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digitalmars.D - type switch

reply "Dennis Ritchie" <dennis.ritchie mail.ru> writes:
Good day to all!

I've been thinking. In D you can write a similar design to the 
generic functions or somewhere, use static if:

// -----
static if (is(T == short) || is(T == int) || is(T == long)) {
	// do anything
} else static if (is(T == real)) {
	// ...
} else static if (is(T == char)) {
	// ...
} else static if (is(T == string)) {
	// ...
} else static if (is(T == int[])) {
	// ...
} else {
	// ...
}

Why not put in Phobos shorter design type switch:

// -----
type switch (T) {
case short, int, long:
	// do anything
case real:
	// ...
case char:
	// ...
case string:
	// ...
case int[]:
	// ...
default:
	// ...
}

This design has been implemented, for example, in Common Lisp 
(typecase):
http://www.lispworks.com/documentation/lw51/CLHS/Body/m_tpcase.htm

What do you think about this?
May 04 2015
next sibling parent "Dennis Ritchie" <dennis.ritchie mail.ru> writes:
On Monday, 4 May 2015 at 22:48:37 UTC, Dennis Ritchie wrote:

 Why not put in Phobos shorter design type switch:


*More precisely, not in Phobos and DMD.
May 04 2015
prev sibling next sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 05/04/2015 03:48 PM, Dennis Ritchie wrote:


 I've been thinking. In D you can write a similar design to the generic
 functions or somewhere, use static if:

 // -----
 static if (is(T == short) || is(T == int) || is(T == long)) {
      // do anything
 } else static if (is(T == real)) {
      // ...
 } else static if (is(T == char)) {
      // ...
 } else static if (is(T == string)) {
      // ...
 } else static if (is(T == int[])) {
      // ...
 } else {
      // ...
 }

 Why not put in Phobos shorter design type switch:

 // -----
 type switch (T) {
 case short, int, long:
      // do anything
 case real:
      // ...
 case char:
      // ...
 case string:
      // ...
 case int[]:
      // ...
 default:
      // ...
 }

 This design has been implemented, for example, in Common Lisp (typecase):
 http://www.lispworks.com/documentation/lw51/CLHS/Body/m_tpcase.htm

 What do you think about this?


Personally, I haven't felt the need for this becuase usually there is 
something different only for one or two specific types.

I don't see that the syntax becomes shorter to warrant a language 
addition either. (However, the risk of writing 'else if' instead of 
'else static if' is a real annoyance that comes with bad consequences.)

Another option is to use template specializations, which can be mixed-in 
(in bar() below) or called directly (in foo() below):

void doSomethingSpecial(T : int)()
{
     // ...
}

void doSomethingSpecial(T : string)()
{
     // ...
}

void foo(T)()
{
     doSomethingSpecial!T();
}

void bar(T)()
{
     mixin doSomethingSpecial!T;
}

void main()
{
     foo!string();
     bar!int();
}

Ali
May 05 2015
parent "Dennis Ritchie" <dennis.ritchie mail.ru> writes:
On Tuesday, 5 May 2015 at 18:12:42 UTC, Ali Çehreli wrote:

 Personally, I haven't felt the need for this becuase usually 
 there is something different only for one or two specific types.

 I don't see that the syntax becomes shorter to warrant a 
 language addition either. (However, the risk of writing 'else 
 if' instead of 'else static if' is a real annoyance that comes 
 with bad consequences.)

 Another option is to use template specializations, which can be 
 mixed-in (in bar() below) or called directly (in foo() below):

 void doSomethingSpecial(T : int)()
 {
     // ...
 }

 void doSomethingSpecial(T : string)()
 {
     // ...
 }

 void foo(T)()
 {
     doSomethingSpecial!T();
 }

 void bar(T)()
 {
     mixin doSomethingSpecial!T;
 }

 void main()
 {
     foo!string();
     bar!int();
 }

 Ali


Maybe it will not much shorter, but in some cases, such a design 
would look more appropriate than static if.
In addition, the keyword "case" may be omitted in this design, 
which will make it even more short and readable. Such as here:
http://dpaste.dzfl.pl/d6643ec8ccd3

In my opinion, the implementation of such a structure will not 
hurt D. While it may be something I do not understand :)
May 05 2015
prev sibling parent reply Justin Whear <justin economicmodeling.com> writes:
How's this? http://dpaste.dzfl.pl/d6643ec8ccd3
May 05 2015
parent "Dennis Ritchie" <dennis.ritchie mail.ru> writes:
On Tuesday, 5 May 2015 at 18:37:52 UTC, Justin Whear wrote:

 How's this? http://dpaste.dzfl.pl/d6643ec8ccd3


Yes, something like that. In my opinion, it's really easier than 
static if in some cases.
May 05 2015