Re: Time for std.reflection

[deadalnix]

On 21/07/2012 23:44, Andrei Alexandrescu wrote:
Walter and I discussed the idea below a long time (years) ago. Most
likely it's also been discussed in this newsgroup a couple of times.
Given the state of the compiler back then, back then it seemed like a
super cool idea that's entirely realizable, it would just take time for
the compiler to become as capable as needed. Nowadays we're in shape to
tackle it.
Here "it" is.

Back when runtime reflection was being discussed, my response was "let's
focus on compile-time reflection, and then we can do run-time reflection
on demand as a library". Though this might sound sensible, I initially
didn't have a design. Now here's what we can do.

Currently we have information about symbols as __traits(...) intrinsics
wrapped in nice but scattered ways. Now that CTFE is good enough to
manipulate structs and arrays thereof, we have the possibility to
finally approach things in a nicely unified, structured way.

First, we need to prime std.reflection with a few abstractions that
characterize entities in a D program.

class ModuleInfo {
@property:
string name();
ImportInfo[] imports();
DataInfo[] data();
FunctionInfo[] functions();
ClassInfo[] classes();
StructInfo[] structs(); // includes unions
TemplateInfo[] templates();
EnumInfo[] enums();
bool hasStaticCtor(), hasStaticDtor(),
hasSharedCtor(), hasSharedDtor();
}

Probably there are a few more pieces of data, but you get the idea. Then
for each of the entities mentioned above we have a similar definition.
For example:

enum Protection { isPublic, isPackage, isProtected, isPrivate }

class ClassInfo {
@property:
string name();
string baseName();
string parentName(); // if applicable, null otherwise
string[] interfaces();
bool isShared();
Protection protection();
DataMemberInfo[] data();
MethodInfo[] methods();
Object defaultConstructor();
...
}

Then for an e.g. method declaration we'd have:

class MethodInfo {
@property:
string name();
bool isStatic(), isFinal(), isOverride();
Protection protection();
string[] parameterTypes();
string[] parameterNames();
}

Some details may vary, e.g. some may be straight members instead of
properties etc. (I used properties to allude to use of lazy gathering of
information).

So so far we have a nice collection of structured data associated with
the entities in a D program. Note how this structuring differs yet has
similar power to the primitives in std.traits; std.traits offers
unstructured bits of information on demand (e.g. ParameterTypeNames)
etc. but the objects above group information together per entity
declared. All of the above goes in std.reflection, of course.

===========

On to primitives that return such data.

Given that D can (since relatively recently) create and manipulate class
objects during compilation too, it follows that the classes above can be
accessed in two ways - through compile-time API and run-time API. When
possible, the APIs may even use the same functions; some other times
they will be necessarily different.

There are two possible approaches to discovering such information. One
is by fetching the ModuleInfo for the whole module and navigating it.
Another one is by using search primitives from strings.

So we should have e.g.

// inside std.reflection
ModuleInfo getModuleInfo(string moduleName);

so a CT call would go like:

// client code
static info = getModuleInfo("std.algorithm");

whereas a run-time call would be:

// client code
auto info = getModuleInfo("std.algorithm");

In the latter case, the module needs to save all needed information for
ri, so it should plant this:

// inside std.algorithm
mixin(makeModuleInfoAvailableDynamically());

The mixin would generate all information needed and would store it for
later dynamic use.

A search API would go like e.g.

ClassInfo getClassInfo(string className);

In this case the class name could be qualified with module information etc.

===========

With this design we unify compile-time and run-time type manipulation in
simple ways, by defining structured information about declarations that
can be queried during compilation or dynamically.

Please chime in with thoughts. Would someone want to pioneer this project?


Andrei

I don't understand. Are theses structures generated by the compiler or 
some kind of libraries at compile time on a per needed basis ?