Hacker News Reader

Comment by BruceIV | Hacker News Reader

It's maybe not quite what you're looking for, but Cforall's polymorphic functions can eliminate nearly-all the unsafety of void-pointer-based polymorphism at little-to-no extra runtime cost (in fact, microbenchmarks in our as-yet-unpublished paper show speedup over void-pointer-based C in most cases due to more efficient generic type layout). As an example:

    forall(dtype T | sized(T))
    T* malloc() {  // in our stdlib
        return (T*)malloc(sizeof(T)); // calls libc malloc
    }

    int* i = malloc(); // infers T from return type

View on HN

Excuse me for my lamerism, but can you tell me what is a polymorphic function?

My idea was that if it is better to do as much compile time checks as possible before you introduce run-time checks. Does that void pointer protection run faster that code that was checked at compile time? How?

A polymorphic function is one that can operate on different types[1]. You would maybe be familiar with them as template functions in C++, though where C++ compiles different versions of the template functions based on the parameters, we pass extra implicit parameters. The example above translates to something like the following in pure C:

    void* malloc_T(size_t sizeof_T, size_t alignof_T) {
        return malloc(sizeof_T);
    }

    int* i = (int*)malloc_T(sizeof(int), alignof(int));

In this case, since the compiler verifies that int is actually a type with known size (fulfilling `sized(T)`), it can generate all the casts and size parameters above, knowing they're correct.

[1] To anyone inclined to bash my definition of polymorphism, I'm mostly talking about parametric polymorphism here, though Cforall also supports ad-hoc polymorphism (name-overloading). The phrasing I used accounts for both, and I simplified it for pedagogical reasons.

Reply on news.ycombinator.com