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)
    {

    }

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

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.

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

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

}

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)
{
    ...
}

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

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.