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digitalmars.D.learn - writeln if not empty

reply Timothee Cour <thelastmammoth gmail.com> writes:
Is there a way to do the following lazily:

writelnIfNotEmpty(T)(T a){
auto b=text(a);
if(b.length)
  writeln(b);
}

ie, without using std.conv.text (which needlessly computes an intermediate
string, which could be quite large) or launching a separate process ?

writelnIfNotEmpty(""); //doesn't print new line
writelnIfNotEmpty("a"); //prints "a\n"
Mar 10 2014
parent reply Nick Sabalausky <SeeWebsiteToContactMe semitwist.com> writes:
On 3/10/2014 9:24 PM, Timothee Cour wrote:

 Is there a way to do the following lazily:

 writelnIfNotEmpty(T)(T a){
 auto b=text(a);
 if(b.length)
    writeln(b);
 }

 ie, without using std.conv.text (which needlessly computes an intermediate
 string, which could be quite large) or launching a separate process ?

 writelnIfNotEmpty(""); //doesn't print new line
 writelnIfNotEmpty("a"); //prints "a\n"


Sounds like what you need is a version of to!string() or text() that 
takes an output sink. Taking a look at std.conv, I'm kinda surprised I 
don't see one :/
Mar 10 2014
next sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Monday, March 10, 2014 21:50:25 Nick Sabalausky wrote:

 On 3/10/2014 9:24 PM, Timothee Cour wrote:

 Is there a way to do the following lazily:
 
 writelnIfNotEmpty(T)(T a){
 auto b=text(a);
 if(b.length)
 
    writeln(b);
 
 }
 
 ie, without using std.conv.text (which needlessly computes an intermediate
 string, which could be quite large) or launching a separate process ?
 
 writelnIfNotEmpty(""); //doesn't print new line
 writelnIfNotEmpty("a"); //prints "a\n"


 
 Sounds like what you need is a version of to!string() or text() that
 takes an output sink. Taking a look at std.conv, I'm kinda surprised I
 don't see one :/


std.format.formattedWrite will do the equivalent of writef to an output range, 
though I'm not sure that that will really do what the OP wants, since it would 
still have to write the result to an output range even if it were empty, and 
odds are that the output range would be something on the heap anyway (e.g. 
Appender is technically on the stack, but it's contents are on the heap), 
making it so that it probably doesn't help much in this case.

Though to be honest, I'm not quite sure why writelnIfNotEmpty would be very 
useful unless what's being passed in would result in the empty string, and I 
would think that that would almost always be detectable (the one exception 
being user-defined types whose toString results in an empty string). Something 
as simple as

void writelnIfNotEmpty(T)(T a)
{
    static if(isInputRange!T)
    {
        if(!a.empty)
            writeln(a);
    }
    else
        writeln(a);
}

would then cover most cases - the one exception being toStrings which can 
result in empty. And if that's a concern, then something like

    else static if(is(T == struct) || is(T == class))
    {
        auto b = to!string(a);
        if(b.length)
            writeln(b);
    }

should take care of the toString case. It doesn't avoid creating an 
intermediate string, but unless the toString takes an output range, it's 
always going to allocate anyway, and if it does take an output range, once 
again, you'd need one which somehow avoided allocating altogether, which isn't 
particularly likely.

Alternatively, you could just assume that no toString will result in the empty 
string, as it's probably pretty rare that it would, but I'm not sure that that 
would actually save you any overhead except in the case where the toString 
takes an output range (since otherwise, it'll allocate a new string 
regardless), but toStrings which take output ranges are fairly uncommon at 
this point.

- Jonathan M Davis
Mar 10 2014
prev sibling parent reply Timothee Cour <thelastmammoth gmail.com> writes:
On Mon, Mar 10, 2014 at 9:14 PM, Jonathan M Davis <jmdavisProg gmx.com>wrote:


 On Monday, March 10, 2014 21:50:25 Nick Sabalausky wrote:

 On 3/10/2014 9:24 PM, Timothee Cour wrote:

 Is there a way to do the following lazily:

 writelnIfNotEmpty(T)(T a){
 auto b=text(a);
 if(b.length)

    writeln(b);

 }

 ie, without using std.conv.text (which needlessly computes an




 intermediate

 string, which could be quite large) or launching a separate process ?

 writelnIfNotEmpty(""); //doesn't print new line
 writelnIfNotEmpty("a"); //prints "a\n"


 Sounds like what you need is a version of to!string() or text() that
 takes an output sink. Taking a look at std.conv, I'm kinda surprised I
 don't see one :/


 std.format.formattedWrite will do the equivalent of writef to an output
 range,
 though I'm not sure that that will really do what the OP wants, since it
 would
 still have to write the result to an output range even if it were empty,
 and
 odds are that the output range would be something on the heap anyway (e.g.
 Appender is technically on the stack, but it's contents are on the heap),
 making it so that it probably doesn't help much in this case.

 Though to be honest, I'm not quite sure why writelnIfNotEmpty would be very
 useful unless what's being passed in would result in the empty string, and
 I
 would think that that would almost always be detectable (the one exception
 being user-defined types whose toString results in an empty string).
 Something
 as simple as

 void writelnIfNotEmpty(T)(T a)
 {
     static if(isInputRange!T)
     {
         if(!a.empty)
             writeln(a);
     }
     else
         writeln(a);
 }

 would then cover most cases - the one exception being toStrings which can
 result in empty. And if that's a concern, then something like

     else static if(is(T == struct) || is(T == class))
     {
         auto b = to!string(a);
         if(b.length)
             writeln(b);
     }

 should take care of the toString case. It doesn't avoid creating an
 intermediate string, but unless the toString takes an output range, it's
 always going to allocate anyway, and if it does take an output range, once
 again, you'd need one which somehow avoided allocating altogether, which
 isn't
 particularly likely.

 Alternatively, you could just assume that no toString will result in the
 empty
 string, as it's probably pretty rare that it would, but I'm not sure that
 that
 would actually save you any overhead except in the case where the toString
 takes an output range (since otherwise, it'll allocate a new string
 regardless), but toStrings which take output ranges are fairly uncommon at
 this point.

 - Jonathan M Davis



Thanks,
that indeed works in many cases but there are still others (eg what if
empty() prints, say in debug mode: writelnIfNotEmpty would not print the
debug information even though writeln() would execute that debug code and
print something).

I was wondering whether there would be a robust way that would check
whether anything was written to stdout (eg via accessing raw file pointer/C
lib), eg something like that :

void writelnIfNotEmpty(T)(T a){
auto file_pos=get_filepos(stdin);
write(a);
if(get_filepos(stdin)!=file_pos)
writeln;
}
Mar 10 2014
next sibling parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Tuesday, 11 March 2014 at 05:37:25 UTC, Timothee Cour wrote:

 void writelnIfNotEmpty(T)(T a){
 auto file_pos=get_filepos(stdin);
 write(a);
 if(get_filepos(stdin)!=file_pos)
 writeln;
 }


You could simply create an sink that forwards to stdout, while 
keeping state:

//----
import std.stdio, std.format;

bool writelnIfNotEmpty(T)(T a)
{
     bool written = false;
     void checkWriter(in char[] s)
     {
         if (s.length)
         {
             written = true;
             write(s);
         }
     }
     formattedWrite(&checkWriter, "%s", a);
     if (written)
     {
         writeln();
         return true;
     }
     return false;
}

void main()
{
     writelnIfNotEmpty(1);
     writelnIfNotEmpty("");
     writelnIfNotEmpty(2);
}
//----

This prints:
//----
1
2
//----

Also, this didn't work up until a few releases ago. I am really 
really happy to see code like this finally "just work". yay!
Mar 11 2014
parent Timothee Cour <thelastmammoth gmail.com> writes:
Thanks! your solution is more robust (minus some caveats i mentioned) and
also trivially extends to variadics.


On Tue, Mar 11, 2014 at 2:07 PM, monarch_dodra <monarchdodra gmail.com>wrote:


 On Tuesday, 11 March 2014 at 05:37:25 UTC, Timothee Cour wrote:


 void writelnIfNotEmpty(T)(T a){
 auto file_pos=get_filepos(stdin);
 write(a);
 if(get_filepos(stdin)!=file_pos)
 writeln;
 }


 You could simply create an sink that forwards to stdout, while keeping
 state:

 //----
 import std.stdio, std.format;

 bool writelnIfNotEmpty(T)(T a)
 {
     bool written = false;
     void checkWriter(in char[] s)
     {
         if (s.length)
         {
             written = true;
             write(s);
         }
     }
     formattedWrite(&checkWriter, "%s", a);
     if (written)
     {
         writeln();
         return true;
     }
     return false;
 }

 void main()
 {
     writelnIfNotEmpty(1);
     writelnIfNotEmpty("");
     writelnIfNotEmpty(2);
 }
 //----

 This prints:
 //----
 1
 2
 //----

 Also, this didn't work up until a few releases ago. I am really really
 happy to see code like this finally "just work". yay!
Mar 11 2014
prev sibling parent "monarch_dodra" <monarchdodra gmail.com> writes:
On Tuesday, 11 March 2014 at 05:37:25 UTC, Timothee Cour wrote:

 Thanks,
 that indeed works in many cases but there are still others (eg 
 what if
 empty() prints, say in debug mode: writelnIfNotEmpty would not 
 print the
 debug information even though writeln() would execute that 
 debug code and
 print something).

 I was wondering whether there would be a robust way that would 
 check
 whether anything was written to stdout (eg via accessing raw 
 file pointer/C
 lib)


I have no idea how you could intercept such information. At 
least, not with "standard D". You'd (probably) have to, as you 
said, have to dig up C's stdlib.
Mar 11 2014