User "Tim Starling" posted a comment on MediaWiki.r81074.
Full URL: http://www.mediawiki.org/wiki/Special:Code/MediaWiki/81074#c21956
Commit summary:
(bug 235) parser function for conversion of units of measurement.
[[Template:Convert]] on enwiki is a behemoth of a construction that just about
manages to do this sort of conversion, taking {{convert|5|mi|km}} and
outputting "5 miles (8 km)", etc. To port this to another wiki requires
copying over three and a half thousand subtemplates. The additional load
produced by including numerous copies of this template is measurable on large
pages on enwiki, and it eats voraciously into the template limits.
This revision introduces {{#convert: 5 mi | km }}, outputting "8 km" or
thereabouts. See http://www.mediawiki.org/wiki/User:Happy-melon/Convert for
more details, or look at the examples in the parser tests.
In a very rough profile, comparing 50 calls to {{convert}} verses the same 50
calls to the wrapper template shown at the link above, the parser function
implementation reduces page load time by 72%, preprocessor node count by 83%,
post-expand include size by 86% and template argument size by 97%. More
detailed profiling would probably reveal places where extra caching could
improve performance further.
The primary reason for putting it in ParserFunctions instead of its own
extension is availability: PFs are already available across the cluster, and
it's accepted as an essential extension for any wiki wishing to emulate or
mirror WMF content. One less separate extension installed on the cluster is
one less extension which has to be matched by reusers.
It's still missing a lot of units, which I ran out of patience to copy from
{{convert}}; I thought I'd get some feedback on the infrastructure first.
Comment:
The reason I made ExprError derive from Exception instead of MWException is
because ExprErrors are never allowed to propagate to the calling code, they are
always caught and converted to an error message. So they are private to the
extension and do not need the MediaWiki-specific formatting code in MWException.
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