Re: the traits trap
On 11/21/14 2:25 AM, Sergei Nosov wrote:
On Friday, 21 November 2014 at 04:08:52 UTC, Steven Schveighoffer wrote:
Can anyone figure out a good solution to this problem? I like template
constraints, but they are just too black-boxy. Would we have to
signify that some enum is actually a trait and so the compiler would
know to spit out the junk of compiling? Would it make sense to add
some __traits function that allows one to signify that this is a
special trait thing?
This is one area that D's templates are very user-unfriendly.
-Steve
I would second this. Personally, I have the same not very pleasant
experience debugging template constraints.
Since more often than not the constraints have the form of:
if (clause1 clause2 clause3 ...)
my naive proposal would be to show which clause was first to be false in
the error message.
That helps, but you still have to figure out why that clause fails.
However, I have no idea if this could be implemented easily.
I think it's a good idea in general. Inevitably, a template constraint
breaks down into a decision tree, and knowing which decisions
contributed to the false at the top is essential. Of course, this is
only needed if it *doesn't* compile.
What I'd like to see is a combination of both determining at the highest
level which part of the if expression caused the failure (this is only
if the thing doesn't compile), and then add a new feature:
static assert(__traits(analyzeTrait, isSomeTrait!x));
The analyzeTrait directive would compile isSomeTrait!x, keeping track of
the decision tree (or optionally, re-compiling it to print the tree),
and whichever pieces caused it to be 0, including __traits(compiles,
...) error messages, and if the result ends up being non-zero, it just
discards that tree. If the result ends up being 0, then the static
assert prints the decision tree.
An example:
enum isFoo(T) = (is(T == int) || is(T == long)) __traits(compiles, (T
t) { blah(t); });
void blah(int x);
static assert(__traits(analyzeTrait, isFoo!int)); // = no messages,
compilation continues
static assert(__traits(analyzeTrait, isFoo!long)); // Compilation stops,
output is:
Error: analyzeTrait returned false for isFoo!(T) where T == long:
is(T == int) = false [1]
is(T == long) = true [2]
[1] || [2] = true [3]
__traits(compiles, ...) = false [4]
Error: no overload of blah for long
[3] [4] = false
Note, the __traits(compiles, ...) line would show the entire compile
expression for reference.
Something like this makes analysis of why the static assert failed so
much better. You could just build this into static assert, but I think
it might be too unwieldy to see all the messages. Sometimes, just
knowing static assert fails some involved test is fine, you don't need
the details.
-Steve
the traits trap
OK, so I'm writing some traits that I'd like my objects to satisfy. And
I'm having the worst time debugging them.
Most of the traits in D look like this:
enum isSomeType(T) = __traits(compiles, (T t){
// some statements using t
// some asserts
// some static asserts
});
All good.
Now, let's test my object:
unittest
{
static assert(isSomeType!SomeObject);
}
Nope. Now, how the hell do I figure out why?
I have found the following technique most valuable:
1. Create a function called testSomeType(T)(T t), make it's body the
same as the trait
2. Instead of static asserting the trait, call the function
Much better results! Whichever part of the trait doesn't work shows up
as a legitimate error, and I can fix the object or the trait.
Now, this idiom of using __traits(compiles, ...) is used everywhere in
phobos. Often times you see things like:
void foo(T)(T t) if (hasSomeTrait!T hasSomeOtherTrait!T
alsoHasThisOne!T) { ...
If this doesn't compile, the compiler says Error template instance blah
blah does not match template declaration blah blah blah
Useless...
Now, even if I want to use my cool technique to figure out where the
issue is, I have to do it one at a time to each trait, and I may have to
temporarily comment out some code to avoid triggering an error before I
get to that point.
When I first came to write this post, I wanted to ask if anyone thought
it was a good idea to replace the __traits(compiles, someLiteral) with
__traits(compiles, someFunctionTemplate!T) somehow, so if one couldn't
do it, you had some easy way to debug by calling someFunctionTemplate.
But I hate that idea. This means you have all these do-nothing functions
whose sole existence is to debug traits. When the traits themselves can
just do it for you.
Can anyone figure out a good solution to this problem? I like template
constraints, but they are just too black-boxy. Would we have to signify
that some enum is actually a trait and so the compiler would know to
spit out the junk of compiling? Would it make sense to add some __traits
function that allows one to signify that this is a special trait thing?
This is one area that D's templates are very user-unfriendly.
-Steve
Re: the traits trap
On Friday, 21 November 2014 at 04:08:52 UTC, Steven Schveighoffer
wrote:
OK, so I'm writing some traits that I'd like my objects to
satisfy. And I'm having the worst time debugging them.
Most of the traits in D look like this:
enum isSomeType(T) = __traits(compiles, (T t){
// some statements using t
// some asserts
// some static asserts
});
All good.
Now, let's test my object:
unittest
{
static assert(isSomeType!SomeObject);
}
Nope. Now, how the hell do I figure out why?
I have found the following technique most valuable:
1. Create a function called testSomeType(T)(T t), make it's
body the same as the trait
2. Instead of static asserting the trait, call the function
Much better results! Whichever part of the trait doesn't work
shows up as a legitimate error, and I can fix the object or the
trait.
Now, this idiom of using __traits(compiles, ...) is used
everywhere in phobos. Often times you see things like:
void foo(T)(T t) if (hasSomeTrait!T hasSomeOtherTrait!T
alsoHasThisOne!T) { ...
If this doesn't compile, the compiler says Error template
instance blah blah does not match template declaration blah
blah blah
Useless...
Now, even if I want to use my cool technique to figure out
where the issue is, I have to do it one at a time to each
trait, and I may have to temporarily comment out some code to
avoid triggering an error before I get to that point.
When I first came to write this post, I wanted to ask if anyone
thought it was a good idea to replace the __traits(compiles,
someLiteral) with __traits(compiles, someFunctionTemplate!T)
somehow, so if one couldn't do it, you had some easy way to
debug by calling someFunctionTemplate.
But I hate that idea. This means you have all these do-nothing
functions whose sole existence is to debug traits. When the
traits themselves can just do it for you.
Can anyone figure out a good solution to this problem? I like
template constraints, but they are just too black-boxy. Would
we have to signify that some enum is actually a trait and so
the compiler would know to spit out the junk of compiling?
Would it make sense to add some __traits function that allows
one to signify that this is a special trait thing?
This is one area that D's templates are very user-unfriendly.
-Steve
There has been a bit of promising work done by Shammah
Chancellor. It's a bit more heavyweight than a template returning
true or false, but it's also more powerful and makes for better
error messages.
http://forum.dlang.org/thread/m219bj$fpa$1...@digitalmars.com
Re: the traits trap
On Friday, 21 November 2014 at 04:08:52 UTC, Steven Schveighoffer wrote: Can anyone figure out a good solution to this problem? I like template constraints, but they are just too black-boxy. Would we have to signify that some enum is actually a trait and so the compiler would know to spit out the junk of compiling? Would it make sense to add some __traits function that allows one to signify that this is a special trait thing? This is one area that D's templates are very user-unfriendly. -Steve I would second this. Personally, I have the same not very pleasant experience debugging template constraints. Since more often than not the constraints have the form of: if (clause1 clause2 clause3 ...) my naive proposal would be to show which clause was first to be false in the error message. However, I have no idea if this could be implemented easily.
Re: the traits trap
On Friday, 21 November 2014 at 04:08:52 UTC, Steven Schveighoffer wrote: This is one area that D's templates are very user-unfriendly. -Steve +1, Well said! --- Paolo
