[Nix-dev] Usefulness of adderrorContext

[Marc Weber]

Run this example and you'll  get:

== result
/tmp/test-nix-add-error-context.nix|1 col 4| error: while evaluating the attribute `<let-body>'
/tmp/test-nix-add-error-context.nix|8 col 8| while evaluating the function
/tmp/test-nix-add-error-context.nix|10 col 7| while evaluating the attribute `take'
/tmp/test-nix-add-error-context.nix|5 col 9| while evaluating the attribute `name'
|| value
|| value is an integer while an attribute set was expected
== code

let 
  inherit (builtins) addErrorContext;

  attr = addErrorContext "attr" {
    name = addErrorContext "value" (2).abc;
  };

  f = x: x;

  take = attr.name;

in f take


So what happened to the error context "attr" ?



You can get both contexts by using addErrorContext2 instead of addErrorContext
  addErrorContext2 = msg: a: addErrorContext msg (strict a);

However this isn't perfect because infinite recursions will be
evaluated etc.

strict could be implemented like this:


  # evaluate everything once so that errors will occur earlier
  # hacky: traverse attrs by adding a dummy
  # ignores functions (should this behavior change?) See strictf
  strict = x :
    let
        traverse = x :
          if isString x then true
          else if isAttrs x then
            if x ? outPath then true
            else all id (mapRecordFlatten (n: traverse) x)
          else if isList x then
            all id (map traverse x)
          else if isBool x then true
          else if isFunction x then true
          else if isInt x then true
          else if x == null then true
          else true; # a (store) path?
    in if (traverse x) then x else throw "else never reached";


So is this a bug or a feature that "attr" is lost in the first example?

Marc Weber