readonly-array ignore return type.

[RebeccaStevens]

Functions that take an Array cannot be passed a ReadonlyArray. This can somethings make it difficult to work with 3rd party libraries.

Say I'm using a 3rd party library that has a function like so:

function some3rdPartyLibrary(data: number[][]): any {
  // Do stuff.
  return someThing;
}

I cannot call this function with a ReadonlyArray so the following code would fail:

function transposeMatrix<T>(input: ReadonlyArray<ReadonlyArray<T>>): ReadonlyArray<ReadonlyArray<T>> {
  return input[0].map((_, index) => input.map(row => row[index]));
}

function processedData(data: ReadonlyArray<ReadonlyArray<number>>): any {
  return some3rdPartyLibrary(transposeMatrix(data));
}

// [ts] Argument of type 'ReadonlyArray<ReadonlyArray<number>>' is not assignable to parameter of type 'number[][]'.
// Property 'push' is missing in type 'ReadonlyArray<ReadonlyArray<number>>'.

Changing the return type of my transposeMatrix function to T[][] would fix this error but cause a tslint issue with the rule readonly-array.

I'd like to suggest adding an ignore-return-type option to the readonly-array rule.

I feel that once a function has finished and returned a value, in most cases, that function should not force the user to keep that data immutable - that should be up to the user.

The proposal outlined in #74 might be a better way to go however.

[jonaskello]

I think the ignore-return-type option would make sense as proposed here. One use case is the one above where you want to produce an Array for input to a third party library. Another as mentioned in #92 (the referenced comment above) would be if you are writing a library and want to return Array. Perhaps for some libraries it would make sense to use ArrayLike or ReadonlyArray as inputs and Array as output.