Numbers in x-expressions are interpreted as XML entities, not as the string
representation of the number. The value of (xexpr->string '(html 1)), to
use your example, is "<html></html>". The 1 represents the character
that Racket would represent as #\u0001, i.e. the value of (integer->char
1). In contrast, (char->integer #\1) produces 49.
On Mon, Mar 13, 2017 at 8:38 PM, Marc Kaufmann <marc.kaufman...@gmail.com>
> I am creating matrices of 0s and 1s that I display in HTML-tables and
> somewhat surprisingly I found out that 0s are not permissible in
> X-expressions, while 1s are:
> (require web-server/http)
> (response/xexpr '(html 1)) ; Fine, no trouble.
> (response/xexpr '(html 0)) ; Blow-up.
> The specific violation is the following:
> "response/xexpr: contract violation;
> Not an Xexpr. Expected a string, symbol, valid numeric entity, comment,
> processing instruction, or list, given 0"
> After some digging around, it turns out that only numbers that satisfy
> valid-char? are acceptable, which means "exact-nonnegative-integer whose
> character interpretation under UTF-8 is from the set ([#x1-#xD7FF] |
> [#xE000-#xFFFD] | [#x10000-#x10FFFF]), in accordance with section 2.2 of
> the XML 1.1 spec."
> First, should 0 really not be accepted? (I am not going to try and figure
> out the UTF-8 interpretation...) And second, is the reason that negative
> numbers are not acceptable that they are not under the XML spec above? This
> took me by surprise and means I have to put number->string in a bunch of
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