Snake Functions

Draw-handler

• Draw-apple: posn image(background) -> image
• Draw-snake: list-of-posns image(background) -> image

Helper Functions

• Hit-apple? posn(snake-head) posn(apple) -> boolean
• Hit-self?: list-of-posns(body) -> boolean
• Hit-wall?: posn(snake-head) -> boolean
• New-head-position: posn(snake-head) posn(direction) -> posn
• Update-snake: list-of-posns(body) posn(direction) -> list-of-posns. Change the list of posns so the snake moves. Add the direction posn to the first posn in the body. Put that result in front of a list made by dropping the last posn from the list of posns.

Tick handler

• Tick-apple: model -> model. Make the apple vanish/move and snake grow longer when it hits the apple.
• Tick-snake: model -> model. Update snake position.

Key handler

• Key-handler: model key -> model. Straightforward. It is good to add a feature so you can’t go back on yourself.

Update-snake

The snake appears to move by removing the last body segment and adding a new segment in the correct location for the head. You should do this in two separate functions before putting it together.