Thanks ... was show stopper. <div class="gmail_quote">On Wed, Aug 1, 2012 at 1:45 PM, Sam Tobin-Hochstadt <<a href="mailto:samth@ccs.neu.edu" target="_blank">samth@ccs.neu.edu</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">This looks like something that should work -- I'm not sure why TR thinks that `Nothing` is the right type to infer for the second parameter to the `Collection` type. However, changing the last line to: (((inst Collection-append D C) coll) elem)) makes it typecheck. <div><div class="h5"> On Wed, Aug 1, 2012 at 12:17 PM, Ray Racine <<a href="mailto:ray.racine@gmail.com">ray.racine@gmail.com</a>> wrote: > The following gives an error of "Expected Nothing, but got D in elem." > Type checking error? > > #lang typed/racket/base > > (struct: (D C) Collection ([append : (D -> Void)] > [build : (-> C)])) > > (: List-Builder (All (D) -> (Collection D (Listof D)))) > (define (List-Builder) > > (: lst (Listof D)) > (define lst '()) > > (Collection (λ: ((datum : D)) > (set! lst (cons datum lst))) > (λ () lst))) > > (: build-Collection (All (D C) (Collection D C)-> C)) > (define (build-Collection collection) > ((Collection-build collection))) > > (: append-Collection (All (D C) (Collection D C) D -> Void)) > (define (append-Collection coll elem) > ((Collection-append coll) elem)) > > </div></div>> ____________________ > Racket Users list: > <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> > -- sam th <a href="mailto:samth@ccs.neu.edu">samth@ccs.neu.edu</a> </blockquote></div>