Now that's a DIP that could use some love

Adam D. Ruppe destructionator at
Mon Sep 14 00:12:34 UTC 2020

On Monday, 14 September 2020 at 00:00:15 UTC, Adam D. Ruppe wrote:
> While that would be possible, I believe a better DIP would be 
> to let the compiler recognize the pattern and work it 
> automatically for us.

Here's a slight refinement: it does the cast to bool already. So 
we can return a struct with opCast. But how to get the message 

It could be a magic type that the compiler recognizes...

Or it could just be a pattern. Suppose template constraints as 
well as static assert (heck maybe even normal assert) just looked 
for a method, maybe __compileErrors. If present, it processes the 
return value as supplemental errors.

Now the library defines

struct MyChecks {
     bool opBool(T:bool) { return errors.length == 0; }
     string[] __compileErrors;

MyChecks isInputRange(T)() {
      MyChecks errors;
      if(!hasMember!(T, "front")
         errors.__compileErrors ~= "missing front";
      if(!hasMember!(T, "popFront")
         errors.__compileErrors ~= "missing popFront";
      if(!hasMember!(T, "empty")
         errors.__compileErrors ~= "missing empty";

      return errors;

Then user code does:

static assert(isInputRange!MyThing); // automatically shows the 
errors if it fails (unless the user specifies something else)

void foo(T)() if(isInputRange!T) {} // shows the errors if it 
fails  in addition to what it has now


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