I had to instantiate the "map" function explicitly, and couldn't use "cons?" because it returns whether something is a Pair, not a Listof - but the following works:<div><div><br></div><div><font face="'courier new', monospace">(: cars : (All (A) (Listof (Listof A)) -> (Listof A)))</font></div> <div><font face="'courier new', monospace">(define (cars ls)</font></div><div><font face="'courier new', monospace"> ((inst map A (Listof A)) car (filter (λ (x) (not (empty? x))) ls)))</font></div><div><br> </div><div><div><font face="'courier new', monospace">> (cars (list (list 1 2 3 4) (list 6 2 3 4)))</font></div><div><font face="'courier new', monospace">- : (Listof Positive-Byte)</font></div><div><font face="'courier new', monospace">'(1 6)</font></div> </div><div><br><div class="gmail_quote">On Wed, Apr 4, 2012 at 4:17 PM, Eric Tanter <span dir="ltr"><<a href="mailto:etanter@dcc.uchile.cl">etanter@dcc.uchile.cl</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Hi,<br> <br> Is there a way to get this to typecheck?<br> <br> (: cars : (All (A) (Listof (Listof A)) -> (Listof A)))<br> (define (cars ls)<br> (map car (filter cons? ls)))<br> <br> By looking at the error, it seems the problem is that it is impossible to map car on a list of lists:<br> > (map car '((1)))<br> Type Checker: Polymorphic function map could not be applied to arguments...<br> <br> <br> Thanks,<br> <br> -- Éric<br> <br> ____________________<br> Racket Users list:<br> <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a><br> </blockquote></div><br></div></div>