Re: overloading an existing operator in Guile

[Maxime Devos]

Op 03-10-2023 om 11:13 schreef Damien Mattei:
> hello,
> is it possible to overload an existing operator in Guile?
>
> example overload + to concatenate vectors.
>
> for example in Scheme (+ i admit) i can do :
> ; first stage overloading
> (define-overload-existing-operator +)
>
> ; second stage overloading
> (overload-existing-operator + vector-append (vector? vector?))
>
> and use it like that:
> > (+ #(1 2 3) #(4 5 6))
> '#(1 2 3 4 5 6)
> > (+ #(1 2 3) #(4 5) #(6 7 8 9))
> '#(1 2 3 4 5 6 7 8 9)
> > {#(1 2) + #(3) + #(4 5 6)}
> '#(1 2 3 4 5 6)
>
> is it possible and how to do it using GOOPS (guile object oriented
> programming system) , i already did some sort of thing with new object
> , but not with an existing operator like + that apply to numbers only.

It's explained in ‘(guile)Methods and Generic Functions’ how to do this.

However, I very much recommend not doing this in your situation.

While this could be a valid interpretation of vector ‘addition’, here is 
another valid interpretation incompatible with yours:

;; element-wise addition
(+ #(1 2 3) #(4 5 6))
#(5 7 9)

Sure, you could choose vector appending in Scheme+ and document that 
appropriately, but so could a hypothetical SchemePlus choose 
element-wise addition, and then if someone imports a library using 
Scheme+ and also a library SchemePlus, there is ambiguity and GOOPS will 
get things wrong!

Instead, I propose using the symbol '++' and defining a new method named 
'++'.  It would unambiguously mean ‘appending’ (*) instead of 
‘addition’, and as an additional benefit, some other languages (^) use 
'++' to mean appending as well.

(*) string-append, append, vector-append, ...
(^) for example, Coq and Haskell

Best regards,
Maxime Devos.

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