[rust-dev] "let mut" <-> "var"

Corey Richardson corey at octayn.net
Wed Jan 29 19:11:37 PST 2014

Last I did a survey, `let mut` was less than half (and more around
30-40%) of the `lets` I found, though it wasn't exhaustive. It's also
important to note that Rust is not a language suited for new
programmers. Far too many concerns; it tackles hard problems and makes
tradeoffs that new programmers don't particularly benefit from, esp.

(I also teach new programmers, at Thinkful)

On Wed, Jan 29, 2014 at 10:09 PM, Samuel Williams
<space.ship.traveller at gmail.com> wrote:
> I agree that it is syntactic salt and that the design is to discourage
> mutability. I actually appreciate that point as a programmer.
> w.r.t. this specific issue: I think what concerns me is that it is quite a
> high burden for new programmers (I teach COSC1xx courses to new students so
> I have some idea about the level of new programmers). For example, you need
> to know more detail about what is going on - new programmers would find that
> difficult as it is one more concept to overflow their heads.
> Adding "var" as a keyword identically maps to new programmer's expectations
> from JavaScript. Writing a program entirely using "var" wouldn't cause any
> problems right? But, could be optimised more (potentially) if using "let"
> for immutable parts.
> Anyway, I'm not convinced either way, I'm not sure I see the entire picture
> yet. But, if I was writing code, I'd certainly get sick of writing "let mut"
> over and over again - and looking at existing rust examples, that certainly
> seems like the norm..
> On 30 January 2014 15:59, Samuel Williams <space.ship.traveller at gmail.com>
> wrote:
>> I guess the main gain would be less typing of what seems to be a
>> reasonably common sequence, and the formalisation of a particular semantic
>> pattern which makes it easier to recognise the code when you visually
>> scanning it.
>> On 30 January 2014 15:50, Kevin Ballard <kevin at sb.org> wrote:
>>> On Jan 29, 2014, at 6:43 PM, Brian Anderson <banderson at mozilla.com>
>>> wrote:
>>> > On 01/29/2014 06:35 PM, Patrick Walton wrote:
>>> >> On 1/29/14 6:34 PM, Samuel Williams wrote:
>>> >>> Perhaps this has been considered already, but when I'm reading rust
>>> >>> code
>>> >>> "let mut" just seems to stick out all over the place. Why not add a
>>> >>> "var" keyword that does the same thing? I think there are lots of
>>> >>> good
>>> >>> and bad reasons to do this or not do it, but I just wanted to propose
>>> >>> the idea and see what other people are thinking.
>>> >>
>>> >> `let` takes a pattern. `mut` is a modifier on variables in a pattern.
>>> >> It is reasonable to write `let (x, mut y) = ...`, `let (mut x, y) = ...`,
>>> >> `let (mut x, mut y) = ...`, and so forth.
>>> >>
>>> >> Having a special "var" syntax would defeat this orthogonality.
>>> >
>>> > `var` could potentially just be special-case sugar for `let mut`.
>>> To what end? Users still need to know about `mut` for all the other uses
>>> of patterns. This would reserve a new keyword and appear to duplicate
>>> functionality for no gain.
>>> -Kevin
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