diaper pattern lurking in ui.scm

[Christopher Allan Webber <cwebber@dustycloud.org>]

I was working on a new package and found that I was very surprised that
suddenly guix told me that "guix package" did not exist:

  cwebber@earlgrey:~/devel/guix$ guix package --help
  guix: package: command not found
  Try `guix --help' for more information.

Whaaaa?

So I dug into the source and found that in ui.scm there's this:

(define (run-guix-command command . args)
  "Run COMMAND with the given ARGS.  Report an error when COMMAND is not
found."
  (define module
    (catch 'misc-error
      (lambda ()
        (resolve-interface `(guix scripts ,command)))
      (lambda -
        (format (current-error-port)
                (_ "guix: ~a: command not found~%") command)
        (show-guix-usage))))

  (let ((command-main (module-ref module
                                  (symbol-append 'guix- command))))
    (parameterize ((program-name command))
      (apply command-main args))))

That catch of 'misc-error was the problem that was hiding the relevant
exception.  I removed the "catch" and got guix to raise the real error.
I had a real, actual problem with my package... I had done this:

    (source (origin
             (method url-fetch)
             (uri (string-append
                   "http://download.gna.org/guile-dbi/guile-dbi-"
                   version
                   ".tar.gz"))
             (sha256
              (base32
               "3545ec6b589eaa601eb1ed275c00be0cc9ba08204a252415b205f1befce7d64a"))))

Oops, I had just copied and pasted `sha256sum guile-dbi-2.1.5.tar.gz'
output into the sexp structure I borrowed from another package.  So the
error that was *really* being thrown was:

In guix/packages.scm:
 182: 4 [#<procedure 21e7600 (str)> #]
In guix/base32.scm:
 260: 3 [base32-string-unfold-right #<procedure 2aaa3a0 at guix/base32.scm:277:28 (chr)> ...]
In unknown file:
   ?: 2 [string-fold-right #<procedure 2a825a0 at guix/base32.scm:260:23 (chr index)> ...]
In guix/base32.scm:
 261: 1 [#<procedure 2a825a0 at guix/base32.scm:260:23 (chr index)> #\e 5]
In unknown file:
   ?: 0 [scm-error misc-error #f "~A ~S" ("invalid base32 character" #\e) #f]
ERROR: In procedure scm-error:
ERROR: invalid base32 character #\e

Aha, of course!  That wasn't a base32 sha256 after all.  No wonder!

Unfortunately, instead of getting this useful error that could have
helped me figure out my mistake above, the catch-all exception handling
hid my real problem from view.

I'm afraid we've fallen victim to the Diaper Pattern here:

  http://mike.pirnat.com/2009/05/09/the-diaper-pattern-stinks/

I think that a more explicit exception should be being thrown and caught
in ui.scm.  I could generate a patch, but I don't know what exception
it's expecting might be raised.

In other words, (error) considered harmful, use (throw 'specific-key)
instead :)

 - Chris