How to implement value() helper method from Laravel in Psalm?

[ppmathis]

Is there any way to represent a function in Psalm which acts like this?

• If first argument is Closure():T, call it with all other arguments passed and return its return value with type T
• If first argument is anything else than Closure():T, return the value directly

The goal at the end of the day is to write a proper stub for the value() helper of Laravel which does exactly that and is often used to allow default arguments of other helpers/functions to either contain a value directly or a Closure returning the desired value.

I already tried https://psalm.dev/r/8ee69ae96b and would have imagined that this works, but for some reason Psalm returns impure-Closure():int|int for the last variable instead of just int as expected.

I noticed that narrowing @template T to @template T as scalar|null fixes the issue, but this is not really a solution as value is not just limited to scalars either. It seems to me that Psalm thinks a Closure():int could either match Closure(...):T or T and does not infer correctly that as soon as T is a Closure(), it can never be directly returned.

[weirdan]

You can get this with a conditional return type and another template parameter for closure return type: https://psalm.dev/r/3d49052546

[ppmathis]

Thank you! I tried various combinations of template and conditional types but never came up with this solution. I now tried integrating this into another method with a similar signature which will internally call value() and while all variables a to e have the correct type, Psalm complaints about InvalidReturnStatement and InvalidReturnType: https://psalm.dev/r/6651aeed3d

Is there any way to get this working without adding /** @var int|null */ above the return value($default); statement?