feat(collections): compile time guarantee on pure functions

[zhangyuannie]

Before 😕

const arr = Object.freeze([1, 2, 3, 4, 5]);
const a = findLast(arr, (e) => e === 1);
                   ^^^
// Argument of type 'readonly number[]' is not assignable to parameter of type 'number[]'.
//   The type 'readonly number[]' is 'readonly' and cannot be assigned to the mutable type 'number[]'.

After 😃

const arr = Object.freeze([1, 2, 3, 4, 5]);
const a = findLast(arr, (e) => e === 1);

On top of that, the compiler will also help std maintainers to not mutate the parameters by mistake to a certain degree.

export function foo(array: readonly number[]) {
  array.push(1);
        ^^^^
  // Property 'push' does not exist on type 'readonly T[]'.

  array[2] = 3;
  ^^^^^^^^
  // Index signature in type 'readonly number[]' only permits reading.

  ...
}

[LionC]

This has actually been on my to do list - looks like I can check that off. Thank you!

I will do a review later today.

[LionC]

Looks good except the one minor thing (which is not your fault but could be easily fixed here) and one question.

Thanks for doing the tedious work to go through this, much appreciated!

[LionC]

LGTM! Pinging @kt3k for final approval :-)

[kt3k]

@zhangyuannie LGTM. Thanks! Nice improvement 👍

And thanks @LionC for reviewing!