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digitalmars.D - Parameterized template value parameter

reply Yuxuan Shui <yshuiv7 gmail.com> writes:
So I was trying to make my template take a value parameter, whose 
type is also a parameter to the template. e.g.:

     template A(Char[] str, Char);

But dmd complains about 'Char' being undefined. I have to write:

     template A(Char, Char[] str);

Which is inconvenient, because now 'Char' can't be deduced by the 
compiler.

Can we make the first case work?
Mar 23
parent reply Dmitry Olshansky <dmitry.olsh gmail.com> writes:
On 3/24/17 12:24 AM, Yuxuan Shui wrote:
 So I was trying to make my template take a value parameter, whose type
 is also a parameter to the template. e.g.:

     template A(Char[] str, Char);

 But dmd complains about 'Char' being undefined. I have to write:

     template A(Char, Char[] str);

 Which is inconvenient, because now 'Char' can't be deduced by the compiler.

 Can we make the first case work?
How about this? template A(alias str) if(is(typeof(str) : Char[], Char)){ alias Char = typeof(str[0]); // ... } In general it's sometimes possible to do the deduction w/o introducing more template arguments. --- Dmitry Olshansky
Mar 24
parent reply Yuxuan Shui <yshuiv7 gmail.com> writes:
On Friday, 24 March 2017 at 20:43:18 UTC, Dmitry Olshansky wrote:
 On 3/24/17 12:24 AM, Yuxuan Shui wrote:
 So I was trying to make my template take a value parameter, 
 whose type
 is also a parameter to the template. e.g.:

     template A(Char[] str, Char);

 But dmd complains about 'Char' being undefined. I have to 
 write:

     template A(Char, Char[] str);

 Which is inconvenient, because now 'Char' can't be deduced by 
 the compiler.

 Can we make the first case work?
How about this? template A(alias str) if(is(typeof(str) : Char[], Char)){ alias Char = typeof(str[0]); // ... }
Yes. This is what I ended up doing (https://github.com/yshui/sdpc/blob/master/sdpc/parsers.d#L45). One problem of this is that 'str' is not longer restricted to be a compile time value. Which is probably fine, but kind of makes me uncomfortable.
 In general it's sometimes possible to do the deduction w/o 
 introducing more template arguments.

 ---
 Dmitry Olshansky
Mar 24
parent Nicholas Wilson <iamthewilsonator hotmail.com> writes:
On Friday, 24 March 2017 at 21:13:26 UTC, Yuxuan Shui wrote:
 On Friday, 24 March 2017 at 20:43:18 UTC, Dmitry Olshansky
 template A(alias str)
 if(is(typeof(str) : Char[], Char)){
 	alias Char = typeof(str[0]);
 //	...
 }
 One problem of this is that 'str' is not longer restricted to 
 be a compile time value. Which is probably fine, but kind of 
 makes me uncomfortable.
To test wether str is a compile time value just check if you can use it to assign to an enum if(is(typeof(str) : Char[], Char) && is(typeof({enum foo = str;})))
Mar 24