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CHAPTER 4. SCHEME SERVLETS

Exercise 4 Modify the date.servlet example above to include more sophisticated

HTML and CSS, then visit the page and hit the refresh button several times. You

should notice that the time changes each time you hit the refresh button.

Exercise 5 Write a servlet that displays a random number using

[(Math.random)]

This scheme expression generates a random decimal number between 0.0 and

1.0. If you want a whole number between say 0 and 100, then you can use the

following expression instead:

[(round (* 100 (Math.random)))]

By multiplying the random number by 100, we get a decimal number between 0.0

and 100.0. The round operator then rounds that number to the nearest whole

number in the range 0 to 100.

Exercise 6 What is the result of evaluating the following Scheme expressions:

{ 3 + 4 = [(+ 3 4)] }

{ [(* 3 3)]   [(+ 2 5)] = [(/ 4 2)] }

{ A day contains [(* 24 60 60)] seconds!}

As mentioned above, the curly braces in a servlet {} indicate that the en 

closed text is to be sent verbatim to the client, except that text enclosed in

square braces

is first evaluated to get some  interesting  value which is then inserted into the

text. This curly brace/square brace notation is called quasi string notation. It

is another example of a Scheme expression.

4.5

Quasi strings and string append

The general form of a quasi string expression is as follows:

{...text...[A].....

...text...[B].....

....

...text...[C].....}

The rule for evaluating such an expression is to first evaluate the embedded

Scheme expressions A, B, ..., C, and then to insert the values one obtains in the

corresponding places in the string. There is a Scheme function string append

which can be used to achieve a similar effect, e.g. instead of writing:

{the sum of 5 and 7 is [(+ 5 7)] the difference is [(  5 7)]}

one could write