It makes code marginally easier to read, and makes it no harder to write. Most general-purpose programming languages are case-sensitive, including PHPâs spiritual ancestors: Perl, C, and Java. It would have been very simple to implement case-insensitive name lookup in the Hack typechecker, and it wouldnât affect the typecheckerâs ability to do type inference. This restriction actually has nothing to do with either type safety or performance. Thatâs because Hack has to be able to interoperate with PHP, in which name lookups are still case-insensitive. Note, however, that although Hack is case-sensitive, itâs not valid to define two functions (or two classes, etc.) that have names differing only in casing. If you try to do that in Hack, however, the typechecker will report an error, saying the function CoMpUtE is undefined. That is, if you define a function named compute, you can call it by writing CoMpUtE(). In PHP, function and class names are looked up case-insensitively. Ideally, that one partial-mode file will only have one top-level statement other than require and definitionsâa function call that is the gateway to the bulk of the programâs logic: execute () } main () If it does rely on being in global scope, itâll need a more substantial rewrite to become valid Hack.Įvery program, whether a script or a web app, starts execution in top-level code, so every program will need at least one partial-mode file to serve as an entry point. You can get rid of top-level code that doesnât rely on being in global scope simply by wrapping it in a function. This is simply because top-level code exists in global scope, 1 so any read or write of a local variable is actually a read or write of a global variable. (It is allowed, but not typechecked, in partial mode.) Youâre allowed to define named entities (functions, classes, etc.) and use the require/ include family of statements at the top level in all modes. This means an additional memory access, which puts pressure on cache memories and incurs more roundtrips to main memory.Īs a corollary to the ban on the global statement, most top-level code is forbidden in strict mode. Thereâs a separate, small way in which references are bad for performance: accessing a variable that is a reference requires an additional pointer dereference, compared to accessing a regular variable. This problem is even worse with references, which is why Hack simply ignores them. As we saw in âInference on Propertiesâ, the typechecker must be very conservative with its inference around object properties, because there are so many ways to act upon properties at a distance. This makes it impossible to ensure type safety around references, and difficult to execute code around them efficiently.Īnother example of âaction at a distanceâ making type inference difficult is the problem of object properties. Passing a variable to a function by reference means that anything can happen to that variable, and the typechecker has no way of knowing what it is (because type inference is function-local, as described in âInference Is Function-Localâ). They allow the possibility of âaction at a distance,â where innocuous-looking code can have unknowable effects. References make it very difficult to do sound static analysis. They are a cross-sectional language feature: they have a deep influence on how PHP engines represent program values, on how variables are handled, on function call and return mechanisms, and on memory management. Of the features that Hack doesnât support, references are the most fundamental. Once more, to be clear: HHVM supports all of these features when running regular PHP code. If youâre simply looking to get started with Hack, itâs enough to skim the section headers of this chapter. In this chapter, weâll explore these unsupported features, analyzing why they are hard or impossible to implement static type analysis for and why theyâre hard to compile into efficient native code. PHP code that uses a feature absent from Hack can still interoperate seamlessly with Hack code. When HHVM is running PHP code (i.e., code in any file with
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