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CHAPTER 4. SCHEME SERVLETS

4.7

The case expression

The echo servlet will behave poorly however if you don't give it any parameter.

It will usually just cause the browser to hang. To rectify this problem, we can

use the following nicer echo servlet:

; niceEcho.servlet

(servlet (a)

(case a

((#null){Please add "?a=Hi" to the URL and hit return})

(("Hi") {Good job. Now try "?a=Hello"})

(("Hello") {Great. Now try "?a=ANYTHING"})

(("ANYTHING") {OK, OK. Now try something more original})

(("original" "something") {Are you trying to be funny?})

(else {You just typed [a]})))

If one visits this page without an  a  parameter, then the value assigned to  a 

by the servlet will be the special value  #null  and it will return some instruc 

tions about how it should be called. Otherwise, it will look at the value passed

in for  a  and if it matches one of the quoted expressions  Hi ,  Hello  etc. it

will return the corresponding expression. Note that  original  and  something 

both trigger the same response  Are you trying to be funny? . If no match is

found, then the servlet just echoes back what the user entered.

The case expression in general has the form

(case a

((value1) result1)

((value2 value3) result2)

...

(else lastresult))

When the case expression is evaluated, the Scheme system tries to find the first

match of a with one of the values. When a match is found, the corresponding

result expression is evaluated. If no match is found then the else clause

applies and the lastresult expression is evaluated. The niceEcho.servlet

shows how to use the case to support an interaction between the user and the

servlet.

Exercise 7 What is the result of evaluating the following Scheme expressions:

(case "b"

(("a" "c") "hello")

(("b" "d") "goodbye")

(("a" "b") "wow")

(else "darn"))

(case (* 2 2)

((1 3 5 7 9) "odd digit")