When should pure function modify parameters?

[Alex Bryan]

On Monday, 17 November 2025 at 14:46:06 UTC, Brother Bill wrote:
> Pure functions in Functional Programming provide the same 
> result when provided the same arguments.
>
> They are not allowed to read or write to system resources such 
> as file system, console, clock, network, etc.  In addition, 
> they may not read/write to module level variables.
>
> In D, they are allowed to mutate parameters which seems to 
> violate purity.  Why did D make this choice and when to best 
> exploit this architectural decision.
>
> Also, would you agree that not mutating parameters to have 
> "true" purity would be preferred?

There's a separate mechanism for describing the immutability of 
parameters, and that's by using `const`/`immutable`/`in` for each 
parameter. The `pure` keyword for me, covers the rest of the 
purity of the function (globals). You could argue that `pure` 
should enforce `const` or `immutable` for each parameter, but I 
don't think that's a good idea as parameters are commonly used to 
return values and I think marking a function that returns values 
in one or more parameters `pure` to enforce that the function 
doesn't change any globals still has value