Improve reflection support

[alfonsogarciacaro]

It'd be great to improve reflection support for Fable 3. We can do it by bringing the reflection changes in #1839, but if possible I'd like to have these changes:

• Have a separate PR for reflection and quotation support.
• Keep the current style in Reflection.ts. That is, use module functions instead of class methods so it works better with tree shaking.
• Instead of adding attribute and member information to all types. We could have the following functions in the Fable.Core.Reflection module:

generateConstructors<'T>()
generateProperties<'T>()
generateProperty<'T>(name)
generateMethods<'T>()
generateMethod<'T>(name)
generateCustomAttributes<'T>()

The Replacements module would replace calls to these methods with the necessary info inline, so we only generate the code where is needed.

What do you think @krauthaufen? Do the changes look good? Would you like to help with this? (If you want to sart working, please note we'll be using the next branch for Fable 3 development).

[lfr]

Did this make it to Fable 3 in the end?

[alfonsogarciacaro]

No unfortunately it didn't. I realized that my original idea of having something like generateMethods<'T>() wouldn't work for many use cases, like quotations, so I'm still unsure how to balance improving reflection support while keeping JS bundle sizes small.

[lfr]

As suggested by @fable-compiler on twitter, I was able to find work arounds using a mix of dynamic code and inline functions. While the dynamic code is more brittle, it's quite contained and can be made somewhat safe with tests.

I find the inline workaround more problematic as it renders the generated JS unreadable, which is a shame as the JS generated by Fable seems otherwise surprisingly readable.

[alfonsogarciacaro]

Closing as this didn't make it into Fable 3 at the end. There are a few other issues still open about increasing reflection support in specific situations. We can try to deal with them in a case-by-case basis.