Print the value in value is X while a Y is expected error

[9999years]

Motivation

Adds the failing value to value is <TYPE> while a <TYPE> is expected error messages.

Context

Partial fix for #561, in particular #561 (comment).

Priorities

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[roberth]

We have to be careful not to print too much. For instance, pkgs isn't just cyclic but diverging, and printValue doesn't have a mitigation for that.
We don't want to fail before printing the actual message!
Value::print has a depth limit, but isn't quite safe.

To make this work well, I we should have a safe and brief printing method that

• only prints one level deep (maybe two levels for lists, after making sure not to print too many items)
• only returns a string up to a certain length, perhaps 120?
• does not evaluate values that haven't been forced yet
• prints the most useful attributes first: _type, name and type (but only if their values have already been forced)
• can print a package name without instantiating the derivation (outPath, drvPath)

[9999years]

@roberth I double-checked this and it doesn't force values, so it's OK:

$ cat bad.nix -p
let
  pkgs = import <nixpkgs> {};
in
  pkgs ++ 1

$ nix-instantiate --eval bad.nix
error:
       … while evaluating one of the elements to concatenate

         at /Users/wiggles/nix/bad.nix:4:8:

            3| in
            4|   pkgs ++ 1
             |        ^
            5|

       error: value is a set while a list was expected: { AAAAAASomeThingsFailToEvaluate = <CODE>; AMB-plugins = <CODE>; ArchiSteamFarm = <CODE>; AusweisApp2 = <CODE>; CHOWTapeModel = <CODE>; ... }

Printing all of pkgs attribute names like this adds about 20ms to the command. Limiting the attributes printed is a good idea -- I'd also like if this code printed lambda locations and derivation paths like nix repl does.

Maybe I'll take a shot at unifying the printing code (it's duplicated in a few places), but even with very large output (like "all of nixpkgs") it's much more helpful than not having anything (IMO).

[edolstra]

Our error messages are already way too verbose, e.g. a typical NixOS evaluation error dumps thousands of lines of stacktrace info on the screen. Printing giant values (like pkgs or config) is only going to make it harder to see the actual error.

[Qyriad]

Your error messages aren't too verbose; they have too much noise and not enough signal. Nix's call traces being difficult to visually parse is arguably helped by this change because in a sea of "while calling …" lines the one that contains a big value is probably at least slightly more important than all the other myriad lines.

This at least puts visual emphasis on something that at least probably gives some helpful information.

Yeah, some smart heuristics for large values would be nice, but this is still better than the status quo.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/whats-nixs-stable-api-is-code-in-nix-instantiate-part-of-it/37986/1