Subject: [PATCH] guix: build: Add a hint for unspecified value

From: Julien Lepiller <[email-masked]>
Date: Wed, 16 Sep 2020 23:40:03 +0200

Hi Guix!

this patch adds a hint to guix build, when you try to build from a file
and it returns #<unspecified>.  The most common error is when you copy
the structure from other guix modules and use `define-public` for your
package. The file returns #<unspecified> and you get this message:

> #<unspecified>: not something we can build.

Which is not very helpful. Here is what happens with this change:

> guix build: error: #<unspecified>: not something we can build
> 
> hint: If you build from a file, make sure the last scheme expression
> returns a package value.  `define-public' defines a variable, but
> returns #<unspecified>.  To fix this, add a scheme expression at the
> end of the file that consists only of the package's variable name you
> defined.  ex:
> 
>      (define-public my-package
>        (package
>          ...))
>      
>      my-package

If instead, you return a value that is not understood, the message is
simpler (as this is not something that happens out of confusion / being
new to guix or guile):

> guix build: error: 1: not something we can build
> 
> hint: If you build from a file, make sure the last scheme expression
> returns a package, gexp, derivation value or a list of such values.

(here the file returned "1" which we can't build :))
>From b29a325ef52d402c9618bfe9f02e478134297c0f Mon Sep 17 00:00:00 2001
From: Julien Lepiller <[email-masked]>
Date: Wed, 16 Sep 2020 23:32:00 +0200
Subject: [PATCH] guix: build: Add a hint for unspecified value.

* guix/scripts/build.scm (options->things-to-build): Add a hint when we
cannot build something.
---
 guix/scripts/build.scm | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/guix/scripts/build.scm b/guix/scripts/build.scm
index 25418661b9..46d1415a4d 100644
--- a/guix/scripts/build.scm
+++ b/guix/scripts/build.scm
@@ -38,6 +38,7 @@
   #:use-module (guix monads)
   #:use-module (guix gexp)
   #:use-module (guix profiles)
+  #:use-module (guix diagnostics)
   #:autoload   (guix http-client) (http-fetch http-get-error?)
   #:use-module (ice-9 format)
   #:use-module (ice-9 match)
@@ -46,6 +47,7 @@
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
   #:use-module (srfi srfi-37)
   #:use-module (gnu packages)
   #:autoload   (guix download) (download-to-store)
@@ -805,7 +807,27 @@ must be one of 'package', 'all', or 'transitive'~%")
 build---packages, gexps, derivations, and so on."
   (define (validate-type x)
     (unless (or (derivation? x) (file-like? x) (gexp? x) (procedure? x))
-      (leave (G_ "~s: not something we can build~%") x)))
+      (raise (make-compound-condition
+               (formatted-message (G_ "~s: not something we can build~%") x)
+               (condition
+                (&fix-hint
+                 (hint
+                   (if (unspecified? x)
+                       (G_ "If you build from a file, make sure the last scheme
+expression returns a package value.  @code{define-public} defines a variable,
+but returns #<unspecified>.  To fix this, add a scheme expression at the end of
+the file that consists only of the package's variable name you defined. ex:
+
+@example
+(define-public my-package
+  (package
+    ...))
+
+my-package
+@end example")
+                       (G_ "If you build from a file, make sure the last scheme
+expression returns a package, gexp, derivation value or a list of such
+values.")))))))))
 
   (define (ensure-list x)
     (let ((lst (match x
-- 
2.28.0