# (lispkit draw turtle) Library `(lispkit draw turtle)` defines a simple "turtle graphics" API. The API provides functionality for creating turtles and for moving turtles on a plane generating *drawings* as a side-effect. A *drawing* is a data structure defined by library `(lispkit draw)`. A *turtle* is defined in terms of the following components: * A position *(x, y)* defining the coordinates where the turtle is currently located within a coordinate system defined by parameters used to create the turtle via `make-turtle` * A heading *angle* which defines the direction in degrees into which the turtle is moving * A boolean flag *pen down* which, if set to `#t`, will make the turtle draw lines on the graphics plane when moving. * A *line width* defining the width of lines drawn by the turtle * A *color* defining the color of lines drawn by the turtle * A *drawing* which records the moves of the turtle while the pen is down. Turtles are mutable objects created via `make-turtle`. The functions listed below change the state of a turtle. In particular, they generate a drawing as a side-effect which can be accessed via `turtle-drawing`. For most functions, the turtle is an optional argument. If it is not provided, the function applies to the turtle provided by the `current-turtle` parammeter object. **current-turtle** Defines the *current turtle*, which is used as a default by all functions for which the turtle argument is optional. If there is no current turtle, this parameter is set to `#f`. **(turtle? *****obj*****)** Returns `#t` if *obj* is a turtle. Otherwise, it returns `#f`. **(make-turtle *****x y scale*****)** Returns a new turtle object. *x* and *y* determine the "home point" of the turtle. This is equivalent to the zero point of the coordinate system in which the turtle navigates. *scale* is a scaling factor. **(turtle-drawing *****turtle*****)** Returns the drawing associated with the given *turtle*. **(turtle-x *****turtle*****)** Returns the x coordinate of the position of the given *turtle*. **(turtle-y *****turtle*****)** Returns the y coordinate of the position of the given *turtle*. **(turtle-angle *****turtle*****)** Returns the current angle of the given *turtle*. **(turtle-angle *****turtle*****)** Returns the current angle of the given *turtle*. **(turtle-pen-down? *****turtle*****)** Returns `#t` if the given *turtle* has its pen down; otherwise `#f` is returned. **(pen-up)** \ \&#xNAN;**(pen-up *****turtle*****)** Lifts *turtle* from the plane. If *turtle* is not provided, the turtle defined by `current-turtle` is used. Subsequent `forward` and `backward` operations don't lead to lines being drawn. Only the current coordinates are getting updated. **(pen-down)** \ \&#xNAN;**(pen-down *****turtle*****)** Drops *turtle* onto the plane. If *turtle* is not provided, the turtle defined by `current-turtle` is used. Subsequent `forward` and `backward` operations will lead to lines being drawn. **(pen-color *****color*****)** \ \&#xNAN;**(pen-color *****color turtle*****)** Sets the drawing color of *turtle* to *color*. If *turtle* is not provided, the turtle defined by `current-turtle` is used. *color* is a color object as defined by library `(lispkit draw)`. **(pen-size *****size*****)** \ \&#xNAN;**(pen-size *****size turtle*****)** Sets the pen size of *turtle* to *size*. If *turtle* is not provided, the turtle defined by `current-turtle` is used. The pen size corresponds to the width of lines drawn by `forward` and `backward`. **(home)** \ \&#xNAN;**(home *****turtle*****)** Moves *turtle* to its home position. If *turtle* is not provided, the turtle defined by `current-turtle` is used. **(move *****x y*****)** \ \&#xNAN;**(move *****x y turtle*****)** Moves *turtle* to the position described by the coordinates *x* and *y*. If *turtle* is not provided, the turtle defined by `current-turtle` is used. **(heading *****angle*****)** \ \&#xNAN;**(heading *****angle turtle*****)** Sets the heading of *turtle* to *angle*. If *turtle* is not provided, the turtle defined by `current-turtle` is used. *angle* is expressed in terms of degrees. **(turn *****angle*****)** \ \&#xNAN;**(turn *****angle turtle*****)** Adjusts the heading of *turtle* by *angle* degrees. If *turtle* is not provided, the turtle defined by `current-turtle` is used. **(right *****angle*****)** \ \&#xNAN;**(right *****angle turtle*****)** Adjusts the heading of *turtle* by *angle* degrees. If *turtle* is not provided, the turtle defined by `current-turtle` is used. **(left *****angle*****)** \ \&#xNAN;**(left *****angle turtle*****)** Adjusts the heading of *turtle* by *-angle* degrees. If *turtle* is not provided, the turtle defined by `current-turtle` is used. **(forward *****distance*****)** \ \&#xNAN;**(forward *****distance turtle*****)** Moves *turtle* forward by *distance* units drawing a line if the pen is down. If *turtle* is not provided, the turtle defined by `current-turtle` is used. **(backward *****distance*****)** \ \&#xNAN;**(backward *****distance turtle*****)** Moves *turtle* backward by *distance* units drawing a line if the pen is down. If *turtle* is not provided, the turtle defined by `current-turtle` is used. **(arc *****angle radius*****)** \ \&#xNAN;**(arc *****angle radius turtle*****)** Turns the turtle by the given *angle* (in radians) and draws an arc with *radius* around the current turtle position if the pen is down. 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