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