );

However, this is just a quick idea how to fix the error. The output type is still just object.

Please take a look at the omit method in fp-ts-std. It has proper type support.

import { omit } from "fp-ts-std/Struct";

const reducedPerson = pipe(person, omit(["name", "dob"]))

[ammarriq]

I think fp-ts-std works well with fp-ts because the name implies that it is an extension for fp-ts. right?

[ankrut]

Yes. At least that's what it is saying:

The missing pseudo-standard library for fp-ts.

And it is official part of the fp-ts ecosystem.