If your game is working and…
-
both the
DangerandTargetreturn to the screen -
your
Playermoves up and down with the arrow keys
you have all the tools you need to begin this challenge!
; update-danger :: Number ‑> Number
; takes in danger’s x-coordinate and produces the next one
The update-danger function only moves our DANGER left or right… because it doesn’t do anything with the y-coordinate!
Suppose we wanted to write a new function, update-danger-2 that moves the DANGER diagonally…
1 What will have to change about the Domain?
2 What, if anything, will have to change about the Range if we want to get our character moving diagonally?
Since an (x, y) coordinate has two Numbers, one idea might be to write the contract this way:
; update-danger-2 :: Number Number ‑> Number Number
; take in danger’s x- and y-coordinate, and produces the next x- and next y-coordinate
…But that contract breaks an important rule about functions:
Given an input, all functions must produce one output!
We need some way to package two Numbers together into a single value.
Fortunately, our programming language has another data type, called a Posn, which utilizes two Numbers to describe a single "position"!
We can make a Posn to represent the position (100, 200) with the following code: (make-posn 100 200)
3 What expression will make a Posn representing the origin?
4 Write the contract for the make-posn function on the line below.
These materials were developed partly through support of the National Science Foundation,
(awards 1042210, 1535276, 1648684, and 1738598). 
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