The Status of Const

[Jonathan M Davis]

On Friday, August 13, 2010 13:37:09 Walter Bright wrote:
> There are many languages which support immutable - but it's hidden away in
> how they handle strings. For example, Perl has immutable strings.
> 
> The great advantage to immutability (aside from threading) is that you can
> treat a reference type as if it were a value type. This is why so many
> languages treat strings that way. With D's immutability in the type
> system, rather than special cased for strings, is you can bring that
> advantage to any of your user defined types.

There are many languages which support immutable but few support it explicitly 
(the most obvious cases where you have immutable are functional languages where 
_everything_ is immutable), and I'm not aware of any other language that tries 
to mix mutable, const, and immutable like D does. For that matter, const is a 
bit of a rarity in my experience (though obviously there are some major 
languages with it). It's more frequent that either all data is mutable, all data 
is immutable, or most data is mutable with certain types being immutable 
(generally in an implicit manner rather than explictly). So, D's attempt to mix 
all of these options (on top of other stuff like shared) in an explicit manner 
is, to my knowledge, unique.

- Jonathan M Davis