Add syntactic holes which raise errors with full context information

[jwiegley]

Add a token for the symbol '_' such that when the evaluator encounters it, we print everything we know about that point in the code.

This feature could also become the basis for supporting name completion in editors.

[shlevy]

Just FYI, there is some nix code here and there that uses _ as "unused argument"... Not sure if any of it is in nixpkgs these days but we definitely have some in our infra.

[jwiegley]

Ah, OK. Maybe __ or ___?

[Profpatsch]

Purescript uses ?name.

Naming “internal” variables like __var or _var is quite common in nixpkgs.

[jwiegley]

For code completion, these holes would be treated as "stop tokens". Immediately upon encountering one, a special exception is throw with the current lexical scope as its argument, allowing a completion list to be built.

Note that completion within dynamic arguments to with blocks would still be very tricky, since the visible names would depend on the context of evaluation. Maybe the symbolic evaluator can be used here to provide the intersection of names that would always be available.

[domenkozar]

nix-repl> ?name: "foobar"
error: syntax error, unexpected '?', at (string):1:1

Purescript notation sounds promising, since "Nix C++ compatibility" would refuse such syntax.

[sjakobi]

What's the status of this issue? I see that some work has been done in #398 and merged in 28ab983.

Is there more to do? What exactly?

[jwiegley]

I'm not sure any work has started on this yet.

[sjakobi]

@jwiegley But the work in #398 is related to this issue, right?!

I ended up finding this issue because I was confused about the purpose of the NSynHole constructor:

hnix/src/Nix/Expr/Types.hs

Lines 123 to 124 in 704323e

	| NSynHole !VarName
	-- ^ Syntactic hole, e.g. @^foo@ , @^hole_name@

(CC @Madelinja)

[jwiegley]

Interesting, I wasn't even aware of NSynHole, so that must have happened recently. :)