• Nicolas F. (37/37) Nov 20 2015 Today I stumbled upon something somewhat confusing to me with
• Jack Stouffer (3/7) Nov 20 2015 Sounds like a good idea! Please file an enhancement request on
• Jonathan M Davis (5/12) Nov 20 2015 Yeah. Whenever you think that an error message is bad, and you
• Jonny (19/19) Nov 23 2015 On Friday, 20 November 2015 at 19:37:09 UTC, Nicolas F. wrote:
• Timon Gehr (2/6) Nov 23 2015 Because the first notation already has a different meaning.

Nicolas F. <ddev fratti.ch> writes:

Today I stumbled upon something somewhat confusing to me with 
regards to type inference and foreach loops.

As hopefully people are aware, declaring variables works like 
this:

int[] foo = [1, 2, 3, 4];

and declaring them with type inference works like this:

auto foo = [1, 2, 3, 4];

However, if we have a foreach loop, the following is the case:

foreach (int e; foo) { } // Works as expected
foreach (auto e; foo) { } // Does not work, "Error: basic type 
expected, not auto"
foreach (e; foo) { } // Works, type inference!

Somebody on #d told me that this is due to "auto" being 
essentially meaningless other than a keyword for saying "we're 
declaring a variable", and since we're always declaring a 
variable in the case of foreach, it'd be redundant.

Allowing "auto" here, as somebody pointed out, would lead to two 
styles which may be confusing as one might mistake "foreach (e" 
to be different from "foreach (auto e", and even worse, might be 
led to believe that "e" was declared earlier in the former case.

Enforcing "auto" inside foreach declarations would break old code 
for no reason other than adding a redundant keyword to make 
things more obvious to people who don't know the semantic 
workings of "foreach" (i.e. don't realise it's always a 
declaration there).

The third, and in my opinion, most pragmatic approach to clearing 
things up is to improve the compiler error message for this 
special case of somebody trying to use "auto" inside foreach. 
"Error: basic type expected, not auto" was misleading to me, 
since it almost seems like it requires a basic type 
syntactically, which it doesn't. This would also make things 
clearer without actually changing the language spec.

TL;DR: My suggestion is to have the compiler output a specific 
error message should it encounter "auto" inside a foreach 
declaration, pointing out that "auto" is neither needed nor 
allowed in this context to get type inference.

Any thoughts or opinions on this?

Jack Stouffer <jack jackstouffer.com> writes:

On Friday, 20 November 2015 at 19:37:09 UTC, Nicolas F. wrote:
 My suggestion is to have the compiler output a specific error 
 message should it encounter "auto" inside a foreach 
 declaration, pointing out that "auto" is neither needed nor 
 allowed in this context to get type inference.

Sounds like a good idea! Please file an enhancement request on 
http://issues.dlang.org/

Jonathan M Davis <jmdavisProg gmx.com> writes:

On Friday, 20 November 2015 at 19:43:16 UTC, Jack Stouffer wrote:
 On Friday, 20 November 2015 at 19:37:09 UTC, Nicolas F. wrote:
 My suggestion is to have the compiler output a specific error 
 message should it encounter "auto" inside a foreach 
 declaration, pointing out that "auto" is neither needed nor 
 allowed in this context to get type inference.

 Sounds like a good idea! Please file an enhancement request on 
 http://issues.dlang.org/

Yeah. Whenever you think that an error message is bad, and you 
have a better suggestion, please create an enhancement request 
for it.

- Jonathan M Davis

Jonny <Jonny nomail.com> writes:

On Friday, 20 November 2015 at 19:37:09 UTC, Nicolas F. wrote:
"
Allowing "auto" here, as somebody pointed out, would lead to two 
styles which may be confusing as one might mistake "foreach (e" 
to be different from "foreach (auto e", and even worse, might be 
led to believe that "e" was declared earlier in the former case.
"

So, doesn't that logically suggest that auto e should be kept and 
e should disregarded or given the error you suggested?

foreach(e; x) looks like e is an unqualified type.
foreach(auto e; x) looks like e has type "auto", which makes 
sense.

I think most C programmers looking at D without any previous D 
experiences would think using auto made more sense.

Else, why not allow stuff like

e = 1;

instead of

auto e = 1;

?

Timon Gehr <timon.gehr gmx.ch> writes:

On 11/24/2015 05:58 AM, Jonny wrote:
 Else, why not allow stuff like

 e = 1;

 instead of

 auto e = 1;

Because the first notation already has a different meaning.