Background

• For more detail on modules refer to Chapter 6 Racket Guide.

What to do

In quick we covered the Quick tutorial section on modules We've also been using modules as part of unit testing. In this section of the lab we we will further explore modules, submodules, require and provide.

• create hello.rkt with the following content

    #lang racket
    (define (hello) (displayln "Hello world!"))

• You can test your new module by running (hello)
• Following the racket guide, replace #lang racket with a (module …) form
• add a test

    (check-equal? (with-output-to-string hello) "Hello world!\n")

• check-equal? is defined in the module rackunit, so to get the above to compile we need to (require rackunit). Experiment a bit. Does it matter where you put (require rackunit)?

• For now just add this to the module body, don't create a test submodule. Temporarily replace displayln with display to see a familiar bug caught by our test.

• Make a second file in the same directory, call it application.rkt.

• in application.rkt, require your module "hello.rkt", and call '(hello)'

• This fails, because we didn't export any bindings from "hello.rkt". Add (provide hello). Again, observe that it doesn't matter too much where you put it: order of definition is less important in Racket than in some languages. By convention, both require and provide should be at the top of the module (or submodule).

• Often unit tests can take a noticable amount of time, or require things like network access not necessarily available to an application. To simulate a long running test, use begin and sleep

    (check-equal? (with-output-to-string hello) (begin (sleep 3) "Hello world!\n"))

• Notice now that running "application.rkt" takes a long time. Fix this using the (module+ test …) form we already used (leave the test body as is).

• compare running "application.rkt" and "hello.rkt" from the command line with "racket filename" and "raco test filename".

• We can see that just running "racket hello.rkt" doesn't seem to do much; it can be useful to have a module that acts a library (i.e. is requireded) and also as a program. Add a main submodule so that "racket hello.rkt" also invokes (hello)