Re: better error messages through assertions

[Philip McGrath <philip@philipmcgrath.com>]

Hi,

On 2/14/22 17:32, Ricardo Wurmus wrote:
> As you can probably tell easily by looking at this message, the
> “service” field of the operating system configuration looked something
> like this:
> 
>    (services (append (list a b c %desktop-services) #;oops))
> 
> instead of this
> 
>    (services (append (list a b c) %desktop-services))
> 
> This is because INSTANTIATE-MISSING-SERVICES — and FOLD-SERVICES, and
> many more — assumes that it is only passed a plain list of services.  It
> then proceeds to call SERVICE-KIND on what may or may not be a service.
> 
> I think we should add simple type checks, something like this:
> 
>    (define (listof pred)
>      (lambda (thing)
>       (and (list? thing) (every pred thing))))
>    …
>    (define (assert-type type-check thing message)
>      (or (false-if-exception (type-check thing))
>          (report-error (G_ "type error: …\n" message))))
> 
>    ;; Use ASSERT-TYPE in an example procedure.
>    (define (do-something-with-services services)
>      (assert-type (listof service?) services
>                   "SERVICES must be a list of <service> values.")
> 
>      ;; Do things…
>      (map service-kind services))
> 
> What do you think?  There are many different ways of implementing this
> (a new variant of DEFINE that also accepts a type declaration, an assert
> like above, a fancier assert that composes a helpful error message by
> itself, a separate type declaration that is looked up only when the
> corresponding procedure is called in a certain context, etc), but I’d
> first like to know if there is consensus that we want something like
> this.
> 

As a Guix user and contributor, I would love better error messages.

As a Racketeer, I think you're half way to reinventing contracts.

In particular, since the operating system services field is thunked, 
this example already points to the desirability of higher-order contracts.

Using "contracts" need not initially involve all the bells and whistles 
of Racket's contract library. For example, higher-order contracts can be 
implemented with `lambda` in the absence of special runtime support for 
chaperones and impersonators, as illustrated in [1]. But the Racket 
community has accumulated a great deal of both theoretical insight and 
implementation experience in many subtle corners of this problem: I hope 
Guix can build on that experience.

-Philip

[1]: https://docs.racket-lang.org/guide/Building_New_Contracts.html