Parametric methods

I have just read the “parametric methods” part of the julia docs:

Is it possible to parametrize the value (but not the type) of arguments of a function?

For example, suppose we want to calculate the nth power of x, and we want to pass the parameter n using the ‘where’ keyword. Is that possible?

Of course, you can say, it is easy with the more conventional way:

function f(x , n )
    return x^n 

It’s unclear to me what that means. Can you elaborate?

You can dispatch on argument values by moving values into the type domain — that is, you wrap the value in a type. A generic type that is provided for this purpose is Val.

This is exactly what is done by the literal_pow function, which gets called when you write x^n where n is a literal constant.


Parameters are always involved in the type domain, even when they’re not types. If you go beyond type-based polymorphism, you start losing the distinction between types and their values.

A rewrite of the original example could look like this:

f(x, n::Number) = f(x, Val(n))

f(x, ::Val{N}) where N = x^N

I really suggest that you don’t use methods like the first method on a regular basis, literal_pow is only used because powers of integer literals can lower to it and leverage constant optimizations. If you use it on a variable whose runtime value can vary without any restrictions, then you’re compiling the second method repeatedly. That can easily ruin any performance benefits.

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