On Tuesday, 16 May 2017 at 15:47:37 UTC, Steven Schveighoffer
wrote:
On 5/16/17 9:54 AM, Stanislav Blinov wrote:
On Tuesday, 16 May 2017 at 12:27:30 UTC, Steven Schveighoffer
wrote:
When we have tests using dummy lambdas, are we to expect
users to
immediately extract the lambda body, parse it, and figure
out what's
wrong?
This is what you have to do today. The task has already been
tried by
the compiler, and the result is known by the compiler. Just
have the
compiler tell you.
:) The compiler does not know what I'm checking for with that
lambda. As
far as the compiler is concerned, I'm interested in whether it
compiles
or not. It doesn't care what that means in the context of my
constraint.
Neither should the user.
You seem to be not understanding that a hand-written message of
"needs to have member x" or something conveys the same
information as the compiler saying "Error in this line: auto
test = T.x"
You are nitpicking again, or, at least, oversimplifying. I am
understanding that perfectly. But we've both shown examples of
constraints that are much more complex than just checking for one
member. As in, the type needs to have these members and/or these
properties. All of them. Or some of them. Or a set of
combinations of them. With those specific traits, etc. It's not
*just* "error in this line". It's "error in this line on this
execution path with these conditions". And nowhere in sight is
*what* all those checks are doing as a unit. Not individually,
but in complex, all of them, in concert.
A fresh user of your code cannot know at a glance what all of
them are doing, nor should they.
Sure, it's not proper English. It gets the job done, and I
don't have to instrument all my functions and template
constraint helpers. I don't need to explode compile times, or
debug why my messages don't match what actually happens.
If your messages don't match what actually happens, there's a bug
in your constraint, simple as that. Which you should catch with
tests.
The compiler, and by extension your hand-written error
checking,
cannot know the true intention of the user.
hand-written == hand-compiled. That is, I translated what I
think this boolean condition means into a human-readable format
by hand. I could have got it wrong. Then a nice message is
useless.
Eh? You *wrote* the constraint. You *know* what all of those
tests mean. You didn't translate what you *think* the condition
means. You wrote the condition, you *know* what it means. If you
don't, you shouldn't be writing this constraint. The only way for
you to "get it wrong" is to write a bug in the constraint. Or in
the compiler.
You know that if you've just did is(typeof((T t) @safe {})) and
it returned false, T's destructor cannot be called in @safe code.
It's obvious, isn't it? Every D user knows that for sure.
There are obscure checks, that's the way the language works.
There are cases when those obscure checks need to be cleanly
explained. That is *all* that I'm proposing.
When the
user violates the contract, I must inform them they did so, by
reporting
*why* their argument does not satisfy me. Not by telling *how*
I figured
that out (compiler output), but by telling *what* is wrong
with the
argument (human-readable error message).
The constraint tells the user why it doesn't work. There is no
extra effort required. It's human readable (I know what
isInputRange!R means). I don't need a specialized full-sentence
message to understand that.
*You* know what isInputRange means. This doesn't mean that all
users do.
And do you know, beforehand, what all possible constraints in all
possible libraries that you may use in the future mean, without
looking at their code? No. No one does.
If the user does want to know how the constraint caught their
mistake,
they're free to inspect what the compiler outputs.
And here is the problem. Your solution doesn't get us any
closer to that. It's actually quite painful to do this today.
Of course it is painful, that's the whole point of my proposal.
When I have a type like this:
struct S
{
int foo;
}
and the hand-written error message says: "Your type must be a
struct that contains an integer property named 'foo'". How does
that help me figure out what I did wrong? We can spend all day
arguing over how nice this message is, but the truth is, what
the constraint writer put in the constraints, and how the
compiler is interpreting it, may be 2 different things. This
helps nobody.
It cannot be that way. Whoever wrote the constraint possesses
knowledge about it's semantics. If they put wrong message there,
then that's a bug, or they're a... well, not a very good person.
That is why (static) asserts have an optional string argument:
to
display a clean an readable error message, instead of just
dumping some
code on the user. The same thing, in my opinion, is needed for
constraints.
If assert(x == 5) would just print "Error: x == 4"
automatically, we could eliminate most needs for a message.
You're taking overly simplified pieces of code and base your
reasoning on them, while you know full well it's almost never
this simple.
assert(x.veryBadlyNamedFunction ==
obscureStateObtainedFromElsewhere);
"Error: x.veryBadlyNamedFunction == 42"
Very helpful message.
You could solve this with a message, but again, this is a huge
task to undertake on ALL code in existence, rather than fixing
I seem to be expressing myself poorly, as I find I must repeat it
the second time already: I never suggested to change any existing
code. I suggested an *optional* mechanism of reporting errors.
it all to a "good enough" degree that we don't need the
messages. And it avoids the "human compiler" that is prone to
error.
This again. No it does not. If there's a bug in the constraint,
there is a bug in the constraint. Messages or no, it's all the
same. Either it does what it's supposed to do, or it doesn't.
Reporting the wrong message is a bug, as is returning false for
compliant type, or true for malicious one.
stack traces. User is informed their type is wrong and *why*
it is
wrong. If they disagree, if they think there's a bug in the
constraint,
or if they're interested in how the constraint works, they're
free to go
through all the hoops you describe.
That doesn't help me if the constraint author didn't put that
message, or the constraint author put the *wrong* message, or
wrote the constraint incorrectly.
And again. If they wrote the constraint incorrectly, it's their
bug. If they didn't put any message, compiler still should
provide it's own diagnostics (better than today ones for sure).
