Using a tuple as a function parameter

[user1234]

On Friday, 22 November 2024 at 16:36:43 UTC, Andrew wrote:
> I'm getting started using D for some small personal projects 
> and one thing I wanted to do was use a helper function for a 
> tuple. I declared the function like this:
>
>     string getOrZeroth(Tuple!(string, string, string) tup, int 
> i) pure {
>         return tup[i] == "" ? tup[0] : tup[i];
>     }
>
> and would like to use it like this:
>
>     auto foo = tuple("a", "", "c");
>     writeln(foo.getOrZeroth(1)); // prints a
>
> but when I try it, I'm getting the below error:
>
>> Error: variable \`i\` cannot be read at compile time
>
> It would probably make more sense to create a class and make 
> that a member function, but at this point I'm mostly just 
> curious about how to use a tuple as a function parameter. I 
> tried using a couple different options like `tuple` and 
> `TypeTuple` as well as trying to convert the function into a 
> template, but didn't have any better luck with those. I feel 
> like I'm probably missing something silly, and any help would 
> be appreciated.

tuples elements must be retrieved with indexes that are known at 
compile time (integer literals or enum members). You pass one but 
this becomes a value that's not known inside the body

so you can use a template value parameter instead:

```
string getOrZeroth(int i)(Tuple!(string, string, string) tup)
{
     return tup[i] == "" ? tup[0] : tup[i];
}
```

and use it like that:

```
writeln(foo.getOrZeroth!(1)); // prints a
```