funcpp

Functional C++, it's fun.

This experiment shows how to use C++ as a functional language. Loops (for, while) and jumps (goto) are not allowed.

Types

The fundamental type is Var (v). It is internally either a numberic value (Number, n) or a list (List, l).

Creation

Var n = Number::number(2);  // = 2.
Var l1 = List::list(std::vector<int>({1, 2, 3})); // = [1, 2, 3]
Var l2 = List::list(1, 3);  // = [1, 2, 3]
Var l3 = List::empty();     // = []

Helpers

int i = as_int(v);     // interprets v as an int
// For internal use only:
auto l = as_list(v);    // interprets v as a list
auto n = as_number(v);  // interprets v as a number

Functions

The functions operate on the Var types. The correctness of the types is checked during run-time.

Generic functions:

• print(v) - prints the string representation of v to the sandard output

List operations:

• length(l) - a number with the value of the number of elements of l
• head(l) - the first element of l
• tail(l) - a list containing the elements of l excluding the first element
• append(n, l) - a list created by appending n to the end of l
• concat(l1, l2) - a list containing the elements of l1 and l2

Arithmetic operation:

• add(n1, n2) - a number with the value of n1 + n2
• sub(n1, n2) - a number with the value of n1 - n2
• mult(n1, n2) - a number with the value of n1 * n2
• idiv(n1, n2) - a number with the value of n1 / n2, where '/' is the integer division

Higher-order functions:

• map(f(n), l) - a list created by applying f to every element of l
• filter(f(n), l) - a list from l where f is true for every element of this list

Examples

Demo:

#include <funcpp.h>

Var add2(Var & value){
    return add(value, Number::number(2));
}

bool odd(Var & value){
    return as_int(value) % 2 != 0;
}

int main() {
    auto l = List::list(std::vector<int>({1, 2, 3}));

    print(l);
    print(head(l));
    print(tail(l));
    print(concat(List::list(head(l)), tail(l)));
    print(map(add2, l));
    print(filter(odd, l));

    return 0;
}

Filtering out prime numbers:

#include <funcpp.h>

bool is_prime(int n, int i = 2){
    if(n <= 1){
        return false;
    } else if(n == 2){
        return true;
    } else if(i * i > n){
        return true;
    } else if(n % i == 0){
        return false;
    } else {
        return is_prime(n, i + 1);
    }
}

bool prime(Var & value){
    return is_prime(as_int(value));
}

int main(){
    Var l = List::list(1, 100);

    print(l);
    print(filter(prime, l));

    return 0;
}