www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - Bug or logic error in if with auto?

reply "js.mdnq" <js_adddot+mdng gmail.com> writes:
Can this be made to work in D?

Error in D, x not found:

if (auto x = "thing" in arr && x == 45)
{

}

Works:

if (auto x = "thing" in arr)
     if (x == 45)
     {

     }
Dec 02 2012
next sibling parent reply "Peter Alexander" <peter.alexander.au gmail.com> writes:
On Sunday, 2 December 2012 at 15:09:17 UTC, js.mdnq wrote:

 Can this be made to work in D?

 Error in D, x not found:

 if (auto x = "thing" in arr && x == 45)
 {

 }


You can't have both a declaration and expression in an 
if-statement.

http://dlang.org/statement.html#IfStatement

IfStatement:
     if ( IfCondition ) ThenStatement
     if ( IfCondition ) ThenStatement else ElseStatement

IfCondition:
     Expression
     auto Identifier = Expression
     BasicType Declarator = Expression
Dec 02 2012
parent "Peter Alexander" <peter.alexander.au gmail.com> writes:
And no, it cannot be made to work, because the grammar would be 
ambiguous:

if (auto x = y && z)

Does this mean?

if (auto x = (y && z))

or

if (auto x = y)
     if (x && z)


You could make it work with new syntax, but given you can just 
use two if-statements, I don't see any point.
Dec 02 2012
prev sibling parent reply Nick Treleaven <ntrel-public yahoo.co.uk> writes:
On 02/12/2012 15:09, js.mdnq wrote:

 Can this be made to work in D?

 Error in D, x not found:

 if (auto x = "thing" in arr && x == 45)
 {

 }

 Works:

 if (auto x = "thing" in arr)
      if (x == 45)
      {

      }


Most programmers would probably just pollute the existing scope:

auto x = "thing" in arr;
if (x && *x == 45)
{
     ...
}

I expect this is because wrapping the above in {} for a new scope is 
just too ugly, introducing more nesting and indentation.

It would be nice if there was a way of introducing a new scope just for 
a declaration and the next statement, something like:

with auto x = "thing" in arr:
if (x && *x == 45)
{
     // x is still visible here
}

That would overload 'with' with a different meaning, but would be clear 
enough IMO. It would be useful with other constructs like while, do, 
foreach, etc. I think it would be more elegant than allowing a 
declaration clause in all constructs (like the 'for' statement has). As 
you have shown, D's 'if' statement declaration form is perhaps not very 
useful.

About while declaration syntax, see:
http://d.puremagic.com/issues/show_bug.cgi?id=5432
Dec 03 2012
next sibling parent reply "Rob T" <rob ucora.com> writes:
On Monday, 3 December 2012 at 14:22:28 UTC, Nick Treleaven wrote:


 Most programmers would probably just pollute the existing scope:

 auto x = "thing" in arr;
 if (x && *x == 45)
 {
     ...
 }


You can always wrap it inside its own scope

main()
{
   // temp scope
   {
     auto x = "thing" in arr;
     if (x && *x == 45)
     {
        ...
     }
   }

   writeln(x); // <- error x is undefined

}
Dec 03 2012
parent Nick Treleaven <ntrel-public yahoo.co.uk> writes:
On 03/12/2012 20:17, Rob T wrote:

 On Monday, 3 December 2012 at 14:22:28 UTC, Nick Treleaven wrote:


 Most programmers would probably just pollute the existing scope:

 auto x = "thing" in arr;
 if (x && *x == 45)
 {
     ...
 }


 You can always wrap it inside its own scope


Yes, that's why I wrote:


 I expect this is because wrapping the above in {} for a new scope is
 just too ugly, introducing more nesting and indentation.


;-)


    // temp scope
    {
      auto x = "thing" in arr;
      if (x && *x == 45)
      {
         ...
      }
    }


I would much prefer this:

with (auto x = "thing" in arr)
if (x && *x == 45)
{
     ...
}
Dec 04 2012
prev sibling parent reply Nick Treleaven <ntrel-public yahoo.co.uk> writes:
On 03/12/2012 14:22, Nick Treleaven wrote:

 Most programmers would probably just pollute the existing scope:

 auto x = "thing" in arr;
 if (x && *x == 45)
 {
      ...
 }

 I expect this is because wrapping the above in {} for a new scope is
 just too ugly, introducing more nesting and indentation.

 It would be nice if there was a way of introducing a new scope just for
 a declaration and the next statement, something like:

 with auto x = "thing" in arr:
 if (x && *x == 45)
 {
      // x is still visible here
 }

 That would overload 'with' with a different meaning, but would be clear
 enough IMO. It would be useful with other constructs like while, do,
 foreach, etc. I think it would be more elegant than allowing a
 declaration clause in all constructs (like the 'for' statement has).


The above syntax was inspired from Tove and Tommi's existing solution (I 
couldn't find the link earlier):
http://forum.dlang.org/post/nyhpuaoxkeunwqongfyk forum.dlang.org

The idea was to use a Tuple to wrap the new variable:

with (Tuple!(int, "a")(getInt()))
if (a > 9)
     ...

I've now improved on this syntax using opDispatch and opAssign so we can 
write:

with (wrap.a = getInt())
if (a > 9)
     ...

Whether using this in real code is good practice or not may be debatable ;-)

Code:
http://dpaste.dzfl.pl/4dbefd84
Dec 07 2012
parent "deadalnix" <deadalnix gmail.com> writes:
On Friday, 7 December 2012 at 14:21:51 UTC, Nick Treleaven wrote:

 On 03/12/2012 14:22, Nick Treleaven wrote:

 Most programmers would probably just pollute the existing 
 scope:

 auto x = "thing" in arr;
 if (x && *x == 45)
 {
     ...
 }

 I expect this is because wrapping the above in {} for a new 
 scope is
 just too ugly, introducing more nesting and indentation.

 It would be nice if there was a way of introducing a new scope 
 just for
 a declaration and the next statement, something like:

 with auto x = "thing" in arr:
 if (x && *x == 45)
 {
     // x is still visible here
 }

 That would overload 'with' with a different meaning, but would 
 be clear
 enough IMO. It would be useful with other constructs like 
 while, do,
 foreach, etc. I think it would be more elegant than allowing a
 declaration clause in all constructs (like the 'for' statement 
 has).


 The above syntax was inspired from Tove and Tommi's existing 
 solution (I couldn't find the link earlier):
 http://forum.dlang.org/post/nyhpuaoxkeunwqongfyk forum.dlang.org

 The idea was to use a Tuple to wrap the new variable:

 with (Tuple!(int, "a")(getInt()))
 if (a > 9)
     ...

 I've now improved on this syntax using opDispatch and opAssign 
 so we can write:

 with (wrap.a = getInt())
 if (a > 9)
     ...

 Whether using this in real code is good practice or not may be 
 debatable ;-)

 Code:
 http://dpaste.dzfl.pl/4dbefd84


That is really an interesting idea.
Dec 07 2012