Silly me... I put a (debug) in each lambda and I could see that my pcase 
is indeed incorrectly matching.

The keyword documentation is strange, however.

On 7/14/23 00:56, Bryce Carson wrote:
>
> I forgot to attach the screenshot of the in-error appearance in the 
> customize buffer when using these lambdas to convert betwixt internal 
> and internal values.
>
> In the customize buffer, the button appears as invalid (nil). Perhaps 
> my pcase is incorrect, but it's rather simple matching so I don't see 
> why it'd be the cause (though it /would/ not match any of the 
> choice-items if none of the patterns are matching [nil would be 
> returned]).
>
> On 7/14/23 00:52, Bryce Carson wrote:
>>
>> Further, I can't seem to get these functions to work /as documented/ 
>> or expected.
>>
>> Without these defined when I'm creating a widget with the following 
>> code, I do get a button but the internal value is displayed in the 
>> clickable button. I'd like the external value to be displayed in the 
>> button %v, just as the menu-tag is is "externally facing".
>>
>> <<widget>>=
>> (define-widget 'project-widget 'list
>>    :tag (let ((s "\n\tProject"))
>>           (put-text-property 0 (length s) 'face 'bold s) s)
>>    :format "%t\n%v "
>>    :offset 0
>>    :indent 0
>>    :convert-widget 'widget-types-convert-widget
>>    :args '((menu-choice
>>             :tag "EmacSQL-supported backend"
>>             :format "%[%t%]: %[%v%]"
>>             :value "sqlite"
>>             :value-to-external <<menu-choice internal value to external lambda>>
>>             :value-to-internal <<menu-choice external value to internal lambda>>
>>             (choice-item :menu-tag "MySQL" :value "mysql")
>>             (choice-item :menu-tag "PostgreSQL" :value "postgresql")
>>             (choice-item :menu-tag "SQLite" :value "sqlite"))))
>> <<menu-choice internal value to external lambda>>=
>> (lambda (widget internal-value)
>>    "Converts lowercase, internal values to the casing of trademarks."
>>    (pcase internal-value
>>      ("mysql" "MySQL")
>>      ("sqlite" "SQLite")
>>      ("postgresql" "PostgreSQL")))
>> <<menu-choice external value to internal lambda>>=
>> (lambda (widget external-value)
>>    "Converts the casing of trademarked names to lowercase, internal values."
>>    (pcase external-value
>>      ("MySQL" "mysql")
>>      ("SQLite" "sqlite")
>>
>> On 7/14/23 00:32, Bryce Carson wrote:
>>>
>>> In the section /Defining New Widgets/, the following quotation 
>>> reveals a bug, I believe. I added emphasis for the last sentence of 
>>> each description of the keywords. I believe that for the 
>>> :value-to-external function, the last sentence is not supposed to be 
>>> present.
>>>
>>> :value-to-internal
>>> 	Function to convert the value to the internal format. The function takes two
>>> 	arguments, a widget and an external value, and returns the internal value./The function is called on the present :value when the widget is 
>>> created, and on any value set later with widget-value-set./
>>> :value-to-external
>>> 	Function to convert the value to the external format. The function takes two
>>> 	arguments, a widget and an internal value, and returns the external value./The function is called on the present :value when the widget is 
>>> created, and on any value set later with widget-value-set./
>>>
>>> It looks like the author or editor of the descriptions coped them, 
>>> which is fine. It doesn't make sense that :value-to-external would 
>>> be called when the widget is created, nor when new values are set 
>>> with widget-value-set.
>>>
>>> Other places in the manual state that the value of the :value 
>>> keyword, when creating or defining widget, should be in the external 
>>> form, more evidence that the last sentence is a bug.
>>>
>> -- 
>> "It's a GNU system, I don't know this!"
>> ||--- Mirror universe Lex Murphy, in Cenozoic Zoo
> -- 
> "It's a GNU system, I don't know this!"
> ||--- Mirror universe Lex Murphy, in Cenozoic Zoo
-- 
"It's a GNU system, I don't know this!"
||--- Mirror universe Lex Murphy, in Cenozoic Zoo