On Friday, 20 May 2016 at 19:34:11 UTC, Walter Bright wrote:
Constraints can address behavior and relationships, concepts do
not.
Wow, TIL. That's so clear once said !
There's been several discussion here and even one phobos PR that
proposes a kind of concepts but I didn't realize before that the
2 things are different.
The problem I see in D is that the constraints, since they
prevent to output a good message, are doing both (in a way):
void foo(T)(T t)
if (constraint)
{
// cannot have the message if constraint fails...
static assert((checkConcept!T).ok, (checkConcept!T).message);
}
At the language level it would work
void foo(T)(T t) @Concept(CheckerTemplate) // use this to output
a smart message
if (constraint)
{
}
But really, without changing much what I'd like to see is a DMD
feature that
would parse and evaluate the constraints to output a message:
such as
void foo(T)(T t)
if ((a || b) && (a || b))
{
}
error:(a || b) is false
instead of throwing the whole constraint text in the output.
