Partial function evaluationΒΆ

Many of the adaptors(such as partial or pipable) in the library supports optional partial evaluation of functions. For example, if we have sum function adapted with partial adaptor:

auto sum = partial([](int x, int y)
    return x+y;

So if we write sum(1, 2) it will return 3, however, if we write sum(1) it will return a new function, which when called again it will evaluate the function and return 3:

int i = sum(1, 2); // Returns 3
auto f = sum(1);
int j = f(2); // returns 3

Of course due to limitations in C++, to decide whether evaluate the function or to partially evaluated it is based on callability of the function and not arity. So if we call sum(1, 2, 3), it will return a function:

auto f = sum(1, 2, 3);

However, this can get out of hande as the function f will never be evaluated. Plus, it would be nice to produce an error at the point of calling the function rather than a confusing error of trying to use a partial function. The limit decorator lets us annotate the function with the max arity:

auto sum = partial(limit_c<2>([](int x, int y)
    return x+y;

So now if we call sum(1, 2, 3), we will get a compiler error. So this improves the situation, but it is not without its limitations. For example if we were to define a triple sum using the pipable adaptor:

auto sum = pipable(limit_c<3>([](int x, int y, int z)
    return x+y+z;

So if we call sum(1), there is no compiler error, not until we try to pipe in a value:

auto f = sum(1); // No error here
auto i = 2 | f; // Compile error

Of course, the goal may not be to use the pipable call, which could lead to some confusing errors. Currently, there is not a good solution to this.