On Sep 2, 2014, at 11:45 AM, Daniel Bastos wrote: > A candidate for a solution. > > Exercise 20.1.1. Assume the Definitions window in DrScheme contains > (define (f x) x). Identify the values among the following expressions: > > (1) (cons f empty) > (2) (f f) > (3) (cons f (cons 10 (cons (f 10) empty))) > > Explain why they are values and why the remaining expressions are not > values. > > Solution. First we consider (1). Here's the relevant part of the > grammar. > > <lst> = empty | (cons <val> <lst>) > > <val> = <boo> | <sym> | <num> | empty | <lst> > | <var> (names of defined functions) > | <prm> > > So (1) is a value because it is <lst> of the form (cons <var> empty), > where f is <var> because it is a defined function. > > Let's consider (3) before (2). (3) is a list of either <var> or <num>, > so (3) is <val> as well.
You checked only one half of the value-grammar for lst. Check the other half, too. Report back whether you want to change the answer or not. -- Matthias ____________________ Racket Users list: http://lists.racket-lang.org/users
