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

Gaetan gaetan at xeberon.net
Wed Jan 29 23:32:33 PST 2014

I like the fact that is more expensive to write let mut than let. So you
initialize mutable variable on purpose
Le 30 janv. 2014 05:31, "Haoyi Li" <haoyi.sg at gmail.com> a écrit :

> Sorry to parachute in to the conversation (long time lurker) but just to
> add to the statistics, the Scala standard library and compiler has
> 11875:54575 ratio of mutable (var) vs immutable (val) declarations, so it
> seems to match pretty well with the numbers you guys are seeing in the rust
> libraries.
> On Thu, Jan 30, 2014 at 12:18 PM, Samuel Williams <
> space.ship.traveller at gmail.com> wrote:
>> Jason, I haven't actually written any rust code (yet). I'm just
>> commenting based on reading other people's code - it was something I
>> noticed in a few examples - it might have been bad form (assuming that
>> mutable data are the "Wrong Thing"?). I brought it up because I wasn't sure
>> if it was something that had been considered. It made the code I was
>> reading look a bit clunky (which appears to be the desired result).
>> On 30 January 2014 17:07, Jason Fager <jfager at gmail.com> wrote:
>>> You *should* get sick of writing 'let mut' all over the place, not
>>> just b/c of the syntax but b/c you're using mutable variables "all over the
>>> place".  Casual mutability kills maintainability.
>>> Affordances matter.  I'm convinced that the reason Option.unwrap() is
>>> used so frequently is b/c it's the shortest method name and requires the
>>> fewest explicit decisions, and so is the easiest thing to reach for.  If it
>>> were unwrap_or_fail("reason"), forcing you to both type more and to think
>>> about a fail message, unwrap_or and unwrap_or_else wouldn't look as
>>> difficult in comparison.  or and or_else would be even better, or
>>> 'do' syntax now that the keyword's free again.
>>> Make the Right Thing the easy thing, and don't put effort into making
>>> the Wrong Thing as easy or easier.
>>> On Wednesday, January 29, 2014, 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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