How to update Associative Array?

kdevel kdevel at
Sun Feb 13 13:58:38 UTC 2022

On Sunday, 13 February 2022 at 01:27:45 UTC, Ali Çehreli wrote:
> [...]
> > If I remove the constraint (the if-stuff) I get
> >
> >     v1.d(12): Error: cannot implicitly convert expression
> `kv.value()`
> > of type `byte` to `string`
> >     v1.d(23): Error: template instance
> `v1.update!(string[int],
> > byte[short])` error instantiating
> That's inferior because now the error message is pointing at 
> the implementation of a library function and confuses everyone.

It confuses me not really. (Function) templates are by their very 
visible to the user and I got used to reading the implementations.
Nonetheless may the compiler-generated checks be moved into the 

    void update(Target, From)(ref Target target, From from, Target 
dummy = From.init)

For this function template dmd reports in case of incompatible 

    cannot implicitly convert expression `null` of type 
`byte[short]` to `string[int]`

> [...]

> After all, everything that the programmer needs is spelled out 
> here. It requires following some types:
>       with `Target = string[int],
>            From = byte[short]`
>       must satisfy the following constraint:
>              isImplicitlyConvertible!(ValueType!From, 
> ValueType!Target)`
> The only thing that's not readily available there is ValueType 
> but at least it's very readable.

The constraints appear artificial to me: Neither are there means 
which ensure
that a list of Constraints is complete nor is it ensured that a 
list of
constraints is not overly restrictive. It seems that these 
Constraints constitute
another class of things which may get out of sync with the actual 
code (besides program documentation, program comments and 
function names).

Doesn't this re-iteration of what is already implicitly contained 
in the line

    target[kv.key] = kv.value;

violate the DRY principle?

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