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digitalmars.D.learn - type set

reply spir <denis.spir gmail.com> writes:
Hello,

I have a template condition that looks like this:

     T check (T) () if (
             is(T == DLogical) ||
             is(T == DNumber) ||
             is(T == DText) ||
             is(T == DList) ||
             is(T == DUnit)
         ) {
         ...
     }

Is there a way to "factor out" such an expression using a kind of type set? If 
only for cleaning the code; but also because such a set may get long.

     T check (T) () if (is (T in validTypeSet)) {
         ...
     }

Denis
-- 
_________________
vita es estrany
spir.wikidot.com
Feb 28 2011
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 28 Feb 2011 08:22:58 -0500, spir <denis.spir gmail.com> wrote:


 Hello,

 I have a template condition that looks like this:

      T check (T) () if (
              is(T == DLogical) ||
              is(T == DNumber) ||
              is(T == DText) ||
              is(T == DList) ||
              is(T == DUnit)
          ) {
          ...
      }

 Is there a way to "factor out" such an expression using a kind of type  
 set? If only for cleaning the code; but also because such a set may get  
 long.

      T check (T) () if (is (T in validTypeSet)) {
          ...
      }


This should probably work:

template isOneOf(X, T...)
{
     static if(!T.length)
        enum bool isOneOf = false;
     else static if(is(X == T[0]))
        enum bool isOneOf = true;
     else
        enum bool isOneOf = isOneOf!(X, T[1..$]);
}

T check(T) () if(isOneOf!(T, DLogical, DNumber, DText, TList, DUnit))
{
    ...
}

Not sure if this exists in std.traits or not, but that's where I'd look.

-Steve
Feb 28 2011
next sibling parent reply spir <denis.spir gmail.com> writes:
On 02/28/2011 02:32 PM, Steven Schveighoffer wrote:

 On Mon, 28 Feb 2011 08:22:58 -0500, spir <denis.spir gmail.com> wrote:


 Hello,

 I have a template condition that looks like this:

 T check (T) () if (
 is(T == DLogical) ||
 is(T == DNumber) ||
 is(T == DText) ||
 is(T == DList) ||
 is(T == DUnit)
 ) {
 ...
 }

 Is there a way to "factor out" such an expression using a kind of type set?
 If only for cleaning the code; but also because such a set may get long.

 T check (T) () if (is (T in validTypeSet)) {
 ...
 }


 This should probably work:

 template isOneOf(X, T...)
 {
 static if(!T.length)
 enum bool isOneOf = false;
 else static if(is(X == T[0]))
 enum bool isOneOf = true;
 else
 enum bool isOneOf = isOneOf!(X, T[1..$]);
 }

 T check(T) () if(isOneOf!(T, DLogical, DNumber, DText, TList, DUnit))
 {
 ...
 }

 Not sure if this exists in std.traits or not, but that's where I'd look.

 -Steve


Waow, great anyway! Didn't even know one can write variadic type/template param 
lists.

By the way, the block of the function is a series of static if-s, one for each 
allowed type. Is there any static switch? Or any other nicer way to write it
than:

     T check (T) () if (
             is(T == DLogical) ||
             is(T == DNumber) ||
             is(T == DText) ||
             is(T == DList) ||
             is(T == DUnit)
         ) {
         TypeCode type;

         static if (is(T == DLogical))
             if (this.code == LOGICAL)
                 return this.logical;
             else
                 type == LOGICAL;
         static if (is(T == DNumber))
             if (this.code == NUMBER)
                 return this.number;
             else
                 type == NUMBER;
         static if (is(T == DText))
             if (this.code == TEXT)
                 return this.text;
             else
                 type == TEXT;
         static if (is(T == DList))
             if (this.code == LOGICAL)
                 return this.list;
             else
                 type == LOGICAL;
         static if (is(T == DUnit))
             if (this.code == UNIT)
                 return this.unit;
             else
                 type == UNIT;

         // type error
         throw new TypeError(type, this);
     }

This func type-checks and returns the current value of a tagged union. I would 
be very pleased with a mapping from types to type codes (tags). I can't do 
without the type param, I guess, because it's the return value's type... or can 
I? But the discriminating code of the union cannot be the type itself (*), 
instead it's a plain code.
I thought at using TypeInfo-s as codes, which can then be mapped from types 
using typeid(). But according to sizeof, this makes the code weigh 1 word 
instead of one byte.

Denis

(*) Indeed. Else it would be a template generating N distinct types, which is 
precisely the opposite of what a union provides.
-- 
_________________
vita es estrany
spir.wikidot.com
Feb 28 2011
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 28 Feb 2011 09:27:36 -0500, spir <denis.spir gmail.com> wrote:


 By the way, the block of the function is a series of static if-s, one  
 for each allowed type. Is there any static switch? Or any other nicer  
 way to write it than:

      T check (T) () if (
              is(T == DLogical) ||
              is(T == DNumber) ||
              is(T == DText) ||
              is(T == DList) ||
              is(T == DUnit)
          ) {
          TypeCode type;

          static if (is(T == DLogical))
              if (this.code == LOGICAL)
                  return this.logical;
              else
                  type == LOGICAL;
          static if (is(T == DNumber))
              if (this.code == NUMBER)
                  return this.number;
              else
                  type == NUMBER;
          static if (is(T == DText))
              if (this.code == TEXT)
                  return this.text;
              else
                  type == TEXT;
          static if (is(T == DList))
              if (this.code == LOGICAL)
                  return this.list;
              else
                  type == LOGICAL;
          static if (is(T == DUnit))
              if (this.code == UNIT)
                  return this.unit;
              else
                  type == UNIT;

          // type error
          throw new TypeError(type, this);
      }


There is a final switch, but I don't know if that works on types.  You may  
be stuck with static if.

When doing things like this, I'd recommend using a mapping template.  For  
example:

private template typeCode(T)
{
    static if(is(T == DLogical)) enum typeCode = LOGICAL;
    else static if(is(T == DNumber)) enum typeCode = NUMBER;
    ...
    else static assert(0);
}

then you almost can use this to generate the right code:

if(this.code == typeCode!T)
{
    static if(is(T == DUnit)) return this.unit;
    else static if(...
}

You can probably replace the inner static if with a mixin, if you name  
your union members properly (i.e. if you can generate the name of the  
union member based on its type name or code, like DUnit dunit_val).

But at least it gives you an idea of how this can be done efficiently.

Plus avoiding large repetitive static ifs can save you from tedious  
copy-pasting bugs like the one in your DList branch ;)

-Steve
Feb 28 2011
parent reply spir <denis.spir gmail.com> writes:
On 02/28/2011 03:50 PM, Steven Schveighoffer wrote:

 On Mon, 28 Feb 2011 09:27:36 -0500, spir <denis.spir gmail.com> wrote:


 By the way, the block of the function is a series of static if-s, one for
 each allowed type. Is there any static switch? Or any other nicer way to
 write it than:

 T check (T) () if (
 is(T == DLogical) ||
 is(T == DNumber) ||
 is(T == DText) ||
 is(T == DList) ||
 is(T == DUnit)
 ) {
 TypeCode type;

 static if (is(T == DLogical))
 if (this.code == LOGICAL)
 return this.logical;
 else
 type == LOGICAL;
 static if (is(T == DNumber))
 if (this.code == NUMBER)
 return this.number;
 else
 type == NUMBER;
 static if (is(T == DText))
 if (this.code == TEXT)
 return this.text;
 else
 type == TEXT;
 static if (is(T == DList))
 if (this.code == LOGICAL)
 return this.list;
 else
 type == LOGICAL;
 static if (is(T == DUnit))
 if (this.code == UNIT)
 return this.unit;
 else
 type == UNIT;

 // type error
 throw new TypeError(type, this);
 }


 There is a final switch, but I don't know if that works on types. You may be
 stuck with static if.

 When doing things like this, I'd recommend using a mapping template. For
example:

 private template typeCode(T)
 {
 static if(is(T == DLogical)) enum typeCode = LOGICAL;
 else static if(is(T == DNumber)) enum typeCode = NUMBER;
 ...
 else static assert(0);
 }

 then you almost can use this to generate the right code:

 if(this.code == typeCode!T)
 {
 static if(is(T == DUnit)) return this.unit;
 else static if(...
 }


That's it! This actually builds an the equivalent of an AA which keys are 
types. I was looking for such a functionality for a while already.
I am in fact *contantly* annoyed in D by the fact there are no types (I mean at 
runtime). On the other hand, this forces me looking for workaround, to express 
my models in "distorted" ways, which lets me discover unusual features and 
idioms in D's semantic dark corners ;-)


 You can probably replace the inner static if with a mixin, if you name your
 union members properly (i.e. if you can generate the name of the union member
 based on its type name or code, like DUnit dunit_val).


Yeah, I could, as shown by the code above: lang type Xyz <--> code XYZ <--> D 
impl type DXyz <--> union member xyz.
Could not be more regular, I guess ;-)
But as you may know, I 100% against string mixins. I prefere keeping the 
mapping explicite without string sorcery.
(Precisely, in the toy lang I'm starting to realise, one could do that 
trivially by manipulating the AST, without any dark magic. But this is another 
story...)


 But at least it gives you an idea of how this can be done efficiently.

 Plus avoiding large repetitive static ifs can save you from tedious
 copy-pasting bugs like the one in your DList branch ;)


Good catch, Steve! And thank you again.

PS: Is it your email client that eats whitespace (see my code above)?

Denis
-- 
_________________
vita es estrany
spir.wikidot.com
Feb 28 2011
parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 28 Feb 2011 12:50:21 -0500, spir <denis.spir gmail.com> wrote:


 PS: Is it your email client that eats whitespace (see my code above)?


Only in your reply does the code appear indented improperly, so I think  
it's your client, not mine.

-steve
Feb 28 2011
prev sibling parent bearophile <bearophileHUGS lycos.com> writes:
Steven Schveighoffer:


 Not sure if this exists in std.traits or not, but that's where I'd look.


std.typetuple.anySatisfy helps here.

-------

spir:


 Is there any static switch?


foreach done on a typetuple is a static foreach.



 Or any other nicer way to write it than:


I suggest to put the types in a typetuple and use a foreach(i, T,
TypeTuple!(t1, t2, ...)), put your values in another typetuple and use the i
index to access and return the values.

import std.stdio, std.typetuple;
int mapper(TX)() {
    alias TypeTuple!(int, float, string) Tkeys;
    enum values = [100, 5, 2];
    //alias TypeTuple!(100, 5, 2) Tvalues; // alternative

    foreach (i, T; Tkeys)
        if (is(T == TX))
            return values[i];
    assert(0, "Can't happen");
}
enum r = mapper!float();
static assert(r == 5);
void main() {}

Bye,
bearophile
Feb 28 2011