20. Posn Util

The functions in 20.5 are all useful later. When you do them, you should save them in a file called posn-util.rkt. (Please use that name.)

The following functions should be in posn-util.rkt:

• posn=?: posn posn -> boolean. Are both positions exactly the same? (Both corresponding coordinates match.)
• add-posns: posn posn -> posn. Add corresponding coordinates.
• sub-posns: posn posn -> posn. Subtract corresponding coordinates. First minus second.
• scale-posn: number posn -> posn. Multiply both coordinates by the same number.
• distance: posn posn -> number. The distance in the plane between the points specified by the two posns.
• place-image/posn: image posn image(background) -> image. Purpose: Place one image at a given set of coordinates on a given background.
• Anything else you want to have available for re-use. (Maybe from physics or trig…)

You should write tests for yourself. You are going to use these functions for a month, so you do not want any mistakes. If you really need them, you can see some tests I wrote.

Require and provide

Require loads functions from another file. Provide makes functions available for other files to load.

Provide

The provide command makes functions available for other files to use. One way to use it is to list all of the functions you want to allow other files to use:

(provide add-posns sub-posns scale-posn distance)

Sometimes you just want to make every function in the file available. There is a shortcut to do that:

(provide (all-defined-out))

Require

In order to use the functions in another file, you need to require that file. You already know (require picturing-programs) but when we use require we need to give the whole filename.

(require "posn-util.rkt")