model() should support a narrower output type

[Harpush]

Which @angular/* package(s) are relevant/related to the feature request?

core

Description

Currently input supports transform but model doesn't although it can be very handy.

Proposed solution

Allow model(2, {transform: fn})
For example if I can accept number | '${number}' from outside but emit only number.
It allows the user to either use:

1. modelName="8" without two way binding
2. [(modelName)]="value" with two way binding while value is only number

Alternatives considered

Probably use model<number | '${number}'>(2). But then inside I need to run the transform function myself

[alxhub]

This is a duplicate of #55166 which has the previous discussion (which concluded that model() should not support transform).

[alxhub]

I'm going to steal this issue and refocus it a bit. transform support isn't necessary to address the problem you're talking about. If we had a way to declare that model() will emit a narrower type than it accepts, then it could support two-way binding to a WritableSignal<number> since Signal<number> is assignable to Signal<number|string> and we'd never try to write a string back.

[Harpush]

I'm going to steal this issue and refocus it a bit. transform support isn't necessary to address the problem you're talking about. If we had a way to declare that model() will emit a narrower type than it accepts, then it could support two-way binding to a WritableSignal<number> since Signal<number> is assignable to Signal<number|string> and we'd never try to write a string back.

But how would you transform the string to the number for input values? At the end the component internally only want number and will always emit number. Only the outside can provide strings.

[alxhub]

But how would you transform the string to the number for input values? At the end the component internally only want number and will always emit number. Only the outside can provide strings.

You can use a computed to go from number|string to just number.

[Harpush]

But how would you transform the string to the number for input values? At the end the component internally only want number and will always emit number. Only the outside can provide strings.

You can use a computed to go from number|string to just number.

But wouldn't something like model(2, {inputTansform: fn}) makes more sense? Especially if we look at it as input and output - a model built from input with transform and output with actual type. The output type must be assigned to the input type.

[Harpush]

@alxhub Just adding that using input and output we can do:

value = input(2, { transform: fn });
valueChange = output<number>();

The only downside is we can't change it from inside and the outside must implement the output and input or two way binding.
Hence the suggested:

value = model(2, {inputTansform: fn});

Which does exactly the same but with model added features.