2 Value Turtles

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2 Value Turtles

package: htdp-lib

The value turtles are a variation on traditional turtles. Rather than having just a single window where each operation changes the state of that window, in the graphics/value-turtles library, the entire turtles window is treated as a value. This means that each of the primitive operations accepts, in addition to the usual arguments, a turtles-window value; instead of returning nothing, each returns a turtles-window value.

procedure
(turtles width
height
[ init-x
init-y
init-angle]) → turtles?
  width : real?
  height : real?
  init-x : real? = (/ width 2)
  init-y : real? = (/ height 2)
  init-angle : real? = 0

Creates a new turtles window with the given width and height. The remaining arguments specify position of the initial turtle and the direction in radians (where 0 is to the right).

procedure
(turtles? v) → boolean?
v : any/c

Determines if v is a turtles drawing.

procedure
(move n turtles) → turtles?
n : real?
turtles : turtles?

Moves the turtle n pixels, returning a new turtles window.

procedure
(draw n turtles) → turtles?
n : real?
turtles : turtles?

Moves the turtle n pixels and draws a line along the path, returning a new turtles window.

procedure
(erase n turtles) → turtles?
n : real?
turtles : turtles?

Moves the turtle n pixels and erases a line along the path, returning a new turtles window.

procedure
(move-offset h v turtles) → turtles?
  h : real?
  v : real?
  turtles : turtles?
procedure
(draw-offset h v turtles) → turtles?
  h : real?
  v : real?
  turtles : turtles?
procedure
(erase-offset h v turtles) → turtles?
  h : real?
  v : real?
  turtles : turtles?

Like move, draw, and erase, but using a horizontal and vertical offset from the turtle’s current position.

procedure
(turn theta turtles) → turtles?
theta : real?
turtles : turtles?

Turns the turtle theta degrees counter-clockwise, returning a new turtles window.

procedure
(turn/radians theta turtles) → turtles?
theta : real?
turtles : turtles?

Turns the turtle theta radians counter-clockwise, returning a new turtles window.

procedure
(merge turtles1 turtles2) → turtles?
turtles1 : turtles?
turtles2 : turtles?

The split and tprompt forms provided by graphics/turtles aren’t needed for graphics/value-turtles, since the turtles window is a value.

Instead, the merge accepts two turtles windows and combines the state of the two turtles windows into a single window. The new window contains all of the turtles of the previous two windows, but only the line drawings of the first turtles argument.

procedure
(clean turtles) → turtles?
turtles : turtles?

Produces a turtles like turtles, but with only a single turtle, positioned in the center.

2.1 Examples

(require graphics/value-turtles-examples)
	package: htdp-lib

The graphics/turtle-examples library’s source is meant to be read, but it also exports the following examples.

procedure
(radial-turtles n turtles) → turtles?
n : exact-nonnegative-integer?
turtles : turtles?

Places 2n turtles spaced evenly pointing radially outward.

procedure
(spaced-turtles n turtles) → turtles?
n : exact-nonnegative-integer?
turtles : turtles?

Places 2n turtles evenly spaced in a line and pointing in the same direction as the original turtle.

procedure
(neato turtles) → turtles?
turtles : turtles?

As the name says...

procedure
(regular-poly sides radius turtles) → turtles?
  sides : exact-nonnegative-integer?
  radius : real?
  turtles : turtles?

Draws a regular poly centered at the turtle with sides sides and with radius radius.

procedure
(regular-polys n s turtles) → turtles?
  n : exact-nonnegative-integer?
  s : any/c
  turtles : turtles?

Draws n regular polys each with n sides centered at the turtle.

procedure
(spokes turtles) → turtles?
turtles : turtles?

Draws some spokes, using radial-turtles and spaced-turtles.

procedure
(spyro-gyra turtles) → turtles?
turtles : turtles?

Draws a spyro-grya reminiscent shape.

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