Introduce top type

[bdsl]

Psalm has a bottom type, no-return, and the dynamic type mixed, but I think it might be useful to also have a top type. Taking a lead from typescript, mixed is comparable to Typescript's any, no-return is comparable to Typescript's never, and we could adopt the name unknown for the top type.

To illustrate the difference, psalm emits MixedMethodCall on the call to bar() below:

<?php

/** @return mixed */
function foo() {
    return "bar";
}

function usesFoo(): void {
  foo()->bar();
}

MixedMethodCall is (with default Psalm settings) an INFO, not an ERROR.

However if the return statement was changed to /** @return unknown */ then instead of MixedMethodCall psalm would emit some form of error.

The top type should be treated as a supertype of all other types, so that the following code should cause no issues:

<?php
/** @param unknown $value */
function is_valid_array_key($value): bool {
    return is_string($value) || is_int($value);
}

unknown would be roughly equivilent to object|scalar|mixed|array|null, except currently psalm seems to treat object as a form of mixed, rather than as a supertype of all other object types.

[muglug]

Flow handles this with a mixed type that's equivalent to TypeScript's unknown.

I think it's an interesting idea, but I wonder whether it would be useful in that many places.

[bdsl]

Right, Hack does the same thing. But I think for PHP mixed probably has to retain its current meaning as used in existing docblocks.

If you don't have something like any / mixed then I think the language stops being gradually typed and becomes statically typed, which we probably don't want.

[ondrejmirtes]

PHPStan's mixed is effectively a top type.

[muglug]

PHPStan's mixed is effectively a top type.

The only deliberate difference I'm aware of is that a union of mixed|string in Psalm is still mixed|string.

The benefit of that is in finding bugs - PossiblyInvalidMethodCall can be emitted for https://psalm.dev/r/80910f8ebf where previously it would just be a MixedMethodCall issue.

[ondrejmirtes]

Yes, I want to tackle this by adding this as a property to MixedType - "it's still mixed, but sometimes I'm sure it's a string".

[muglug]

Closing as not really relevant