, takes an expression and puts it in directly<div><br></div><div>`(something ,(list 1 2 3) else) ===> '(something (1 2 3) else)</div><div><br></div><div>,@ takes an expression that evaluates to a list and "splices" it in place</div> <div><br></div><div>`(something ,@(list 1 2 3) else) ===> '(something 1 2 3 else)</div><div><br></div><div>I typically do something like</div><div><br></div><div>`(something ,@(if condition (list answer) empty) else)</div> <div><br></div><div>and even may make a macro out of it</div><div><br></div><div>(define-syntax-rule (when-xexpr cond expr)</div><div> (if cond (list expr) empty))</div><div><br></div><div>Jay<br><br><div class="gmail_quote"> On Fri, Dec 9, 2011 at 11:12 PM, Jordan Schatz <span dir="ltr"><<a href="mailto:jordan@noionlabs.com">jordan@noionlabs.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> I think thats there must be a "lisp" way of doing this, but I'm still too<br> new to see how. Anyone care to enlighten me?<br> <br> I'm trying to create a function that produces an xexpr:<br> <br> (define (custom-input name<br> #:class [class "xlarge"]<br> #:id [id #f]<br> #:size [size 30]<br> #:type [type "text"]<br> #:placeholder [placeholder #f]<br> #:autofocus [autofocus #f]<br> #:label [label #f])<br> `(div ((class "clearfix"))<br> ,(if label<br> `(label ((for ,name)) ,name)<br> '())<br> (div ((class "input"))<br> (input ((name ,name) (class ,class)<br> (size ,(number->string size)) (type ,type)<br> ,(if id<br> `(id ,id)<br> `(id ,name))<br> ,(when placeholder<br> `(placeholder ,placeholder))<br> ,(when autofocus<br> '(autofocus "autofocus")))))))<br> <br> But using "when" produces #<void> like so:<br> <br> (validate-xexpr<br> (custom-input "Company"<br> #:label #t<br> #:placeholder "Your company name (optional)"))<br> <br> > Expected a symbol as the element name, given (name "Company")<br> <br> Substituting an<br> (if condition<br> then-branch<br> empty)<br> for the "when" also produces an invalid xexpr, though I'd rather not use<br> if and have an else branch unless I really wanted it to be there.<br> <br> Thanks,<br> Jordan<br> <br> <br> <br> <br> _________________________________________________<br> For list-related administrative tasks:<br> <a href="http://lists.racket-lang.org/listinfo/users" target="_blank">http://lists.racket-lang.org/listinfo/users</a><br> </blockquote></div><br><br clear="all"><div><br></div>-- <br>Jay McCarthy <<a href="mailto:jay@cs.byu.edu" target="_blank">jay@cs.byu.edu</a>><br>Assistant Professor / Brigham Young University<br><a href="http://faculty.cs.byu.edu/~jay" target="_blank">http://faculty.cs.byu.edu/~jay</a><br> <br>"The glory of God is Intelligence" - D&C 93<br> </div>