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digitalmars.D.learn - Nullable instantiator anyone?

reply =?UTF-8?B?Ik5vcmRsw7Z3Ig==?= <per.nordlow gmail.com> writes:
I'm missing an instantiator function for std.typecons:Nullable. 
Is this is intentional? If not, is this

Nullable!T nullable(T)(T a)
{
     return typeof(return)(a);
}

sufficient for our needs?
Aug 17 2014
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Nordlöw:


 I'm missing an instantiator function for std.typecons:Nullable. 
 Is this is intentional? If not, is this

 Nullable!T nullable(T)(T a)
 {
     return typeof(return)(a);
 }

 sufficient for our needs?


It could be sufficient, but note that in Phobos there are two 
different versions of Nullable, one of them doesn't require extra 
memory, it uses one value as the "null" value.

Bye,
bearophile
Aug 17 2014
parent reply =?UTF-8?B?Ik5vcmRsw7Z3Ig==?= <per.nordlow gmail.com> writes:
On Sunday, 17 August 2014 at 18:51:38 UTC, bearophile wrote:

 It could be sufficient, but note that in Phobos there are two 
 different versions of Nullable, one of them doesn't require 
 extra memory, it uses one value as the "null" value.


Ok, thanks for the reminder.

Do you have a suggestion of an instantiator for this variant? I'm 
not sure how to define this. So far I have


Nullable!T nullable(T nullValue, T)(T a, T nullValue)
{
     return Nullable!nullValue(a);
}

unittest
{
     auto x = nullable!(int.max)(3);
}


but it fails as


mangling.d(50,21): Error: undefined identifier T


How do I make the template parameter nullValue infer type T with 
a single template parameter? Is it even possible?
Aug 17 2014
next sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 08/17/2014 12:05 PM, "Nordlöw" wrote:


 On Sunday, 17 August 2014 at 18:51:38 UTC, bearophile wrote:

 It could be sufficient, but note that in Phobos there are two
 different versions of Nullable, one of them doesn't require extra
 memory, it uses one value as the "null" value.


 Ok, thanks for the reminder.

 Do you have a suggestion of an instantiator for this variant? I'm not
 sure how to define this. So far I have


 Nullable!T nullable(T nullValue, T)(T a, T nullValue)
 {
      return Nullable!nullValue(a);
 }

 unittest
 {
      auto x = nullable!(int.max)(3);
 }


 but it fails as


 mangling.d(50,21): Error: undefined identifier T


 How do I make the template parameter nullValue infer type T with a
 single template parameter? Is it even possible?


I don't think it is possible.

The following solution relies on an alias template parameter and a 
template constraint. The code covers both Nullable variants:

import std.typecons;

auto nullable(T)(T value)
{
     return Nullable!T(value);
}

unittest
{
     auto x = nullable(42.5);
     assert(is(typeof(x) == Nullable!double));
}

auto nullable(alias nullValue, T)(T value)
     if (is (typeof(nullValue) == T))
{
     return Nullable!(T, nullValue)(value);
}

unittest
{
     auto x = nullable!(int.max)(3);
     assert(is (typeof(x) == Nullable!(int, int.max)));
}

void main()
{}

Ali
Aug 17 2014
parent =?UTF-8?B?Ik5vcmRsw7Z3Ig==?= <per.nordlow gmail.com> writes:
On Monday, 18 August 2014 at 01:22:00 UTC, Ali Çehreli wrote:

 The following solution relies on an alias template parameter 
 and a template constraint. The code covers both Nullable 
 variants:

 import std.typecons;

 auto nullable(T)(T value)
 {
     return Nullable!T(value);
 }

 unittest
 {
     auto x = nullable(42.5);
     assert(is(typeof(x) == Nullable!double));
 }

 auto nullable(alias nullValue, T)(T value)
     if (is (typeof(nullValue) == T))
 {
     return Nullable!(T, nullValue)(value);
 }

 unittest
 {
     auto x = nullable!(int.max)(3);
     assert(is (typeof(x) == Nullable!(int, int.max)));
 }

 void main()
 {}

 Ali


Great! Thx.
Aug 18 2014
prev sibling parent "Phil Lavoie" <maidenphil hotmail.com> writes:
On Sunday, 17 August 2014 at 19:05:11 UTC, Nordlöw wrote:

 On Sunday, 17 August 2014 at 18:51:38 UTC, bearophile wrote:

 It could be sufficient, but note that in Phobos there are two 
 different versions of Nullable, one of them doesn't require 
 extra memory, it uses one value as the "null" value.


 Ok, thanks for the reminder.

 Do you have a suggestion of an instantiator for this variant? 
 I'm not sure how to define this. So far I have


 Nullable!T nullable(T nullValue, T)(T a, T nullValue)
 {
     return Nullable!nullValue(a);
 }

 unittest
 {
     auto x = nullable!(int.max)(3);
 }


 but it fails as


 mangling.d(50,21): Error: undefined identifier T


 How do I make the template parameter nullValue infer type T 
 with a single template parameter? Is it even possible?


Hi Nordlöw,

I'm not sure I fully understand your question but I'll try and 
answer it. First of all, yes, you can have everything inferred as 
long as your produce a default value for your null value, a.k.a:

Nullable!(T, nullValue) nullable(T, T nullValue = T.init)(T val)
{
  return Nullable!(T, nullValue)(val);
}

unittest{
  auto myNullableInt = nullable(5);
  assert(!myNullableInt.isNull() && myNullableInt == 5);
}

However, keep in mind that using a sentinel value as the null 
value can produce surprises:

unittest{
  auto myNullableInt = nullable(0);
  assert(myNullableInt.isNull()); //Since T.init == 0
}

Phil
Aug 17 2014