[web-devel] Re: [Haskell-cafe] Google Summer of Code: BlazeHTML RFC

Johan Tibell johan.tibell at gmail.com
Thu May 27 06:30:06 EDT 2010

On Thu, May 27, 2010 at 10:53 AM, Michael Snoyman <michael at snoyman.com>wrote:

> In other words, here's what I think the three different benchmarks are
> really doing:
> * String: generates a list of Strings, passes each String to a relatively
> inefficient IO routine.
> * ByteString: encodes Strings one by one into ByteStrings, generates a list
> of these ByteStrings, and passes each ByteString to a very efficient IO
> routine.
> : Text: encodes Strings one by one into Texts, generates a list of these
> Texts, calls a UTF-8 decoding function to decode each Text into a
> ByteString, and passes each resulting ByteString to a very efficient IO
> routine.

If Text used UTF-8 internally rather than UTF-16 we could create Texts from
string literals much more efficiently, in the same manner as done in
Char8.pack for bytestrings:

    {-# RULES
       "FPS pack/packAddress" forall s .
          pack (unpackCString# s) = inlinePerformIO (B.unsafePackAddress s)

This rule skips the creation of an intermediate String when packing a string
literal by having the created ByteString point directly to the memory GHC
allocates (outside the heap) for the string literal. This rule could be
added directly to a builder monoid for lazy Texts so that no copying is done
at all. In addition, if Text was internally represented using UTF-8
encodeUtf8 would be free.

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