# Scope/block behaviour

Andrei Alexandrescu SeeWebsiteForEmail at erdani.org
Thu Aug 19 06:54:57 PDT 2010

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}

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}

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