Re: Best practices of using const

2019-06-22 Thread Bart via Digitalmars-d-learn

On Wednesday, 13 February 2019 at 16:40:18 UTC, H. S. Teoh wrote:
So ironically, the iron-clad semantics of D's const system 
turns out to be also its own downfall.





Such things are not ironic. There is always a trade off. You get 
nothing for free in this universe. Physics tells us this. 
Conservation laws apply to energy and everything is energy. Hence 
your computer cannot violate these laws nor can the D 
specification(whatever it ends up meaning) nor can the D const 
system, so to speak...


That is, any time there something is inversely related to 
something else then there will be a conservation relationship. If 
you restrict something too much then something else in direct 
opposition is becoming too unrestricted.


It's not that D's const system is bad, it is that it creates too 
much restriction without any other option. The usual way too 
solve these problems is granularity. this way you can choose the 
right tool for the job.


Maybe D needs different levels of const. constN where constN can 
always be cast to constn for n <= N. One would need to properly 
define the levels to maximize utility and minimize the 
granularity. D probably only needs 3-5 levels to be effective.














Re: Best practices of using const

2019-06-22 Thread Yatheendra via Digitalmars-d-learn

On Saturday, 22 June 2019 at 05:10:14 UTC, Yatheendra wrote:
It feels disingenous to want to call a caching object even 
"logically" const. There has to be a scaffolding-based but 
hopefully generic compromise. I haven't yet tested this belief, 
but I believe "physical" const is of good use wherever it can 
be applied.


On Friday, 21 June 2019 at 23:39:20 UTC, H. S. Teoh wrote:

The problem with this is that you cannot use const(Wrapper).

In particular, if you have a function that wants to document 
that it does not mutate its argument, you cannot write:


auto func(in Wrapper data) { ... }

because const(Wrapper) does not allow lazy initialization.
...

...


"physical" const has to be applicable & good in many/most other 
use-cases than caching (citation needed). Somehow, wanting to 
call mutating code on logically const values sounds to be the 
wrong want.


Lazy initialization sounds like it will be a good DIP :-) 
Generate (once) on (first) read, just like copy (once) on (first) 
write. But there are other ways.


Heavy computation called at most once: bite the bullet, eagerly 
construct an immutable value ahead of time. "physical" const 
might have just enough optimization opportunity to offset biting 
the bullet.


Called more than once: same thing.


Re: Best practices of using const

2019-06-21 Thread Yatheendra via Digitalmars-d-learn
It feels disingenous to want to call a caching object even 
"logically" const. There has to be a scaffolding-based but 
hopefully generic compromise. I haven't yet tested this belief, 
but I believe "physical" const is of good use wherever it can be 
applied.


On Friday, 21 June 2019 at 23:39:20 UTC, H. S. Teoh wrote:

The problem with this is that you cannot use const(Wrapper).

In particular, if you have a function that wants to document 
that it does not mutate its argument, you cannot write:


auto func(in Wrapper data) { ... }

because const(Wrapper) does not allow lazy initialization.
...


IMHO, in parameters are a more important scenario than const in 
ranges (of course, same constraints).


Just for the heck of it, I'll try to get a snippet "working" but 
I see out parameters snaking all through the call chain!


Re: Best practices of using const

2019-06-21 Thread H. S. Teoh via Digitalmars-d-learn
On Fri, Jun 21, 2019 at 06:32:33PM +, Yatheendra via Digitalmars-d-learn 
wrote:
[...]
>struct CostlyComputeResult {
>   ... // data fields
>   // constructor takes compute results, no postblit
>}
> 
>struct Wrapper {
>   const (CostlyComputeResult) *cachee = 0;
>   ... // data fields storing compute inputs
>   // constructor takes compute inputs
>   // pointer to function(compute inputs)
>   const ref get() {
>  if (!cachee) {
> cachee = new(function(inputs));
>  }
>  return cachee;
>   }
>}
> 
> Hopefully jumping through these hoops is worth the while. Instead,
> maybe just wait until the compiler grows a 'cache pure' function
> qualifier (move constructor required?).

The problem with this is that you cannot use const(Wrapper).

In particular, if you have a function that wants to document that it
does not mutate its argument, you cannot write:

auto func(in Wrapper data) { ... }

because const(Wrapper) does not allow lazy initialization.  Basically
you have to resort to convention (e.g., name it ReadOnly or something
similar) rather than actually mark it const.  This generally isn't a big
problem if you're using func in isolation, but as soon as you need to
compose func with other const code, you quickly find yourself in a
gordian knot of const incompatibilities that percolate throughout the
entire call chain, because D's const is transitive.

I.e., you want func to interact with other functions that trade in const
data, but you cannot because of the constant(!) need to keep Wrapper
mutable.  The non-constness of Wrapper will then percolate up the call
chain, "tainting" all functions that call it so that they cannot be
marked const, even though *logically* they are const.  This mix is
already bad enough (try it on a non-trivial codebase sometime and see
for yourself), but once you add generic functions to the mix, the whole
thing simply becomes unusable -- because generic functions expect to
write const types as const(T), but that will break if T is Wrapper. OK,
so you can try to make it mutable as a workaround.  But then that breaks
const-ness attribute inference so the generic function becomes
non-const, which in turn recursively causes its callers to be non-const,
etc.  Somewhere at the top of the call chain you'll have a const method
that wants to call a const function, passing some higher-level data
structure that eventually contains Wrapper somewhere deep down -- and it
simply doesn't work without making the *entire* structure mutable, due
to the infectiousness of const.

So as soon as you use Wrapper in any data structure of arbitrary
complexity, the entire thing must be mutable -- otherwise const
percolates all the way down to Wrapper and the caching doesn't work
anymore.

tl;dr: using a wrapper works fine for relatively simple cases.  But as
soon as you add any meaningful complexity to it, the scheme quickly
becomes either impractically convoluted, or outright impossible to use
without a hard cast to cast away const (thereby invoking the lovely UB).


T

-- 
INTEL = Only half of "intelligence".


Re: Best practices of using const

2019-06-21 Thread Yatheendra via Digitalmars-d-learn
That is a comprehensive reply. No pointers to other material 
required :-)


On Friday, 21 June 2019 at 16:35:50 UTC, H. S. Teoh wrote:
On Fri, Jun 21, 2019 at 06:07:59AM +, Yatheendra via 
Digitalmars-d-learn wrote:
Actually, optimizers work best when there is minimal mutation 
*in the original source*.  The emitted code, of course, is free 
to use mutation however it wants.  But the trouble with 
mutation at the source level is that it makes many code 
analyses very complex, which hinders the optimizer from doing 
what it might have been able to do in the absence of mutation 
(or a reduced usage of mutation).

[...]


(aside: I hope we don't end up advocating the Haskell/Erlang way 
or the Clojure way!)


Yes, the hindrances of non-const code are documented (are most 
programmers listening!). I was only pointing out that mutation 
being part of the design limits what can be logically const. Is 
the trade-off clear, between (mythical) guaranteed C++-like-const 
at all the points we remember to put it, versus guaranteed 
D-const at the fewer points we manage to put it? Does D-const 
distort the design (but you get all the optimizations possible in 
that scenario)?


The inability to have a const caching object seems correct. 
The way around would be to have a wrapper that caches (meh). 
If that is not possible, then maybe caching objects just 
aren't meant to be const by their nature? Isn't memoize a 
standard library feature? I should look at it, but I wouldn't 
expect it to be const.


It's not as simple as it might seem.  Here's the crux of the 
problem: you have an object that logically never changes 
(assuming no bugs, of course).  Meaning every time you read it, 
you get the same value, and so multiple reads can be elided, 
etc.. I.e., you want to tell the compiler that it's OK to 
assume this object is const (or immutable).


However, it is expensive to initialize, and you'd like it to be 
initialized only when it's actually needed, and once 
initialized you'd like it to be cached so that you don't have 
to incur the initialization cost again.  However, declaring a 
const object in D requires initialization, and after 
initialization it cannot be mutated anymore. This means you 
cannot declare it const in the first place if you want caching.


It gets worse, though.  Wrappers only work up to a certain 
point.  But when you're dealing with generic code, it becomes 
problematic.  Assume, for instance, that you have a type Costly 
that's logically const, but lazily initialized (and cached).  
Since you can't actually declare it const -- otherwise lazy 
initialization doesn't work -- you have to declare it mutable.  
Or, in this case, declare a wrapper that holds a const 
reference to it, say something like this:


struct Payload {
// lazily-initialized data
}

struct Wrapper {
const(Payload)* impl;
...
}

However, what if you're applying some generic algorithms to it?
 Generic code generally assume that given a type T, if you want 
to declare a const instance of it, you simply write const(T).  
But what do you pass to the generic function? If you pass 
Wrapper, const(Wrapper) means `impl` cannot be rebound, so 
lazily initialization fails.  OK, then let's pass 
const(Payload) directly.  But that means you no longer have a 
wrapper, so you can't have lazy initialization (Payload must be 
constructed before you can pass it to the function, thus it 
must be eagerly initialized at this point).




I should check on std memoize & maybe code something up for 
understanding before writing more than this - would you mind 
pointing to an example range algorithm that we would have trouble 
passing a caching wrapper to?


I hadn't considered pointers as an option. Why wouldn't the 
following work, if expressible in D?

   struct CostlyComputeResult {
  ... // data fields
  // constructor takes compute results, no postblit
   }

   struct Wrapper {
  const (CostlyComputeResult) *cachee = 0;
  ... // data fields storing compute inputs
  // constructor takes compute inputs
  // pointer to function(compute inputs)
  const ref get() {
 if (!cachee) {
cachee = new(function(inputs));
 }
 return cachee;
  }
   }

Hopefully jumping through these hoops is worth the while. 
Instead, maybe just wait until the compiler grows a 'cache pure' 
function qualifier (move constructor required?).


Re: Best practices of using const

2019-06-21 Thread H. S. Teoh via Digitalmars-d-learn
On Fri, Jun 21, 2019 at 06:07:59AM +, Yatheendra via Digitalmars-d-learn 
wrote:
> Am I mistaken in saying that we are conflating:
>"anything that is logically const should be declared const"

No, not in D.  D does not have logical const; it has "physical" const,
which is a strict subset of logical const, and therefore there are some
uses of logical const for which D's const is unsuitable.


>// makes perfect sense
>// e.g. the lowest 2, and some branches of the 3rd and 4th, levels
>// of members (and a subset of the overall methods) in a 5-deep type 
> hierarchy are const
> with:
>"most code/data should be declared const"
>// no! isn't efficient code all about mutation?
>// no grounds for, e.g.: "ideally, no more than 40% of code should be
> doing mutation"

Actually, optimizers work best when there is minimal mutation *in the
original source*.  The emitted code, of course, is free to use mutation
however it wants.  But the trouble with mutation at the source level is
that it makes many code analyses very complex, which hinders the
optimizer from doing what it might have been able to do in the absence
of mutation (or a reduced usage of mutation).  Aliasing is one example
that hampers optimizers from emitting optimal code. Aliasing plus
mutation makes the analysis so complex that the optimizer has a hard
time deciding whether a particular construct can be optimized away or
not.

Having minimal mutation in the original source code allows the optimizer
to make more assumptions, which in turn leads to better optimizations.
It also makes the source code easier to understand.  Paradoxically,
having less mutation in the source code means it's easier for the
compiler to optimize it into mutation-heavy optimal code -- because it
doesn't have to worry about arbitrary mutations in the source code, and
therefore can be free(r) to, e.g., eliminate redundant copies, redundant
movement of data, etc., which ultimately results in in-place
modification of values, i.e., mutation-heavy emitted code.

Conversely, if the source code is heavy on mutations, then the compiler
cannot confidently predict the overall effect of the mutations, and
therefore is forced to err on the safe side of assuming the worst, i.e.,
don't apply aggressive optimizations in case the programmer's mutations
invalidate said optimizations. The result is less optimal code.


[...]
> The inability to have a const caching object seems correct. The way
> around would be to have a wrapper that caches (meh). If that is not
> possible, then maybe caching objects just aren't meant to be const by
> their nature? Isn't memoize a standard library feature? I should look
> at it, but I wouldn't expect it to be const.

It's not as simple as it might seem.  Here's the crux of the problem:
you have an object that logically never changes (assuming no bugs, of
course).  Meaning every time you read it, you get the same value, and so
multiple reads can be elided, etc.. I.e., you want to tell the compiler
that it's OK to assume this object is const (or immutable).

However, it is expensive to initialize, and you'd like it to be
initialized only when it's actually needed, and once initialized you'd
like it to be cached so that you don't have to incur the initialization
cost again.  However, declaring a const object in D requires
initialization, and after initialization it cannot be mutated anymore.
This means you cannot declare it const in the first place if you want
caching.

It gets worse, though.  Wrappers only work up to a certain point.  But
when you're dealing with generic code, it becomes problematic.  Assume,
for instance, that you have a type Costly that's logically const, but
lazily initialized (and cached).  Since you can't actually declare it
const -- otherwise lazy initialization doesn't work -- you have to
declare it mutable.  Or, in this case, declare a wrapper that holds a
const reference to it, say something like this:

struct Payload {
// lazily-initialized data
}

struct Wrapper {
const(Payload)* impl;
...
}

However, what if you're applying some generic algorithms to it?  Generic
code generally assume that given a type T, if you want to declare a
const instance of it, you simply write const(T).  But what do you pass
to the generic function? If you pass Wrapper, const(Wrapper) means
`impl` cannot be rebound, so lazily initialization fails.  OK, then
let's pass const(Payload) directly.  But that means you no longer have a
wrapper, so you can't have lazy initialization (Payload must be
constructed before you can pass it to the function, thus it must be
eagerly initialized at this point).

It's an impasse.  Cached / lazily-initialized objects and D's const
simply don't mix.  Well, you can try to mix them, but it's like trying
to mix water and oil.  They just don't work well together.


T

-- 
Notwithstanding the eloquent discontent that you have 

Re: Best practices of using const

2019-06-21 Thread Yatheendra via Digitalmars-d-learn

Am I mistaken in saying that we are conflating:
   "anything that is logically const should be declared const"
   // makes perfect sense
   // e.g. the lowest 2, and some branches of the 3rd and 4th, 
levels
   // of members (and a subset of the overall methods) in a 
5-deep type hierarchy are const

with:
   "most code/data should be declared const"
   // no! isn't efficient code all about mutation?
   // no grounds for, e.g.: "ideally, no more than 40% of code 
should be doing mutation"


On Wednesday, 13 February 2019 at 16:40:18 UTC, H. S. Teoh 
wrote:
On Wed, Feb 13, 2019 at 11:32:46AM +, envoid via 
Digitalmars-d-learn wrote:
Unfortunately, that guarantee also excludes a lot of otherwise 
useful idioms, like objects that cache data -- a const object 
cannot cache data because that means it's being mutated, or 
lazily-initialized objects -- because once the ctor has run, 
the object can no longer be mutated. Most notably, D's 
powerful range idiom is pretty much unusable with const 
because iteration over a range requires mutating the range 
(though having const *elements* in a range is fine).  This 
doesn't seem as bad at first glance, but it wreaks havoc on 
generic code, another thing that D is purportedly good at. 
It's very hard (and often impossible) to write generic code 
that works with both const and mutable objects.


So ironically, the iron-clad semantics of D's const system 
turns out to be also its own downfall.



T


The point about generic code (reiterated by many) is intriguing 
on its own; until now, I hadn't explicitly thought about const 
even for my C++ template library code (whatever little I have of 
those). Any pointers to other posts or articles elaborating this 
a little bit?


I believe the other points probably matter when interacting with 
every other feature (I would have to write some of my "real" code 
in D to see if I hit it on my own), but there doesn't seem to be 
anything unusable about them on their own.


The inability to have a const caching object seems correct. The 
way around would be to have a wrapper that caches (meh). If that 
is not possible, then maybe caching objects just aren't meant to 
be const by their nature? Isn't memoize a standard library 
feature? I should look at it, but I wouldn't expect it to be 
const.


On Monday, 18 February 2019 at 06:50:32 UTC, Marco de Wild wrote:


I agree that const by nature unfortunately kills lazy 
initialization.


Lazy initialization - is this the same as post-blit? At the cost 
of copying (justifiable? maybe), doesn't D have a way to 
copy-construct a const/immutable struct object from a mutable 
one? If there is a way (or will be - there is a recent posting 
and a Dconf talk about copy constructors), does the copying 
negate the benefits of lazy initialization?


However, I don't really understand why const is a problem with 
ranges. Const elements are not a problem. Iterating over a 
range consumes it (if I understand correctly). It does not make 
sense to be able to consume a const object, so from my point of 
view it's perfectly logical to disallow iterating const ranges. 
If I'm missing something, please correct me.

...


+1.

Or I haven't understood why ranges would ever ever need to be 
const.

After all, in C++, what use is:
   std::vector::const_iterator const iter = sequence.begin();
About the only kind of use would be:
   std::vector::const_iterator iter = sequence.begin();
   std::vector::const_iterator const iterEnd = sequence.end();
What are ranges if not an encapsulation of the above 
functionality?




Re: Best practices of using const

2019-02-20 Thread drug via Digitalmars-d-learn

On 20.02.2019 11:05, Kagamin wrote:

On Tuesday, 19 February 2019 at 16:38:17 UTC, drug wrote:
The same I can say about properties - for example I use them in meta 
programming to detect what to serialize/process - I skip methods but 
serialize properties and for me this is a nice language feature.


Serialization of arbitrary stuff is a bad practice anyway, it was the 
cause of vulnerabilities in serialization libraries. DTO is the way to go.
serialization is just an example here. But using properties lets me to 
avoid using DTO except really complex cases and lets me decrease 
maintenance cost. In my case (I develop a prototype and very often 
change its data structures) they work really well.


Re: Best practices of using const

2019-02-20 Thread Kagamin via Digitalmars-d-learn

On Tuesday, 19 February 2019 at 16:38:17 UTC, drug wrote:
The same I can say about properties - for example I use them in 
meta programming to detect what to serialize/process - I skip 
methods but serialize properties and for me this is a nice 
language feature.


Serialization of arbitrary stuff is a bad practice anyway, it was 
the cause of vulnerabilities in serialization libraries. DTO is 
the way to go.


Re: Best practices of using const

2019-02-19 Thread drug via Digitalmars-d-learn

On 19.02.2019 19:19, Kagamin wrote:

On Tuesday, 19 February 2019 at 15:30:22 UTC, Atila Neves wrote:
I keep hearing how const is nigh unusable in D, and except for ranges 
I litter my code with const everywhere, pretty much just as often as I 
used in C++.


I once spent a good amount of effort to annotate my code with pure and 
inout only to find a compiler bug, then I realized that annotations 
aren't really needed, because the collection is inherently mutable 
anyway (appender).


I use const all over the place too. And I made PR to other libraries to 
add const qualifier.
Yes, it sometimes forces me to make a copy of data to mutate it - but 
I'm pretty sure this is the purpose of the qualifier. This helps me to 
catch/prevent bug. So I don't agree with people who do not use const at 
all. Definitely const qualifier in D is usable and is useful. The same I 
can say about properties - for example I use them in meta programming to 
detect what to serialize/process - I skip methods but serialize 
properties and for me this is a nice language feature.


Re: Best practices of using const

2019-02-19 Thread Kagamin via Digitalmars-d-learn

On Tuesday, 19 February 2019 at 15:30:22 UTC, Atila Neves wrote:
I keep hearing how const is nigh unusable in D, and except for 
ranges I litter my code with const everywhere, pretty much just 
as often as I used in C++.


I once spent a good amount of effort to annotate my code with 
pure and inout only to find a compiler bug, then I realized that 
annotations aren't really needed, because the collection is 
inherently mutable anyway (appender).


Re: Best practices of using const

2019-02-19 Thread Atila Neves via Digitalmars-d-learn

On Wednesday, 13 February 2019 at 16:40:18 UTC, H. S. Teoh wrote:
On Wed, Feb 13, 2019 at 11:32:46AM +, envoid via 
Digitalmars-d-learn wrote:

[...]


Const in D is very restrictive because it's supposed to provide 
real compiler guarantees, i.e., it's statically verifiable that 
the data cannot be changed.


[...]


I keep hearing how const is nigh unusable in D, and except for 
ranges I litter my code with const everywhere, pretty much just 
as often as I used in C++. I normally only use `auto` for return 
types and input ranges, and nearly all of my function parameters 
are `in`.


It's true that a lot of people don't use `const` because I keep 
finding and filing bugs in dub libraries as soon as I try using 
them, but other than that: const is fine.


Re: Best practices of using const

2019-02-17 Thread Marco de Wild via Digitalmars-d-learn

On Wednesday, 13 February 2019 at 16:40:18 UTC, H. S. Teoh wrote:
On Wed, Feb 13, 2019 at 11:32:46AM +, envoid via 
Digitalmars-d-learn wrote:
Unfortunately, that guarantee also excludes a lot of otherwise 
useful idioms, like objects that cache data -- a const object 
cannot cache data because that means it's being mutated, or 
lazily-initialized objects -- because once the ctor has run, 
the object can no longer be mutated. Most notably, D's powerful 
range idiom is pretty much unusable with const because 
iteration over a range requires mutating the range (though 
having const *elements* in a range is fine).  This doesn't seem 
as bad at first glance, but it wreaks havoc on generic code, 
another thing that D is purportedly good at. It's very hard 
(and often impossible) to write generic code that works with 
both const and mutable objects.


So ironically, the iron-clad semantics of D's const system 
turns out to be also its own downfall.



T


I agree that const by nature unfortunately kills lazy 
initialization. However, I don't really understand why const is a 
problem with ranges. Const elements are not a problem. Iterating 
over a range consumes it (if I understand correctly). It does not 
make sense to be able to consume a const object, so from my point 
of view it's perfectly logical to disallow iterating const 
ranges. If I'm missing something, please correct me.


I use const quite thoroughly in my project (a mahjong board game) 
and in fact I am writing a blog post explaining how it helped me 
understand what was happening in my code base. It enforces 
encapsulated mutations. In classic OOP languages, mutable objects 
propagate through the entire system, unless you actively create 
an immutable copy of it (which is a lot of work for little gain). 
If someone modifies your object on a place you don't expect (e.g. 
creating and persisting data when rendering a read-only view), it 
becomes hard to impossible to reason about the problem and debug 
it.
Refactoring in const was a lot of work, but I think it made my 
code better in the end. I didn't run into any problems when using 
it, except when I tried to modify an object where I should not 
have (e.g. sorting a hand when rendering the view). I was able to 
untangle the spaghetti because the compiler poked me about it. As 
I didn't run into any problems and it helped clean up my code 
base, I would recommend trying it.


Re: Best practices of using const

2019-02-14 Thread envoid via Digitalmars-d-learn

Thank you for such a comprehensive answer.


Re: Best practices of using const

2019-02-13 Thread psycha0s via Digitalmars-d-learn

On Wednesday, 13 February 2019 at 11:32:46 UTC, envoid wrote:
Is there an article that explains best practices of using const 
in D?


You can find some information here:
https://dlang.org/articles/const-faq.html




Re: Best practices of using const

2019-02-13 Thread H. S. Teoh via Digitalmars-d-learn
On Wed, Feb 13, 2019 at 11:32:46AM +, envoid via Digitalmars-d-learn wrote:
> In C++ we have const correctness in some way. A compiler can make
> optimizations whenever it doesn't find a const_cast and the mutable
> specifier marks members that aren't a part of the object state. Of
> course, it's not perfect but one can document their intentions and
> it's possible to use synchronization primitives without an issue. On
> the opposite side, D has a stricter (transitive) const and it's almost
> useless in many cases. Is there an article that explains best
> practices of using const in D? The statement "In C++ const isn't
> transitive so we fixed that." alone isn't convincing. The only way I
> see right now is omitting the const keyword completely which is
> ridiculous.

Const in D is very restrictive because it's supposed to provide real
compiler guarantees, i.e., it's statically verifiable that the data
cannot be changed.

Unfortunately, that guarantee also excludes a lot of otherwise useful
idioms, like objects that cache data -- a const object cannot cache data
because that means it's being mutated, or lazily-initialized objects --
because once the ctor has run, the object can no longer be mutated. Most
notably, D's powerful range idiom is pretty much unusable with const
because iteration over a range requires mutating the range (though
having const *elements* in a range is fine).  This doesn't seem as bad
at first glance, but it wreaks havoc on generic code, another thing that
D is purportedly good at. It's very hard (and often impossible) to write
generic code that works with both const and mutable objects.

In practice, I've found that using const is really only sustainable at
the lowest levels of code, to guarantee low-level non-mutability of PODs
and other low-level objects.  It's also useful for representing a
reference to data that could be either mutable or immutable, in this
"type inheritance" diagram that's very helpful for D learners to
understand how D's const system works:

   const
   /   \
mutable immutable

I.e., mutable and immutable are implicitly convertible to const, but
const is not implicitly convertible to either.  Immutable in D is a hard
guarantee that the data cannot ever be changed by anyone in any thread.
Const means the holder of the const reference cannot mutate it, but a
3rd party could possibly hold a mutable reference to it and mutate it
that way. So it's a somewhat weaker guarantee.  But that's beside the
point.  The point is that when your code doesn't touch the data but you
want to be able to pass both mutable and immutable arguments to it,
const is the ticket.

But given the restrictiveness of const, it's a rare occasion when you
actually have to do this. The most notable exception being D strings,
which are defined to be immutable(char)[], i.e., a (mutable) array of
immutable chars (meaning the array itself can be changed, e.g., by
slicing, changing length, etc., but the underlying char data is
immutable).  Some of my own projects use const(char)[] quite often, in
order for the code to be able to accept both mutable char[] and string
(i.e., immutable(char)[]).

Outside of this, I only use const rarely, maybe in the occasional query
method in a low-level type where I'm sure mutation will never be
necessary.  Even in such cases, I rarely use const, because it's
infectious and a seemingly small change of adding const to a getter
method sometimes percolates throughout the entire codebase and requires
const correctness everywhere else, usually ending in a stalemate when it
reaches something like a range that needs to be mutable and cannot be
made const without onerous refactoring.  It's *possible* in theory to
make everything const-correct, but it's quite onerous and honestly only
of limited benefit relative to the sheer amount of effort required to
pull it off.  So most of the time I just don't bother except in the
lowest levels of code where the scope of const's infectiousness is
limited.

So ironically, the iron-clad semantics of D's const system turns out to
be also its own downfall.


T

-- 
Obviously, some things aren't very obvious.


Re: Best practices of using const

2019-02-13 Thread XavierAP via Digitalmars-d-learn

On Wednesday, 13 February 2019 at 11:32:46 UTC, envoid wrote:


Is there an article that explains best practices of using const 
in D?


Chapter 8 of Andrei Alexandrescu's book The D Programming 
Language.


Re: Best practices of using const

2019-02-13 Thread Alex via Digitalmars-d-learn

On Wednesday, 13 February 2019 at 11:32:46 UTC, envoid wrote:
Is there an article that explains best practices of using const 
in D?


http://jmdavisprog.com/articles/why-const-sucks.html


Re: Best practices of using const

2019-02-13 Thread Kagamin via Digitalmars-d-learn
D has immutable data, const allows to consume both mutable and 
immutable data.