There are two questions in this Guided Practice. The first is an easy exercise using the alternating lists as we have defined them. The second is intended to give you more insight into the nature of data definitions.

**Question 1:** Design a function `lasn-number-of-strings` that takes a `lasn` and returns the number of strings in the **lasn**.

**Question 2:** Our data definitions for lasn and lans
make two assumptions:

- We care equally about lasn's and lans's.
- There is no relationship between the numbers and the strings in any particular lans or lasn.

If our intended application violates either of those assumptions, then we need to go back and re-do our data analysis accordingly.

For example, consider the following problem: Given an alternating
list of strings and numbers, starting with a string, return
`true` if each number is equal to the length of the string
preceding it, and `false` otherwise. For example:

(begin-for-test (check-equal? (right-lengths? empty) true) (check-equal? (right-lengths? (list "foo" 3 "april" 5 "george" 6)) true) (check-equal? (right-lengths? (list "foo" 3 "april" 4 "george" 6)) false) (check-error (right-lengths? (list 3 "april" 4 "george" 6)) "this should raise an error, because the argument is not a lasn"))

(`check-error`, defined in extras.rkt, passes if and only if its
argument raises an error.)

Here both our assumptions are violated: we are only interested in LASNs, not LANSs, and we are very much interested in the relation between the strings and the numbers.

There's no obvious way to write this function using the tools we have and the data definition we gave in this lesson.

To represent the data in these examples, we might write the following data definition:

;; A LASI is one of: ;; -- empty ;; -- (cons String (cons Integer LASI))

Here we are calling these LASI's so there will be no confusion with LASN's.

Write out the template for LASI's, using this data definition, and
write `right-lengths?` . Follow the template recipe.
Notice that `(cons String (cons Integer LASI))` is a
"structured value."

Last modified: Fri Sep 4 11:50:26 Eastern Daylight Time 2015