Re: Poor quality error messages

[Stefan Monnier]

Alan Mackenzie [2022-01-05 18:37:18] wrote:
> During a bootstrap (not of the standard code), the make failed,
> presenting me with the following half error message:
>
>   In toplevel form:
>   cedet/ede.el: Error: No applicable method: initialize-instance, nil, nil
>   make[2]: *** [Makefile:308: cedet/ede.elc] Error 1
>   make[2]: *** Waiting for unfinished jobs....
>
> ..  Is this really the best we can do?
>
> The following faults with this half message are readily apparent:
>
> (ii) There is no preposition or verb indicating the relationship between
>   the missing method (whatever that is) and initialize-instance.

`initialize-instance` is a "generic function" (i.e. a function defined
via `cl-defgeneric` and which can have different implementations for
different argument types, each implementation being called a "method")
and the error says that this function was called with a set of arguments
for which there is no matching method.

[ Side note: this nomenclature comes from CLOS and is a bit different
  from that used in mainstream class-based OO where what CLOS calls
  a "generic function" is often called a "method" and what CLOS calls
  a "method" is just a specific implementation of
  a method/generic-function in a particular class.  ]

> (iii) Is initialize-instance the method that is missing?

According to the error message, `initialize-instance` was called with
arguments nil and nil.  The first argument should be an object whose
slots needs to be initialized, so it shouldn't be nil, and indeed none
of the methods are applicable for the case where the first arg is nil.

> (iv) To what is the method (whatever that is) meant to be applicable?  To
>   initialise-instance, perhaps?  Or to "nil, nil"?  Or to something else?

The generic function by defaults comes with basically only one method
which handles the case where the first arg is an EIEIO object.

> (v) It is not clear what the "nil, nil" at the end of the half message is
>   meant to mean.

I think it's the list of arguments passed to `initialise-instance`.

> Just for info, grep finds no matches for "initialize-instance" in ede.el.

`initialize-instance` is used internally by `make-instance` (and
`make-instance` is used internally by the constructor defined by
`defclass`).

I.e. when the code defines a class with (defclass FOO ...) it defines
a constructor called FOO which calls `make-instance` which in turn calls
`initialize-instance` (as well as `shared-initialize`, IIRC).


        Stefan