How do I multiply an integer and a double in Q#?

Question

If I write this:

function f(n: Int) : Double {
    return 1.5*n;
}

I get an error:

The arguments to the binary operator do not have a common base type.

Apparently I need a function to turn n from an Int into a Double. There are lots of functions going from double to int, like Microsoft.Quantum.Extensions.Math.Floor, but I wasn't able to find anything from int to double.

Answer 1

You want the Microsoft.Quantum.Extensions.Convert.ToDouble function (deprecated in favor of Microsoft.Quantum.Convert.IntAsDouble in 0.6 release).

open Microsoft.Quantum.Extensions.Convert;

function f(n: Int) : Double {
    return 1.5*ToDouble(n);
}

The reason it works this way is because in Q# (Num a) => a -> a -> a is not the same as (Num a,Num b) => a -> b -> a as Haskell would denote it.