[Haskell-cafe] PROPOSAL: New efficient Unicode string library.

Duncan Coutts duncan.coutts at worc.ox.ac.uk
Wed Sep 26 13:46:35 EDT 2007

In message <1190825044.9435.1.camel at jcchost> Jonathan Cast <jcast at ou.edu> writes:
> On Wed, 2007-09-26 at 09:05 +0200, Johan Tibell wrote:

> > If UTF-16 is what's used by everyone else (how about Java? Python?) I
> > think that's a strong reason to use it. I don't know Unicode well
> > enough to say otherwise.
> I disagree.  I realize I'm a dissenter in this regard, but my position
> is: excellent Unix support first, portability second, excellent support
> for Win32/MacOS a distant third.  That seems to be the opposite of every
> language's position.  Unix absolutely needs UTF-8 for backward
> compatibility.

I think you're talking about different things, internal vs external representations.

Certainly we must support UTF-8 as an external representation. The choice of
internal representation is independent of that. It could be [Char] or some
memory efficient packed format in a standard encoding like UTF-8,16,32. The
choice depends mostly on ease of implementation and performance. Some formats
are easier/faster to process but there are also conversion costs so in some use
cases there is a performance benefit to the internal representation being the
same as the external representation.

So, the obvious choices of internal representation are UTF-8 and UTF-16. UTF-8
has the advantage of being the same as a common external representation so
conversion is cheap (only need to validate rather than copy). UTF-8 is more
compact for western languages but less compact for eastern languages compared to
UTF-16. UTF-8 is a more complex encoding in the common cases than UTF-16. In the
common case UTF-16 is effectively fixed width. According to the ICU implementors
this has speed advantages (probably due to branch prediction and smaller code size).

One solution is to do both and benchmark them.


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