Re: Emulation macros and pattern matching on D

[Steven Schveighoffer via Digitalmars-d-learn]

On 6/5/15 10:15 AM, Dennis Ritchie wrote:

On Friday, 5 June 2015 at 13:13:15 UTC, Steven Schveighoffer wrote:

string foo(string mode, string value)
{
   return `writefln(mode ` ~ mode ~ `: %s, ` ~ value ~ `);`;
}

void main()
{
   mixin(foo(Y, 3));
   mixin(foo(X, 2));
}


Thanks. It looks really simple, but I still do not understand the
concept of using mixins in full.

I do not understand why in this line:
return `writefln(mode ` ~ mode ~ `: %s, ` ~ value ~ `);`;

use the following syntax:
~ mode ~ , ~ value ~


Because what foo is constructing is a string that makes sense in the 
*caller*, not inside foo. What those statements do is concat the *value* 
of mode (i.e. Y or X) and the *value* of value (i.e. 3 or 2) to 
the string.


It's equivalent to rust using the ${e} to do variable substitution.


For example, why here I can simply write:

void main() {
 int b = 5;
 mixin(`int a = b;`);
 assert(a == 5);
}


Because b makes sense in the context of main.


Why should not I write like this:

void main() {
 int b = 5;
 mixin(`int a =  ` ~ b ~ ` ;`);
 assert(a == 5);
}


Because it won't compile :) Mixin strings must be constructable at 
compile time, the value of b depends on runtime. Not to mention that you 
can't concat strings with ints.


-Steve

Re: Append to array of strings within array of structs

[Adam D. Ruppe via Digitalmars-d-learn]

On Friday, 5 June 2015 at 19:44:29 UTC, Paul wrote:

someRecords ~= Rec();
someRecords.fileLines ~= someText;


You could condense them this way:

someRecords ~= Rec([someText]);


The Rec() can take arguments for the initial values, so an array 
starting with the someText should be cool