Re: Is there a simple way to check if value is null for every case?
On Monday, 27 August 2018 at 21:48:04 UTC, Alex wrote:
On Monday, 27 August 2018 at 19:36:29 UTC, aliak wrote:
Then Nullable!(int*) would be the same as int*. Or even better
maybe is to give a compiler error when you try and stuff a
nullable type inside a Nullable. Because ... why?
Isn't it arguable, whether this is desired? I mean, in the
present state you can separate the cases, whether the Nullable
is set or not. This is a feature.
If Nullable!(int*) behaves like int*, it would be somewhat more
straight forward, which is maybe also a feature. But you loose
something you can do right now...
Certainly arguable :)
I'm unsure about of what you lose is worth having, is the
problem. And I have trouble thinking of a use case?
If we implement opEquals(typeof(null)) as is now, then this code:
Nullable!(int*) n = null;
assert(n != null);
passes.
And if we have no opEquals then we can't test for native null on
a 'null'able type O_o
Another options is:
bool opEquals(typeof(null)) {
enum isNullInvalid = is(T == class) || is(T == interface) ||
isSomeFunction!T;
static if (isNullInvalid)
return isNull || data is null
else
return isNull;
}
I also did a quick search on google "allintext: "nullable"
site:github.com filetype:d" and there were 0 instances of T*
(just looked thorugh search results didn't click in).
Re: Is there a simple way to check if value is null for every case?
On Monday, 27 August 2018 at 19:36:29 UTC, aliak wrote: Then Nullable!(int*) would be the same as int*. Or even better maybe is to give a compiler error when you try and stuff a nullable type inside a Nullable. Because ... why? Isn't it arguable, whether this is desired? I mean, in the present state you can separate the cases, whether the Nullable is set or not. This is a feature. If Nullable!(int*) behaves like int*, it would be somewhat more straight forward, which is maybe also a feature. But you loose something you can do right now...
Re: Is there a simple way to check if value is null for every case?
On Monday, 27 August 2018 at 14:11:32 UTC, SG wrote:
On Monday, 27 August 2018 at 13:02:28 UTC, rikki cattermole
wrote:
So Nullable in D and C# is basically the same except C#'s has
language support.
The big difference is that in there I could do:
int? i = null;
string j = null;
var k = null;
and test all like:
i == null;
j == null;
k == null;
but in D:
Nullable!int i;
auto j = null;
string k = null;
writefln("%s", i.isNull); // I need to invoke Nullable
property isNull;
writefln("%s", i == null); // I can't just do this.
writefln("%s", j == null);
writefln("%s", k == null);
I hear you. IMO Nullable should be modified a bit to serve the
purpose of turning non-nullable types in to nullable types and
only that (right now it's confusing itself with an optional
type). Then we could implement opEquals(typeof(null)) so that
nullable == null would work.
I.e.
struct Nullable(T) {
static if (isPointer!T) {
private PointerTarget!T _value = PointerTarget!T.init;
} else {
private T _value = T.init;
}
bool opEquals(typeof(null)) {
return isNull;
}
}
Then Nullable!(int*) would be the same as int*. Or even better
maybe is to give a compiler error when you try and stuff a
nullable type inside a Nullable. Because ... why?
Cheers,
- Ali
Re: Is there a simple way to check if value is null for every case?
On Monday, 27 August 2018 at 14:11:32 UTC, SG wrote: On Monday, 27 August 2018 at 13:02:28 UTC, rikki cattermole wrote: So Nullable in D and C# is basically the same except C#'s has language support. Shouldn't it be in the standard library? I think it's worth it to create a feature request in Phobos for that. Or at least make a bug report. Such inconsistencies must be handled by a standard library, not manually, I believe.
Re: Is there a simple way to check if value is null for every case?
On Monday, 27 August 2018 at 13:02:28 UTC, rikki cattermole wrote:
So Nullable in D and C# is basically the same except C#'s has
language support.
The big difference is that in there I could do:
int? i = null;
string j = null;
var k = null;
and test all like:
i == null;
j == null;
k == null;
but in D:
Nullable!int i;
auto j = null;
string k = null;
writefln("%s", i.isNull); // I need to invoke Nullable
property isNull;
writefln("%s", i == null); // I can't just do this.
writefln("%s", j == null);
writefln("%s", k == null);
Re: Is there a simple way to check if value is null for every case?
On Monday, 27 August 2018 at 12:54:59 UTC, SG wrote:
On Monday, 27 August 2018 at 03:21:04 UTC, rikki cattermole
wrote:
Templates make it the easiest way, since common patterns, like
arrays, classes and pointers have the exact same null check
syntax.
I see.
That code is only for classes. C# also has structs which are a
value type. Which it would not work for.
The same thing for struct in C#
Struct S{
public int? i;
}
S.i == null; // This works nicely.
You don't need isNull function for Nullable because it has a
method called it. That will be preferred (hence I specifically
used isNull as the name).
For Variant, use hasValue.
bool isNull(Variant value) {
return !value.hasValue;
}
The problem I'm trying to solve is beyond that. This is just an
example. But bear with me, right now all I want is a function
to check the value from 'a' type and return if it is null.
The type could be a: Class, Struct, a Basic data type, Variant,
Nullable and so.
And what I see, these types has different behaviors, while in
C# at least for this same case, I would write the same thing in
few lines to perform it, in D I found very hard.
Isn't counter intuitive the way D works? Because for each type
Class/Nullable you check with .isNull, for Variant with
.hasValue, for string (variable is null).
Thanks.
hasValue isn't equivalent of isNull or is null. 'null' is valid
value in Variant:
import std.variant;
Variant v;
v = null;
assert(v.hasValue);//v has value
assert(v.get!(typeof(null)) is null); //value in v is
null
Nullable!T.isNull isn't equivalent of is null:
int* i = null;
Nullable!(int*) ni;
ni = i;
assert(ni.isNull == false); ///ni has value
assert(ni is null); ///ni value is null
string is null isn't equivalent of empty:
string str = "test";
str = str[0 .. 0];
assert(str !is null); ///str isn't null
assert(str.empty); ///str is empty
Re: Is there a simple way to check if value is null for every case?
On 28/08/2018 12:54 AM, SG wrote:
The same thing for struct in C#
Struct S{
public int? i;
}
S.i == null; // This works nicely.
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/nullable-types/index
So Nullable in D and C# is basically the same except C#'s has language
support.
Re: Is there a simple way to check if value is null for every case?
On Monday, 27 August 2018 at 03:21:04 UTC, rikki cattermole wrote:
Templates make it the easiest way, since common patterns, like
arrays, classes and pointers have the exact same null check
syntax.
I see.
That code is only for classes. C# also has structs which are a
value type. Which it would not work for.
The same thing for struct in C#
Struct S{
public int? i;
}
S.i == null; // This works nicely.
You don't need isNull function for Nullable because it has a
method called it. That will be preferred (hence I specifically
used isNull as the name).
For Variant, use hasValue.
bool isNull(Variant value) {
return !value.hasValue;
}
The problem I'm trying to solve is beyond that. This is just an
example. But bear with me, right now all I want is a function to
check the value from 'a' type and return if it is null.
The type could be a: Class, Struct, a Basic data type, Variant,
Nullable and so.
And what I see, these types has different behaviors, while in C#
at least for this same case, I would write the same thing in few
lines to perform it, in D I found very hard.
Isn't counter intuitive the way D works? Because for each type
Class/Nullable you check with .isNull, for Variant with
.hasValue, for string (variable is null).
Thanks.
Re: Is there a simple way to check if value is null for every case?
On 27/08/2018 12:51 PM, SG wrote:
On Sunday, 26 August 2018 at 16:39:53 UTC, rikki cattermole wrote:
UFCS function called isNull.
e.g.
import std.traits : isPointer;
bool isNull(T)(T value) if (is(T == class) || isPointer!T) {
return value is null;
}
Hi Rikki,
I'm still confused, I want to create a extension for checking null for
every type, so in D I'll need to handle each case with Templates?
Templates make it the easiest way, since common patterns, like arrays,
classes and pointers have the exact same null check syntax.
Because in C# I just need to compare an object/variable with null,
x == null;
myObject == null;
myObject.i == null;
myStruct == null;
myStruct.j == null;
This works fine in C#.
That code is only for classes. C# also has structs which are a value
type. Which it would not work for.
But again, in D I'll need the check the type? Is possible to make one
Template for all cases?
The code below works, but I don't think this is a right thing to do, right?
You don't need isNull function for Nullable because it has a method
called it. That will be preferred (hence I specifically used isNull as
the name).
For Variant, use hasValue.
bool isNull(Variant value) {
return !value.hasValue;
}
Writing a Unified Function Call Syntax function isn't really an
extension. You're defining a function that is to be preferred when you
ask for a method of the same name. So that it appears as if it was
actually described as a method instead of a free-function (not attached
to class/interface/struct/union).
Re: Is there a simple way to check if value is null for every case?
On Sunday, 26 August 2018 at 16:39:53 UTC, rikki cattermole wrote:
UFCS function called isNull.
e.g.
import std.traits : isPointer;
bool isNull(T)(T value) if (is(T == class) || isPointer!T) {
return value is null;
}
Hi Rikki,
I'm still confused, I want to create a extension for checking
null for every type, so in D I'll need to handle each case with
Templates?
Because in C# I just need to compare an object/variable with null,
x == null;
myObject == null;
myObject.i == null;
myStruct == null;
myStruct.j == null;
This works fine in C#.
But again, in D I'll need the check the type? Is possible to make
one Template for all cases?
The code below works, but I don't think this is a right thing to
do, right?
import std.stdio, std.typecons, std.variant, std.conv;
import std.traits : isPointer;
bool foo(T)(T value) if (is(T == VariantN!32LU)) {
return value == null;
}
bool foo(T)(T value) if (is(T == Nullable!int) || isPointer!T) {
return value.isNull;
}
bool foo(T)(T t) if (!is(T == Nullable!int) && !is(T ==
VariantN!32LU)){
if (is(T == typeof(null))){
return true;
}
return (t is null);
}
class S{
Nullable!int i;
}
void main(){
Variant v = null;
writeln(v == null," - " ,v.foo);
Variant v2 = "a";
writeln(v2 == null," - " ,v2.foo);
string x = "a";
writeln(x is null," - " ,x.foo);
string y = null;
writeln(y is null," - ", y.foo);
auto z = null;
writeln(z is null," - ", z.foo);
S s = new S();
writeln(s.i.isNull," - ", s.i.foo);
}
Is there a simple way to check if value is null for every case?
Hi again,
The code below works for some cases but not for the Nullable!Type.
A way to fix it should be check the type and use ".isNull" for
Nullabe!Type. But is there a simple way to test if value is null
for every case?
import std.stdio, std.typecons, std.variant, std.conv;
bool foo(T)(T t){
return (t is null);
}
class S{
Nullable!int i;
}
void main(){
string x = "a";
writeln(x is null," - " ,x.foo);
string y = null;
writeln(y is null," - ", y.foo);
auto z = null;
writeln(z is null," - ", z.foo);
S s = new S();
writeln(s.i.isNull); // Ok
//writeln(s.i is null); // Error
//s.i.foo(2); // Error
}
Re: Is there a simple way to check if value is null for every case?
On 27/08/2018 4:37 AM, SG wrote:
Hi again,
The code below works for some cases but not for the Nullable!Type.
A way to fix it should be check the type and use ".isNull" for
Nullabe!Type. But is there a simple way to test if value is null for
every case?
import std.stdio, std.typecons, std.variant, std.conv;
bool foo(T)(T t){
return (t is null);
}
class S{
Nullable!int i;
}
void main(){
string x = "a";
writeln(x is null," - " ,x.foo);
string y = null;
writeln(y is null," - ", y.foo);
auto z = null;
writeln(z is null," - ", z.foo);
S s = new S();
writeln(s.i.isNull); // Ok
//writeln(s.i is null); // Error
//s.i.foo(2); // Error
}
UFCS function called isNull.
e.g.
import std.traits : isPointer;
bool isNull(T)(T value) if (is(T == class) || isPointer!T) {
return value is null;
}
