Re: Local constants

On Wed, Sep 4, 2024, at 22:17, John Bafford wrote: > On Sep 4, 2024, at 15:23, Rob Landers <rob@bottled.codes> wrote: > > > > On Tue, Sep 3, 2024, at 05:20, HwapX wrote: > >> Hello internals! > >> > >> I was wondering, has there been any discussion about supporting local constants > >> (variables that cannot be reassigned, perhaps even function parameters)? > > > > Out of curiosity, what value would this bring to PHP? In my experience, modern php methods > > and functions tend to fit on a single screen, at most being a few hundred lines for complex logic. > > > > — Rob > > This is about semantic clarity. If you define a variable as a constant, then, not only do you > know for certain that it cannot change, you are also stating "it is my intent to not change > this variable after setting it", and so if someone later tries to do so, they (should) have to > answer the question, "*why* do I need to make this variable change?". > > If it's useful to be able to annotate class members as being "readonly", it is > likewise useful to do that on a local scope. > > With sufficient compiler support, *typed* constant variable declarations might also allow for > this: > > let SomeType $foo; //or readonly, or writeonce, or whatever > > if($something) { > $foo = ...; > } else if($somethingElse) { > $foo = getSomeString(); //Error, type mismatch; possibly catchable at compile time, definitely > with static analysis > } else if($thirdCondition) { > $foom = ...; //oops, typo > } else { > throw new Exception(...) > } > > doSomething($foo); //Compiler error: $foo not initialized on all call paths. > } > > You might also tighten scoping with such variables: > > foreach(...) { > let $foo = //local working variable; possibly even shadowing a variable in the parent scope > } > > //$foo is undefined; you can't access the last-set value from the loop outside of the loop > > Static analysis can catch all these errors, but it'd be nicer to be able to do it without > requiring docblocks. The language providing tools for being more explicit about intent in code is > never bad. > > -John I think, conceptually, this makes sense, but maybe I'm the only one who writes $arr = doSomething(); $arr = array_map(fn($x) => $x->prop, $arr); $arr = array_filter($arr, $filterFunc); instead of $arr = array_filter(array_map(fn($x) => $x->prop, doSomething()), $filterFunc); And I feel like having constant variables would result in more of the latter, which I feel is more unreadable. Though I do note that my opinion of what is readable might be different from other people's. That being said, I would much rather see block-scoped variables than local variables via let or var or something: var $aNumber = 12; foreach($arr as var $item) { echo $item; // item exists here } echo $item; // item doesn't exist here PHP is simply too verbose to really benefit from local constants, but would benefit from block-scope far more. For example, with local constants, you couldn't write that foreach because that variable exists in the scope of the function and can only be defined once. — Rob