(lisppad turtle)

Library (lisppad turtle) implements a simple graphics pane (a graphics window on macOS, a canvas on iOS) for displaying turtle graphics. The library supports one graphics pane per LispPad session which gets initialized by invoking init-turtle. init-turtle will create a new turtle and display its drawing on a graphics pane. If there is already an existing graphics pane with the given title, it will be reused. Turtle graphics can be reset via reset-turtle.

As opposed to library (lispkit draw turtle), this library supports an indicator, i.e. a symbol that shows where the turtle is currently located and whether the pen is up or down. By default, an arrow is used as an indicator. A yellow arrow indicates that the pen is down, a white translucent arrow indicates that the pen is up.

Once init-turtle was called, the following functions can be used to move the turtle across the plane:

• (indicator-on): Show the turtle indicator

• (indicator-off): Hide the turtle indicator

• (pen-up): Lifts the turtle

• (pen-down): Drops the turtle

• (pen-color color): Sets the current color of the turtle

• (pen-size size): Sets the size of the turtle pen

• (home): Moves the turtle back to the origin

• (move x y): Moves the turtle to position (x, y)

• (heading angle): Sets the angle of the turtle (in radians)

• (turn angle): Turns the turtle by the given angle (in radians)

• (left angle): Turn left by the given angle (in radians)

• (right angle): Turn right by the given angle (in radians)

• (forward distance): Moves forward by distance units drawing a line if the pen is down

• (backward distance): Moves backward by distance units drawing a line if the pen is down

• (arc angle radius): Turns the turtle by the given angle (in radians) and draws an arc with radius around the current turtle position.

This library provides a simplified, interactive version of the API provided by library (lispkit draw turtle).

Setup

(init-turtle) (init-turtle scale) (init-turtle scale title)

Initializes a new turtle and displays its drawing in a graphics pane (a graphics window on macOS, a canvas on iOS). init-turtle gets two optional arguments: scale and title. scale is a scaling factor which determines the size of the turtle drawing. title is a string that defines the name of the graphics pane used by the turtle graphics. It also acts as the identify of the turtle graphics pane; i.e. it won't be possible to have two sessions with the same name but a different graphics pane.

(reset-turtle)

Closes the graphics pane and resets the turtle library.

(turtle-window)

Returns the window associated with the current turtle. Returns #f if there is no associated window.

(turtle-drawing)

Returns the drawing associated with the current turtle.

(turtle-indicator)

Returns the turtle indicator that is currently used.

Indicators

Turtle indicators are defined and configured via record <indicator>. Instances are created using make-indicator. empty-indicator is a predefined indicator showing nothing. arrow-indicator is a constructor for creating arrow indicators of different sizes.

indicator-type-tag

Symbol representing the indicator type. The type-for procedure of library (lispkit type) returns this symbol for all indicator objects.

(indicator? obj)

Returns #t if obj is a turtle indicator object; returns #f otherwise.

(make-indicator shape width stroke-color up-color down-color)

Returns a new turtle indicator of given shape and stroke width. stroke-color is the color used to draw the shape, up-color is the color used to fill the shape when the pen is up, and down-color is the color used to fill the shape when the pen is down.

empty-indicator

An indicator which is not drawing anything. This is used to disable indicators fully.

(make-arrow-indicator) (make-arrow-indicator size) (make-arrow-indicator size width) (make-arrow-indicator size width stroke-color) (make-arrow-indicator size width stroke-color up-color) (make-arrow-indicator size width stroke-color up-color down-color)

Returns a new arrow indicator. size is the size of the arrow shape (12 is the default), width is the stroke width used to draw the shape, stroke-color is the color used to draw the shape, up-color is the color used to fill the shape when the pen is up, and down-color is the color used to fill the shape when the pen is down.

Drawing

(indicator-on)

Show the turtle indicator.

(indicator-off)

Hide the turtle indicator.

(pen-up)

Lifts the turtle from the plane. Subsequent forward and backward operations don't lead to lines being drawn. Only the current coordinates are getting updated.

(pen-down)

Drops the turtle onto the plane. Subsequent forward and backward operations will lead to lines being drawn.

(pen-color color)

Sets the drawing color of the turtle to color. color is a color object as defined by library (lispkit draw).

(pen-size size)

Sets the pen size of the turtle to size. The pen size corresponds to the width of lines drawn by forward and backward.

(home)

Moves the turtle to its home position.

(move x y)

Moves the turtle to the position described by the coordinates x and y.

(heading angle)

Sets the heading of the turtle to angle. angle is expressed in terms of degrees.

(turn angle)

Adjusts the heading of the turtle by angle degrees.

(right angle)

Adjusts the heading of the turtle by angle degrees.

(left angle)

Adjusts the heading of the turtle by -angle degrees.

(forward distance)

Moves the turtle forward by distance units drawing a line if the pen is down.

(backward distance)

Moves the turtle backward by distance units drawing a line if the pen is down.

(arc angle radius)

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.