[dart-misc] What's possible with Dart Generics at runtime in Flutter

[mythz]

I'm developing a serialization library that works in flutter so I'm
avoiding Mirrors and would like to know what's possible with runtime
generics in Dart/flutter to find out what needs to be code-gen'ed and what
can be automated with generic routines.

Firstly how can we can get the runtime time of a generic argument?

T create<T>() {
var runtimeTypeOfT = ... ?
}

Is it possible to create a new instance of T where T has an empty
constructor, e.g:

T create<T>() {
var instance = new T();
return instance;
}

This is possible in C# without reflection by using the `where T : new()`
type constraint, how can we do this in Dart/Flutter?

Can we create a generic factory function which creates a generic list with
the runtime argType?

List createList(String argType) {
var newGenericList = ... // create List<argType>?
return newGenericList;
}

So that we can could use it to create generic lists like:

List<int> intList = createList("int");
List<String> stringList = createList("String");


As a current workaround/hack I'm generating a dictionary of factory
constructors like:

{
'List<String>': () => new List<String>(),
'Map<String,int>': () => new Map<String,int>(),
}

But I would like to avoid needing to do this and would like to be able to
use a generic routine if possible.

Thanks!

['Bob Nystrom' via Dart Misc]

Post by mythz
I'm developing a serialization library that works in flutter so I'm
avoiding Mirrors and would like to know what's possible with runtime
generics in Dart/flutter to find out what needs to be code-gen'ed and what
can be automated with generic routines.
Firstly how can we can get the runtime time of a generic argument?
T create<T>() {
var runtimeTypeOfT = ... ?
}

"T":

T create<T>() {
var runtimeTypeOfT = *T*;
}

Post by mythz
Is it possible to create a new instance of T where T has an empty
T create<T>() {
var instance = new T();
return instance;
}
This is possible in C# without reflection by using the `where T : new()`
type constraint, how can we do this in Dart/Flutter?

No, there's no way to do this in Dart currently without using some sort of
code generation approach.

Post by mythz
Can we create a generic factory function which creates a generic list with
the runtime argType?
List createList(String argType) {
var newGenericList = ... // create List<argType>?
return newGenericList;
}
List<int> intList = createList("int");
List<String> stringList = createList("String");

No, there's no way to do this either. Supporting something like this in
general would make it *very* hard to do tree shaking, which is vital for
controlling code size on the web.

Post by mythz
As a current workaround/hack I'm generating a dictionary of factory
{
'List<String>': () => new List<String>(),
'Map<String,int>': () => new Map<String,int>(),
}

This is, I think, the best current solution. It's not ideal, but
metaprogramming is generally pretty hard in a language that is compiled
ahead of time and where users also care deeply about generated code size.
Dart 2 is more like C++ than, say, C# where you can rely on a VM and a JIT
and where generated code size isn't usually a big concern.

I hope that eventually we can add some more powerful static metaprogramming
features to Dart, but it's a really challenging problem and there are few
successful languages out there in the same space that we can learn from.

Cheers!

– bob