03/06: ui: Add an 'unbound-variable' exception printer.

[Ludovic Courtès]

civodul pushed a commit to branch master
in repository guix.

commit dc856223f5eab57d8a4881782ec0f50abd12afa3
Author: Ludovic Courtès <address@hidden>
Date:   Thu Nov 9 23:31:18 2017 +0100

    ui: Add an 'unbound-variable' exception printer.
    
    * guix/ui.scm (print-unbound-variable-error): New variable.
    Use it as the 'unbound-variable' printer.
---
 guix/ui.scm | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/guix/ui.scm b/guix/ui.scm
index 9f790b6..05782a5 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -149,6 +149,18 @@ messages."
     (report-error args ...)
     (exit 1)))
 
+(define (print-unbound-variable-error port key args default-printer)
+  ;; Print unbound variable errors more nicely, and in the right language.
+  (match args
+    ((proc message (variable) _ ...)
+     ;; We can always omit PROC because when it's useful (i.e., different from
+     ;; "module-lookup"), it gets displayed before.
+     (format port (G_ "~a: unbound variable") variable))
+    (_
+     (default-printer))))
+
+(set-exception-printer! 'unbound-variable print-unbound-variable-error)
+
 (define (make-user-module modules)
   "Return a new user module with the additional MODULES loaded."
   ;; Module in which the machine description file is loaded.