Actually numbers are allowed, but not all numbers. If you look at <a href="http://docs.racket-lang.org/xml/index.html?q=xexpression#(def._((lib._xml/main..rkt)._valid-char~3f))">http://docs.racket-lang.org/xml/index.html?q=xexpression#(def._((lib._xml/main..rkt)._valid-char~3f))</a> It tells you which numbers are allowed, and 2 is not. I'll change the error message to say "valid numeric entity" rather than "number". Jay <div class="gmail_quote">On Tue, Feb 28, 2012 at 2:13 AM, Jens Axel Søgaard <<a href="mailto:jensaxel@soegaard.net">jensaxel@soegaard.net</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">2012/2/28 Johannes Brauer <<a style href="mailto:brauer@nordakademie.de" target="_blank">brauer@nordakademie.de</a>> <div class="gmail_quote"><div class="im"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Hi! In DrRacket version 5.02 this x-expression (validate-xexpr '(add x 2)) is legal. In version 5.2 I get the error message Expected a string, symbol, number, comment, processing instruction, or list, given 2 Can anyone explain this to me? </blockquote><div> </div></div><div>According to the grammar in the documentation a lone number is not an x-expression. Use '(add x "2") instead.</div> <table cellspacing="0"><tbody><tr><td style="padding-left:0px;padding-right:0px" valign="baseline" align="right"> xexpr</td> <td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="center">=</td> <td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left">string</td> </tr><tr><td style="padding-left:0px;padding-right:0px" valign="baseline" align="right"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="center">|</td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left">(<a href="http://docs.racket-lang.org/reference/pairs.html?q=xexpression#%28def._%28%28quote._%7E23%7E25kernel%29._list%29%29" style="text-decoration:none;color:blue" target="_blank">list</a> symbol (<a href="http://docs.racket-lang.org/reference/pairs.html?q=xexpression#%28def._%28%28quote._%7E23%7E25kernel%29._list%29%29" style="text-decoration:none;color:blue" target="_blank">list</a> (<a href="http://docs.racket-lang.org/reference/pairs.html?q=xexpression#%28def._%28%28quote._%7E23%7E25kernel%29._list%29%29" style="text-decoration:none;color:blue" target="_blank">list</a> symbol string) ...) xexpr ...)</td> </tr><tr><td style="padding-left:0px;padding-right:0px" valign="baseline" align="right"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="center">|</td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left">(<a href="http://docs.racket-lang.org/reference/pairs.html?q=xexpression#%28def._%28%28quote._%7E23%7E25kernel%29._cons%29%29" style="text-decoration:none;color:blue" target="_blank">cons</a> symbol (<a href="http://docs.racket-lang.org/reference/pairs.html?q=xexpression#%28def._%28%28quote._%7E23%7E25kernel%29._list%29%29" style="text-decoration:none;color:blue" target="_blank">list</a> xexpr ...))</td> </tr><tr><td style="padding-left:0px;padding-right:0px" valign="baseline" align="right"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="center">|</td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left">symbol</td> </tr><tr><td style="padding-left:0px;padding-right:0px" valign="baseline" align="right"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="center">|</td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"><a href="http://docs.racket-lang.org/xml/index.html?q=xexpression#%28def._%28%28lib._xml/main..rkt%29._valid-char%7E3f%29%29" style="text-decoration:none;color:blue" target="_blank">valid-char?</a></td> </tr><tr><td style="padding-left:0px;padding-right:0px" valign="baseline" align="right"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="center">|</td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left">cdata</td> </tr><tr><td style="padding-left:0px;padding-right:0px" valign="baseline" align="right"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="center">|</td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left"> </td><td style="padding-left:0px;padding-right:0px" valign="baseline" align="left">misc</td> </tr></tbody></table><div>-- </div><div>Jens Axel Søgaard</div><div> </div></div> ____________________ Racket Users list: <a style href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> </blockquote></div> -- Jay McCarthy <<a style href="mailto:jay@cs.byu.edu" target="_blank">jay@cs.byu.edu</a>> Assistant Professor / Brigham Young University <a style href="http://faculty.cs.byu.edu/%7Ejay" target="_blank">http://faculty.cs.byu.edu/~jay</a> "The glory of God is Intelligence" - D&C 93