Mutable defaults

[Chris Angelico]

On Thu, Feb 11, 2021 at 12:56 PM J. Pic <jpic at yourlabs.org> wrote:
>
> I just meant removing the whole "default value mutating" story, not removing mutable variables. Really, I was wondering if there was a use case where this actually turns to an advantage, in which case it would be a designed feature rather than an undesirable side effect, which it seems to be.
>

Right, I know you weren't asking about removing mutables altogether.
But how would you remove "default value mutating"? Do you disallow any
mutable values from being argument defaults? Because that's a huge
category of values that are no longer available.

Mutable defaults most certainly ARE used deliberately, although often
it's more as a sort of "static variable" rather than actually a
parameter. But it's common enough as an idiom that it can't be changed
without breaking a lot of code.

> Now I'm wondering if you have considered copy on write, to save the overhead from copying every mutable default at every call when it is not necessary.
>

That's its own level of mess.

The only way to move forward with this is something
backward-compatible, which probably means an explicit request for late
binding (or copying, which comes to much the same thing). But it needs
an elegant syntax. The proposal lives or dies on that syntax, and so
far, it's always died.

ChrisA