Module:Category pair/sandbox

 require('strict')
 local getArgs = require('Module:Arguments').getArgs
 local hatnote = require('Module:Hatnote')._hatnote
 local formatLink = require('Module:Format link')._formatLink

 local p = {}

 local catNS = mw.site.namespaces.Category.id -- category namespace number

 -- Lua implementation of [[Template:CategoryPair]]
 -- Arguments:
 -- prevTitle -- mw.title.Title object for preceding category
 -- nextTitle -- mw.title.Title object for succeeding category
 -- Returns:
 -- hatnote that says "see also" for one or both of prev/next (depending on whether they exist)
 function p._pair(prevTitle, nextTitle)
	prevTitle = prevTitle and prevTitle.exists and formatLink{link = prevTitle.fullText}
	nextTitle = nextTitle and nextTitle.exists and formatLink{link = nextTitle.fullText}
	local note = ''
	if prevTitle and nextTitle then -- if both
		note = mw.ustring.format('See also the preceding %s and the succeeding %s',prevTitle, nextTitle)
	elseif prevTitle then -- if only prevTitle
		note = mw.ustring.format('See also the preceding %s', prevTitle)
	elseif nextTitle then -- if only nextTitle
		note = mw.ustring.format('See also the succeeding %s', nextTitle)
	else -- otherwise neither
		return mw.title.getCurrentTitle().namespace == catNS and '[[Category:Pages using category pair with no output]]' or ''
	end
	return hatnote(note, {extraclasses = 'seealso'})
 end

 function p.catPair(frame)
	local args = getArgs(frame, {wrappers={'Template:Category pair'}})
	local prevTitle = args[1] and mw.title.new(args[1],catNS)
	local nextTitle = args[2] and mw.title.new(args[2],catNS)
	return p._pair(prevTitle, nextTitle)
 end

 function p.prevCat(frame)
	local args = getArgs(frame, {wrappers={'Template:Preceding category'}})
	local prevTitle = args[1] and mw.title.new(args[1], catNS)
	return p._pair(prevTitle, nil)
 end

 function p.nextCat(frame)
	local args = getArgs(frame, {wrappers={'Template:Succeeding category'}})
	local nextTitle = args[1] and mw.title.new(args[1], catNS)
	return p._pair(nil, nextTitle)
 end

 return p