Subject: Re: bug#23975: 25.0.94: defcustom error message is wrong when :type field has a :match attribute

From: Mauro Aranda <[email-masked]>
Date: Fri, 4 Sep 2020 09:48:01 -0300

Lars Ingebrigtsen <[email-masked]> writes:

> Mauro Aranda <[email-masked]> writes:
>
>> I wonder if we could just document the :type-error property.  So
>> anybody that uses a custom :match function with additional checks can
>> put there the information they like to show the user when something goes
>> wrong.  So the defcustom posted would be something like:
>>
>> (defcustom bounded-num 999
>>   "Positive, bounded number"
>>   :type '(integer :match (lambda (widget value) (and (integerp value)
>>
          (> value 0)
          (< value 1000)))
>>                         :type-error "Value should be an integer between
0 and 1000"))
>>
>> That's easy, and would solve the main problem here.  WDYT?
>
> I think that's a very good idea.

Great! Here's my attempt:
From 84ee1868febb22ee8e43792dbb042def6e2023c7 Mon Sep 17 00:00:00 2001
From: Mauro Aranda <[email-masked]>
Date: Wed, 2 Sep 2020 08:44:01 -0300
Subject: [PATCH] Document :type-error property for customization types

* doc/lispref/customize.texi (Type Keywords): Document :type-error, so
Lisp programs can display a more correct message when the value of a
user option doesn't match its type.  (Bug#23975)
---
 doc/lispref/customize.texi | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/doc/lispref/customize.texi b/doc/lispref/customize.texi
index b9c9130a92..c35444f581 100644
--- a/doc/lispref/customize.texi
+++ b/doc/lispref/customize.texi
@@ -1197,6 +1197,13 @@ Type Keywords
 the widget containing the invalid data, and set that widget's
 @code{:error} property to a string explaining the error.
 
+@item :type-error @var{string}
+@kindex type-error@r{, customization keyword}
+@var{string} should be a string that describes why a value doesn't
+match the type, as determined by the @code{:match} function.  When the
+@code{:match} function returns @code{nil}, the widget's @code{:error}
+property will be set to @var{string}.
+
 @ignore
 @item :indent @var{columns}
 Indent this item by @var{columns} columns.  The indentation is used for
-- 
2.28.0