Implement builtin special forms

[spy16]

• (do <expr>*)
• (if test then else?)
• (def sym value)
• (quote expr)
• (let ( binding* ) expr*)
• (fn name? ( param* ) expr*)

[lthibault]

Quick note about the if expression: I'm unable to use it or to replicate it in Wetware because

1. parseIf is not exported (so I can't reuse it in my own Analyzer type)
2. env.analyzer is not exported (so I can't re-implement it)

In this particular case, I could embed BuiltinAnalyzer into my custom implementation, but I think that would miss the larger point: it feels wrong to have certain special forms only be implementable in parens itself.

[spy16]

Yes, I am aware of this issue but yet to decide on a solution. (Like I had mentioned on the other thread, I am always skeptical of making things public unless they are absolutely needed 😅). Do let me know if you have any other suggestions on this.

[lthibault]

Yup, makes sense! BTW I share your skepticism on the issue of exporting symbols, so 👍.

I don’t have a solution in mind either — just wanted to flag the issue. I’ll let you know if I come up with anything.

[lthibault]

@spy16 The latest commit breaks Wetware's def special form since parens.DefExpr now depends on env.analyzer under the hood.

I've given this a bit of thought and haven't been able to come up with a solution other than exporting Analyze() from Env, so I'm going to open a PR with this straightforward fix. If we come up with a better approach, we can integrate it later (ideally before the 1.0 release).

Have you had any breakthroughs on your end?

[spy16]

Ohh. Yea, def was previously invoking Eval(env) during analyse which was a bug. So i had to change it to only analyse the form provided for binding.

I haven't come up with any solution too (probably because exposing Analyse is the right solution).. Go ahead with the PR.

Another problem to keep in mind (while implementing your custom analyser/exprs for example):

currently Analyse takes an Env argument. And also the Expr.Eval(env) takes an env argument. Env should actually be bound to the expr at analyse time and Eval() should simply use that. Which means, all the Expr types would need to keep the env used during analyse inside them. So the Expr contract will probably change to:

type Expr interface {
    Eval() (Any, error)
}