log time instead of linear for case matching

Simon Peyton-Jones simonpj at microsoft.com
Mon Oct 10 10:14:42 CEST 2011


In GHC, big cases are done as tables (if dense) or trees (if sparse).  If you have some examples where things go bad, do submit a bug report.

For big dispatches on strings, I'm pretty sure we do something linear, top to bottom.   I'd be strongly inclined to use a proper Map structure if you want good performance on that.   Is there some reason you don't want to?


From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-bounces at haskell.org] On Behalf Of Greg Weber
Sent: 09 October 2011 17:39
To: GHC users
Subject: log time instead of linear for case matching

We have a couple use cases in Yesod that can potentially match many different patterns. Routing connects the url of an http request to a Haskell function. The current code just uses a pattern match, which scales linearly. So if a site has a hundred different routes (urls), it could take 100 comparisons to finally match the url to a function. Michael Snoyman is writing some code to make this issue obsolete. One of the things it does is use a Map lookup instead of a case match.

More worrying is our system for internationalization of a website. A user is supposed to make a sum type with every custom message as a constructor in that sum type.

data Msg = Model
         | Year
         | Please

-- Rendering function for English.
renderEnglish Model  = "Model"
renderEnglish Year   = "Year"
renderEnglish Please = "Please fill in your car's details"

-- Rendering function for Swedish.
renderSwedish Model  = "Modell"
renderSwedish Year   = "Årgång"
renderSwedish Please = "Vänligen fyll i uppgifterna för din bil"

So if there are 100 custom messages on a site, that will setup a case match with potentially 100 comparisons.

Note that we are using this technique for type safety- switching to a map here would be difficult. We want to know at compile time that a translation exists for every message.

So first of all I am wondering if a sum type comparison does in fact scale linearly or if there are optimizations in place to make the lookup constant or logarithmic. Second, I as wondering (for the routing case) if Haskell can make a string case comparison logarithmic so that users can use case comparisons instead of maps for string collections that are known at compile time and won't change.
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