[Haskell-cafe] Efficient use of ByteString and type classes in template system

[Thomas Hartman]

Created wiki page

http://haskell.org/haskellwiki/String_Interpolation

and referenced various topics mentioned in this thread, there.

2007/4/16, Donald Bruce Stewart <dons at cse.unsw.edu.au>:
> johan.tibell:
> > Hi Haskell Caf?!
> >
> > I'm writing a perl/python like string templating system which I plan
> > to release soon:
> >
> > darcs get http://darcs.johantibell.com/template
> >
> > The goal is to provide simple string templating; no inline code, etc..
> > An alternative to printf and ++.
>
> Ok. You might also want to briefly look at the other templating system I
> know of in Haskell, this small module by Stefan Wehr,
>
>     http://www.cse.unsw.edu.au/~dons/code/icfp05/tests/unit-tests/VariableExpansion.hs
>
> Just a quick thing he did for the ICFP contest, but does indicate one
> way to do it (i.e. via pretty printing).
>
> >
> > Example usage:
> >
> > >import qualified Data.ByteString as B
> > >import Text.Template
> > >
> > >helloTemplate = "Hello, $name! Would you like some ${fruit}s?"
> > >helloContext = [("name", "Johan"), ("fruit", "banana")]
> > >
> > >test1 = B.putStrLn $ substitute (B.pack helloTemplate) helloContext
> >
> > I want to make it perform well, especially when creating a template
> > once and then rendering it multiple times. "Compiling" the template is
> > a separate step from rendering in this use case:
> >
> > >compiledTemplate = template $ B.pack helloTemplate
> > >
> > >test2 = B.putStrLn $ render compiledTemplate helloContext
> >
> > A template is represented by a list of template fragments, each
> > fragment is either a ByteString literal or a variable which is looked
> > up in the "context" when rendered.
> >
> > >data Frag = Lit ByteString | Var ByteString
> > >newtype Template = Template [Frag]
> >
> > This leads me to my first question. Would a lazy ByteString be better
> > or worse here? The templates are of limited length. I would say the
> > length is usually between one paragraph and a whole HTML page. The
> > Template data type already acts a bit like a lazy ByteString since it
> > consists of several chunks (although the chunck size is not adjusted
> > to the CPU cache size like with the lazy ByteString).
>
> Probably lazy bytestrings are better here, since you get O(n/k) append
> cost, rather than O(n).  If most strings are small, it mightn't be
> noticeable.
>
> > Currently the context in which a template is rendered is represented
> > by a type class.
> >
> > >class Context c where
> > >    lookup :: ByteString -> c -> Maybe ByteString
> > >
> > >instance Context (Map String String) where
> > >    lookup k c = liftM B.pack (Map.lookup (B.unpack k) c)
> > >
> > >instance Context (Map ByteString ByteString) where
> > >    lookup = Map.lookup
> > >
> > >-- More instance, for [(String, String)], etc.
> >
> > I added this as a convenience for the user, mainly to work around the
> > problem of not having ByteString literals. A typical usage would have
> > the keys in the context being literals and the values some variables:
>
> note sure if it is relevant, but:
>
>     pack "Foo"
>
> will be converted via rewrite rules to a bytestring literal at compile
> time. So there's no overhead for having String literals.
>
> >
> > >someContext = Map.fromList [("name", name), ("fruit", fruit)]
> >
> > I'm not sure if this was a good decision, With this I'm halfway to the
> > (in)famous Stringable class and it seems like many smarter people than
>
> Yes, seems a little worrying.
>
> > me have avoided introducing such a class. How will this affect
> > performace? Take for example the rendering function:
> >
> > >render :: Context c => Template -> c -> ByteString
> > >render (Template frags) ctx = B.concat $ map (renderFrag ctx) frags
> > >
> > >renderFrag :: Context c => c -> Frag -> ByteString
> > >renderFrag ctx (Lit s) = s
> > >renderFrag ctx (Var x) = case Text.Template.lookup x ctx of
> > >                           Just v  -> v
> > >                           Nothing -> error $ "Key not found: " ++
> > >                           (B.unpack x)
> >
> > How will the type dictionary 'c' hurt performance here? Would
> > specializing the function directly in render help?
>
> Hmm. Hard to say: look at the Core code and we will know.
>
> Really though, you'll need some stress test cases to be able to make
> resonable conclusions about performance.
>
> >
> > >render (Template frags) ctx = B.concat $ map (renderFrag f) frags
> > >    where f = flip Text.Template.lookup ctx
> > >
> > >renderFrag f (Var x) = case f x of
> >
> > I can see the implementation taking one of the following routes:
> > - Go full Stringable, including for the Template
> > - Revert to Context = Map ByteString ByteString which was the original
> > implementation.
> > - Some middle road, without MPTC, for example:
> > >class Context c where
> > >    lookup :: ByteString -> c ByteString ByteString -> Maybe ByteString
> > This would allow the user to supply some more efficient data type for
> > lookup but not change the string type. Having a type class would allow
> > me to provide things like the possibility to create a Context from a
> > record where each record accessor function would server as key.
> > Something like:
> >
> > >data Person { personName :: String, personAge :: Int }
> > would get converted (using Data?) to:
> > >personContext = [("personName", show $ personName aPerson),
> > >                 ("personAge", show $ personAge aPerson)]
> > but not actually using a Map but the record itself.
> >
> > I guess my more general question is: how do I reason about the
> > performance of my code or any code like this? Are there any other
> > performance improvements that could be made?
> >
> > Also, I would be grateful if someone could provide some feedback on
> > the implementation, anything goes!
> >
> > I still have some known TODOs:
> >
> > - Import error messages for invalid uses of "$".
> > - Improve the regex usage overall.
> > - Add some more functions; the plan is to add those function which
> > could be expressed in efficiently with the current interface. An
> > example is things like renderAndWrite, when writing doing a B.concat
> > first is unnecessary.
>
> I'd suggest: keep it simple and fast. Then work out what extra stuff you
> need.
>
> -- Don
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