digitalmars.D.learn - How to simplify nested ifs

Ozan =?UTF-8?B?U8O8ZWw=?= (13/13) Mar 13 2018 Hi

rikki cattermole (9/29) Mar 13 2018 if (source !is null &&
rumbu (6/19) Mar 13 2018 You need the null conditional operator which is not available in

aliak (19/45) Mar 13 2018 This library may help as well:

Satoshi (5/18) Mar 16 2018 null conditional operators are not implemented in D because, as a

drug (4/29) Mar 16 2018 I think that null conditional operator is intended for OOP only. It's

drug (2/5) Mar 16 2018 Oops. I mean *isn't intended for OOP only*

bauss (3/8) Mar 16 2018 Tbh. the real reason is because there is no real backing and

bauss (55/68) Mar 16 2018 Kind of an ugly hack, but:
Tony (5/6) Mar 16 2018 That type of chain is sometimes referred to as a "train wreck"

Ozan =?UTF-8?B?U8O8ZWw=?= <ozan.sueel gmail.com> writes:

Hi

I have a construction like the following

if (source) {
   if (source.pool) {
     if (source.pool.repository) {
       if (source.pool.repository.directory) {
	if (source.pool.repository.directory.users) {
	  // do something

Any chance to simplify this nested ifs?
I know some languages has a way like.

if (source?pool?repository?directory?users) // do something


Similar ways in D?

Thanks and Regards, Ozan

Mar 13 2018

rikki cattermole <rikki cattermole.co.nz> writes:

On 14/03/2018 1:23 AM, Ozan Süel wrote:
 Hi
 
 I have a construction like the following
 
 if (source) {
    if (source.pool) {
      if (source.pool.repository) {
        if (source.pool.repository.directory) {
      if (source.pool.repository.directory.users) {
        // do something
 
 Any chance to simplify this nested ifs?
 I know some languages has a way like.
 
 if (source?pool?repository?directory?users) // do something
 
 
 Similar ways in D?
 
 Thanks and Regards, Ozan

if (source !is null &&
	source.pool !is null &&
	source.pool.repository !is null &&
	source.pool.repository.directory !is null &&
	source.pool.repository.directory.users !is null) {
	//...
}

That is what I'd do.

Mar 13 2018

rumbu <rumbu rumbu.ro> writes:

On Tuesday, 13 March 2018 at 12:23:06 UTC, Ozan Süel wrote:
 Hi

 I have a construction like the following

 if (source) {
   if (source.pool) {
     if (source.pool.repository) {
       if (source.pool.repository.directory) {
 	if (source.pool.repository.directory.users) {
 	  // do something

 Any chance to simplify this nested ifs?
 I know some languages has a way like.

 if (source?pool?repository?directory?users) // do something


 Similar ways in D?

 Thanks and Regards, Ozan

You need the null conditional operator which is not available in 
D.

A library solution can be found there: 
https://forum.dlang.org/post/mailman.2562.1403196857.2907.digitalmars-d puremagic.com

SafeDeref(source).pool.repository.directory.users

Mar 13 2018

aliak <something something.com> writes:

On Tuesday, 13 March 2018 at 13:13:07 UTC, rumbu wrote:
 On Tuesday, 13 March 2018 at 12:23:06 UTC, Ozan Süel wrote:
 Hi

 I have a construction like the following

 if (source) {
   if (source.pool) {
     if (source.pool.repository) {
       if (source.pool.repository.directory) {
 	if (source.pool.repository.directory.users) {
 	  // do something

 Any chance to simplify this nested ifs?
 I know some languages has a way like.

 if (source?pool?repository?directory?users) // do something


 Similar ways in D?

 Thanks and Regards, Ozan

 You need the null conditional operator which is not available 
 in D.

 A library solution can be found there: 
 https://forum.dlang.org/post/mailman.2562.1403196857.2907.digitalmars-d puremagic.com

 SafeDeref(source).pool.repository.directory.users

This library may help as well: 
https://code.dlang.org/packages/optional

It will handle safe dispatching of reference types and pointer 
types and give you back the actual return result as well wrapped 
up in an optional value.

class C0 {
     C1 f() { return new C1(); }
     class C1 {
         C2 f() { return new C2(); }
         class C2 {
             int f() { return 0; }
         }
     }
}

auto res1 = some!C0(null).dispatch.f.f.f; // all no ops
auto res2 = some!C0(new C0()).dispatch.f.f.f // calls all fs

assert(res1 == none);
assert(res2 == some(0));

Mar 13 2018

Satoshi <satoshi rikarin.org> writes:

On Tuesday, 13 March 2018 at 12:23:06 UTC, Ozan Süel wrote:
 Hi

 I have a construction like the following

 if (source) {
   if (source.pool) {
     if (source.pool.repository) {
       if (source.pool.repository.directory) {
 	if (source.pool.repository.directory.users) {
 	  // do something

 Any chance to simplify this nested ifs?
 I know some languages has a way like.

 if (source?pool?repository?directory?users) // do something


 Similar ways in D?

 Thanks and Regards, Ozan

null conditional operators are not implemented in D because, as a 
(I think Walter) said, D is not language designed to work with 
classes or advanced OOP stuff. Nobody uses it, so please, if you 
are using it, stop and use structs and meta programming instead.

Mar 16 2018

drug <drug2004 bk.ru> writes:

On 16.03.2018 09:34, Satoshi wrote:
 On Tuesday, 13 March 2018 at 12:23:06 UTC, Ozan Süel wrote:
 Hi

 I have a construction like the following

 if (source) {
   if (source.pool) {
     if (source.pool.repository) {
       if (source.pool.repository.directory) {
     if (source.pool.repository.directory.users) {
       // do something

 Any chance to simplify this nested ifs?
 I know some languages has a way like.

 if (source?pool?repository?directory?users) // do something


 Similar ways in D?

 Thanks and Regards, Ozan

 null conditional operators are not implemented in D because, as a (I
 think Walter) said, D is not language designed to work with classes or
 advanced OOP stuff. Nobody uses it, so please, if you are using it, stop
 and use structs and meta programming instead.

I think that null conditional operator is intended for OOP only. It's 
really useful if your data field may be nullable. May be start review 
about this?

Mar 16 2018

drug <drug2004 bk.ru> writes:

On 16.03.2018 09:51, drug wrote:
 I think that null conditional operator is intended for OOP only. It's
 really useful if your data field may be nullable. May be start review
 about this?

Oops. I mean *isn't intended for OOP only*

Mar 16 2018

bauss <jj_1337 live.dk> writes:

On Friday, 16 March 2018 at 09:34:38 UTC, Satoshi wrote:
 null conditional operators are not implemented in D because, as 
 a (I think Walter) said, D is not language designed to work 
 with classes or advanced OOP stuff. Nobody uses it, so please, 
 if you are using it, stop and use structs and meta programming 
 instead.

Tbh. the real reason is because there is no real backing and 
there has never been proposed a DIP on it.

Mar 16 2018

bauss <jj_1337 live.dk> writes:

On Tuesday, 13 March 2018 at 12:23:06 UTC, Ozan Süel wrote:
 Hi

 I have a construction like the following

 if (source) {
   if (source.pool) {
     if (source.pool.repository) {
       if (source.pool.repository.directory) {
 	if (source.pool.repository.directory.users) {
 	  // do something

 Any chance to simplify this nested ifs?
 I know some languages has a way like.

 if (source?pool?repository?directory?users) // do something


 Similar ways in D?

 Thanks and Regards, Ozan

Kind of an ugly hack, but:

string createCondition(string[] entries)
{
     string result = "";
     string lastEntry = "";

     foreach (entry; entries)
     {
         if (lastEntry && lastEntry.length) lastEntry ~= "." ~ 
entry;
         else lastEntry = entry;

         result ~= lastEntry ~ " !is null &&";
     }

     result.length -= 2;

     return result;
}

bool isDefined(alias symbol,T)(T arg)
{
     import std.array : split;

     enum symbolEntries = symbol.split(".");

     enum entriesCondition = createCondition(["arg"] ~ 
symbolEntries[1..$]);

     mixin("return " ~ entriesCondition ~ ";");
}

...

Usage:

Let's say we have these:

class A {}

class B { A a; }

class C { B b; }

class D { C c; }

class E { D d; }

Then instead of:

auto e = new E;

if (e !is null && e.d !is null && e.d.c !is null && e.d.c.b !is 
null && e.d.c.b.a !is null)
{
     // ...
}

Then we can just do:

if (isDefined!((e.d.c.b.a).stringof)(e))
{
    // ...
}

...

The ugly part tbh.

is that we need to stringof when passing the symbol and we also 
have to pass the object itself to the function.

A preferred version would have been:

if (isDefined!(e.d.c.b.a))
{
    // ...
}

Unfortunately that's not possible.

You can see it live here: https://run.dlang.io/is/e2ACNc

Mar 16 2018

Tony <tonytdominguez aol.com> writes:

On Tuesday, 13 March 2018 at 12:23:06 UTC, Ozan Süel wrote:

 if (source?pool?repository?directory?users) // do something

That type of chain is sometimes referred to as a "train wreck" 
(see Law of Demeter).

If this is a common lookup it could be:

if (source && source.GotSomeUsers() )

Mar 16 2018

D Programming

C/C++ Programming

Other

digitalmars.D.learn - How to simplify nested ifs