How to hide a function return type in order to wrap several functions into an associated array?

[Paul Backus]

On Sunday, 27 September 2020 at 18:54:11 UTC, tastyminerals wrote:
> This is rather a generic implementation question not 
> necessarily related to D but I'd like to get some opinions.
> I have a collection of functions that all have the same input, 
> a string. The output however is different and depending on what 
> the function does it can be ulong, double or bool. The problem 
> is that for each line of text I'd like to apply all these 
> functions, collect the results and write them into some file. 
> For example,
>
> [...]
>
> As you can see, I created a templated Feature struct. This does 
> not help much because I also want to create an associative 
> array of ["functionName": &numberOfPunctChars]. How can I 
> define such an array when "Feature!T function(string)[string] 
> allFuns" requires defining T beforehand and using auto is not 
> possible?
>
> I was thinking of having a Feature struct with 3 fiels of 
> ulong, double and bool members but then each Feature init would 
> look ugly imho "Feature("name", null, 1.5, null)". There should 
> be a another way.

You can use an Algebraic [1] or SumType [2] for this:

alias Feature = SumType!(ulong, double, bool);

Feature numberOfPunctChars(string text)
{
     // ...
     return Feature(cnt);
}

Feature ratioOfDigitsToChars(string text)
{
     // ...
     return Feature(ratio);
}

Feature hasUnbalancedParens(string text)
{
     // ...
     return Feature(!isBalanced);
}

[1] 
http://dpldocs.info/experimental-docs/std.variant.Algebraic.html
[2] https://code.dlang.org/packages/sumtype