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digitalmars.D - Scope/block behaviour

reply Eduardo Cavazos <wayo.cavazos gmail.com> writes:
Hello,

I was surprised that these seem to not be allowed in D:

void main ()
{
   auto a = 20 ;

   {
     auto a = 30 ;
   }
}

void main ()
{
   { int f0 () { return 10 ; } }

   { int f0 () { return 20 ; } }
}

Perhaps I missed something in the FAQ.

Is there anywhere (manual or TDPL) I can read up on this language design 
decision? What other contemporary (or classic) languages feature this 
behaviour? Scheme and C both allow the above.

It seems like this would be something that might be nice for certain 
shops to enforce via a compiler switch, but not on by default.

Ed
Aug 19 2010
next sibling parent Stewart Gordon <smjg_1998 yahoo.com> writes:
Eduardo Cavazos wrote:

 Hello,
 
 I was surprised that these seem to not be allowed in D:
 
 void main ()
 {
   auto a = 20 ;
 
   {
     auto a = 30 ;
   }
 }


Correct.

http://www.digitalmars.com/d/1.0/statement.html#ScopeStatement
"Even though a new scope is introduced, local symbol declarations cannot 
shadow (hide) other local symbol declarations in the same function."


 void main ()
 {
   { int f0 () { return 10 ; } }
 
   { int f0 () { return 20 ; } }
 }


Looks like a bug.


 Perhaps I missed something in the FAQ.
 
 Is there anywhere (manual or TDPL) I can read up on this language design 
 decision? What other contemporary (or classic) languages feature this 
 behaviour? Scheme and C both allow the above.
 
 It seems like this would be something that might be nice for certain 
 shops to enforce via a compiler switch, but not on by default.


What use case have you for this feature?

Many things are legal in C(++), but nearly always mistakes and so 
compilers may generate warnings about them.  This is another example. 
Generally, the route D has taken has been to make them illegal.  IINM, 
there are always workarounds for those cases where it really is what you 
meant.

Stewart.
Aug 19 2010
prev sibling next sibling parent reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
On 08/19/2010 07:06 AM, Eduardo Cavazos wrote:

 Hello,

 I was surprised that these seem to not be allowed in D:

 void main ()
 {
 auto a = 20 ;

 {
 auto a = 30 ;
 }
 }

 void main ()
 {
 { int f0 () { return 10 ; } }

 { int f0 () { return 20 ; } }
 }

 Perhaps I missed something in the FAQ.

 Is there anywhere (manual or TDPL) I can read up on this language design
 decision? What other contemporary (or classic) languages feature this
 behaviour? Scheme and C both allow the above.

 It seems like this would be something that might be nice for certain
 shops to enforce via a compiler switch, but not on by default.

 Ed


Raw text from TDPL:

However,  it  is
illegal to  define a symbol that  would mask a symbol  in an enclosing
compound statement:

\begin{D-nocheck}
void main() {
    auto widgetCount = getWidgetCount();
    // Let's now open a nested block
    {
       auto widgetCount = getWidgetCount(); // /*[\codeError]*/
    }
}
\end{D-nocheck}

\indexes{masking}%
As long as masking does not occur, it's legal to reuse the same symbol
in different compound statements:

\begin{D}
void main() {
    {
       auto i = 0;
       ...
    }
    {
       auto i = "eye"; // Fine
       ...
    }
    double i = 3.14;   // Fine too
}
\end{D}
\begin{D-expect}
\end{D-expect}

\indexes{masking, hiding, name hiding, modularity}%
The rationale of this setup is simple.  Allowing global symbol masking
is necessary  for writing  good modular code  that's assembled  out of
separately compiled  parts; you  don't want the  addition of  a global
variable to suddenly  render various innocent bystanders uncompilable.
On the other hand, enclosing-scope  masking is useless as a modularity
device  (as there's  never a  case  of a  compound statement  spanning
multiple modules in~\dee) and most often indicates either an oversight
aspiring to become a bug, or  a cancerous function that's grown out of
control.


Andrei
Aug 19 2010
parent bearophile <bearophileHUGS lycos.com> writes:
Andrei Alexandrescu:


 Allowing global symbol masking
 is necessary  for writing  good modular code  that's assembled  out of
 separately compiled  parts; you  don't want the  addition of  a global
 variable to suddenly  render various innocent bystanders uncompilable.


When you import modules you need to ask for what you want, and not receive a
lot of names you don't want. So if you add one variable and you don't import
it, it shall not give the problems you say.

I'd like a tidier management of global variable names. So I am not sure I agree
with what is written there.

Currently DMD accepts code like:

int x = 1;
// ... lot of code here
void foo() {
    x++; // bug, local x undefined
    // ... more code here
    int x;
    x++;
}
void main() {}


Generally in a system a big source of unwanted complexity comes from unwanted
interactions between its subsystems. So I'd like D functions to be a bit more
strict in where they look for names. Global variables are sometimes useful and
I don't want to remove them from the language, as I think the Delight language
has done. But I'd like them to be asked for in a more explicit way inside
functions (in Python there is the 'global' attribute for this, but its
semantics is not tidy. In D there is the leading "." to denote an outer name,
but its usage is optional unless an equal local name exists).

Bye,
bearophile
Aug 19 2010
prev sibling parent reply Graham Fawcett <fawcett uwindsor.ca> writes:
Hi Eduardo,

I just wanted to welcome you to the list! I've come across your posts
(on Scheme and Factor) on various other lists, and I've enjoyed the
self-contained experiments you've posted about those languages. (You
may have a compatriot in our resident member Bearophile, whose posts
sometimes remind me of yours.)

I hope you'll stay for a while, and share your enthusiasm and design
sensibilities with the D community.

Cheers,
Graham


On Thu, 19 Aug 2010 07:06:50 -0500, Eduardo Cavazos wrote:


 Hello,
 
 I was surprised that these seem to not be allowed in D:
 
 void main ()
 {
    auto a = 20 ;
 
    {
      auto a = 30 ;
    }
 }
 
 void main ()
 {
    { int f0 () { return 10 ; } }
 
    { int f0 () { return 20 ; } }
 }
 
 Perhaps I missed something in the FAQ.
 
 Is there anywhere (manual or TDPL) I can read up on this language design
 decision? What other contemporary (or classic) languages feature this
 behaviour? Scheme and C both allow the above.
 
 It seems like this would be something that might be nice for certain
 shops to enforce via a compiler switch, but not on by default.
 
 Ed
Aug 19 2010
parent reply "Yao G." <nospamyao gmail.com> writes:
On Thu, 19 Aug 2010 10:22:25 -0500, Graham Fawcett <fawcett uwindsor.ca>  
wrote:


 Hi Eduardo,

 I just wanted to welcome you to the list! I've come across your posts
 (on Scheme and Factor) on various other lists, and I've enjoyed the
 self-contained experiments you've posted about those languages. (You
 may have a compatriot in our resident member Bearophile, whose posts
 sometimes remind me of yours.)

 I hope you'll stay for a while, and share your enthusiasm and design
 sensibilities with the D community.

 Cheers,
 Graham


Eduardo Cavazos sounds more like a spanish/latin american name, no?

-- 
Yao G.
Aug 19 2010
parent reply Stanislav Blinov <stanislav.blinov gmail.com> writes:
Yao G. wrote:

 On Thu, 19 Aug 2010 10:22:25 -0500, Graham Fawcett <fawcett uwindsor.ca> 
 wrote:
 

 Hi Eduardo,

 I just wanted to welcome you to the list! I've come across your posts
 (on Scheme and Factor) on various other lists, and I've enjoyed the
 self-contained experiments you've posted about those languages. (You
 may have a compatriot in our resident member Bearophile, whose posts
 sometimes remind me of yours.)

 I hope you'll stay for a while, and share your enthusiasm and design
 sensibilities with the D community.

 Cheers,
 Graham


 
 Eduardo Cavazos sounds more like a spanish/latin american name, no?
 


So? :)
Aug 19 2010
parent "Yao G." <nospamyao gmail.com> writes:
On Thu, 19 Aug 2010 14:57:09 -0500, Stanislav Blinov  
<stanislav.blinov gmail.com> wrote:


 So? :)


So what?

-- 
Yao G.
Aug 19 2010