Writing templates for numerical types poses a challenge, because some types do not support the unary - operator.
Thus, the following example will not compile for unsigned types, because -i is not allowed:
func dummy<T>(i T) {
i = -i
}
However, the following example compiles for unsigned types:
func itoa<T>(i T) {
if (i < 0) {
i = -i
}
}
Before the compiler performs a type check on i = -i, it determines that i < 0 must be false for unsigned types.
If-clauses which are known to be never executed are not compiled and especially: they are not even type checked.
Hence, the typechecker will never see i = -i in the unsigned case and therefore it compiles.
This is in effect a conditional compilation, where some if-clause is ignored based on a type parameter T.
In the above example the if-clause executes if T is signed and i < 0.
To execute a clause only if T is signed, we can write:
func atoi<T>() {
if (min<T> < 0) {
}
}
In this example the compiler infers that min<T> < 0 is true if and only if T is signed and will perform conditional compilation.