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

[Johan Tibell]

Hi Haskell Café!

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 ++.

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).

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:

> 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
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?

> 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.

Cheers,

Johan Tibell