[Haskell-cafe] surrogate code points in a Char

[Mark Lentczner]

On Nov 18, 2009, at 7:28 AM, Manlio Perillo wrote:
> The Unicode Standard (version 4.0, section 3.9, D31 - pag 76) says:
> 
> """Because surrogate code points are not included in the set of Unicode
> scalar values, UTF-32 code units in the range 0000D800 .. 0000DFFF are
> ill-formed"""

The current version of Unicode is 5.1. This text is now in D90, though otherwise the same. My references below are to the 5.1 documents (freely available on line at: http://www.unicode.org/versions/Unicode5.1.0/ )

> However GHC does not reject this code units:
> 
> Prelude> print '\x0000D800'
> '\55296'
> 
> Is this a correct behaviour?

I don't think you should consider Char to be UTF-32.

Think of Char as representing a Unicode code point. Unicode code points are defined as all in integers in the range \x0 through \x10FFFF, inclusive. Values in the range \xD800 through \xDFFF are all valid code points. (§2.4 in general; §3.4, D9, D10)

Not all Unicode code points are "Unicode scalar values". Only Unicode scalar values can be encoded in the standard Unicode encodings. Unicode scalar values are defined a \x0 through \xD7FF and \xE000 through \x10FFFF - All code points except the surrogate pair area. (§3.9, D76)

Not all code points are characters. In particular, \xFFFE, \xFFFF are "Noncharacters": They are representable in Unicode encodings, but should not be interchanged.  Less well known is the range \xFDD0 though \xFDEF which are also noncharacters. (§2.4, Table 2-3; §3.4, D14, §16.7)

In particular, note the stance of Unicode toward application internal forms:
	"Applications are free to use any of these noncharacter code points.
	internally but should never attempt to exchange them." - §16.7 ¶3

Accordingly, it is fine for Haskell's Char to support these values, as they are code points. The place to impose any special handling is in Haskell's various Unicode encoding libraries: When decoding, code points \xD800 through \xDFFF cannot be received, and noncharacters can be either retained or silently dropped (Unicode conformance allows this.) When encoding, code points \xD800 through \xDFFF and noncharacters should either error or just be silently dropped.

	- Mark

Mark Lentczner
http://www.ozonehouse.com/mark/
mark at glyphic.com