1 Textual Language

Racket

1 Textual Language🔗ℹ

Here’s a program for a fish that swims randomly:

1.1 Example🔗ℹ

#lang scratchy

-------------------------------------
fish

image is @fish-image

do
forever {
   wait 0.02
   forward by 2
   turn by (random 5) - 2
}

The ---...- starts a sprite declaration, where the sprite is named fish. In addition, if the file name is "fish-land.rkt", then fish-land is defined as the land where fish starts.

1.2 Grammar🔗ℹ

Here’s the grammar of this textual language (click on a keyword for more information):

‹prog›	::=	‹use›* ‹sprite›*
‹use›	::=	use ‹id›
‹sprite›	::=	---...- ‹id› ‹clause›*
‹clause›	::=	image is ‹expr›
	\|	x is ‹expr›
	\|	y is ‹expr›
	\|	size is ‹expr›
	\|	direction is ‹expr›
	\|	on ‹key› key ‹statement›*
	\|	on ‹string› message ‹statement›*
	\|	variable ‹id› is ‹expr›
	\|	do ‹statement›*
‹expr›	::=	‹number› \| ‹image› \| ‹string›
	\|	‹id›
	\|	‹expr› ‹binary-op› ‹expr›
	\|	‹unary-op› ‹expr›
	\|	random ‹expr›
	\|	touches ‹expr›
	\|	( ‹expr› )
‹statement›	::=	move x by ‹expr›
	\|	move y by ‹expr›
	\|	move x to ‹expr›
	\|	move y to ‹expr›
	\|	turn by ‹expr›
	\|	turn to ‹expr›
	\|	forward by ‹expr›
	\|	change size by ‹expr›
	\|	change size to ‹expr›
	\|	wait ‹expr›
	\|	say ‹expr›
	\|	hush
	\|	hide
	\|	show
	\|	change image to ‹expr›
	\|	send ‹expr› to ‹expr›
	\|	send ‹expr› to everyone
	\|	watch ‹expr›
	\|	move to ‹expr›
	\|	forever { ‹statement›* }
	\|	while ‹expr› { ‹statement›* }
	\|	if ‹expr› { ‹statement›* }
	\|	‹id› = ‹expr›
‹unary-op›	::=	-
	\|	+
‹binary-op›	::=	+ \| - \| * \| / \| < \| > \| <= \| >= \| =
‹key›	::=	‹id› \| + \| - \| * \| / \| < \| > \| =
‹id›	::=	a letter (in a/A to z/Z) followed by letters, numbers, and _s
	\|	@ followed by a sequence of letters, numbers, _s, and -s
‹number›	::=	a decimal number
‹string›	::=	sequence of characters between "s

1.3 Syntactic Forms🔗ℹ

syntax
use ‹id›

Imports the sprite, variable, and land name of a land defined in the file whose name matches ‹id› with a ".rkt" suffix.

syntax
---...-

Starts a sprite declaration. Any number of -s can appear, as long as there are at least three.

syntax
image is ‹expr›

Determines the initial image for a sprite.

syntax
x is ‹expr›
syntax
y is ‹expr›

Determines the initial placement of a sprite.

syntax
size is ‹expr›

Determines the initial size of a sprite, as a multiple of its image’s size.

syntax
direction is ‹expr›

Determines the initial direction of a sprite, in degrees where 0 is north, 90 is east, 180 is south, and 270 is west.

syntax
on ‹key› key ‹statement›*
on ‹string› message ‹statement›*

Declares actions to take when a key is pressed or a message is sent or broadcast.

syntax
variable ‹id› is ‹expr›

Declares a variable, which is visible in the whole land.

syntax
do ‹statement›*

Starts a concurrent task.

syntax
random ‹expr›

Generates a random number between 0 and one less than the integer produced by ‹expr›.

syntax
touches ‹expr›

Produces true if this sprite is touching the one named by ‹expr›.

syntax
turn by ‹expr›
turn to ‹expr›

Changes the sprite’s direction either by a number of degrees clockwise or to a number of degrees like direction.

syntax
move x by ‹expr›
move y by ‹expr›
move x to ‹expr›
move y to ‹expr›
move to ‹expr›

Changes the sprite’s x or y location either by a number of pixels left/up or to a number of pixels from the screen’s center. If neither x or y is indicated, the sprite instead moves to the specified land (keeping its relative position from the current land).

syntax
forward by ‹expr›

Moves the sprite in its current direction by the specified number of pixels.

syntax
change size by ‹expr›
change size to ‹expr›
change image to ‹expr›

Changes the sprite’s size either by an amount to add to its current size multiplier or to a multiple of its image’s size, or sets the sprite image to a new image.

syntax
wait ‹expr›

Pauses a task for the specified number of seconds.

syntax
say ‹expr›

Causes the sprite to have a speech bubble with the specified content.

syntax
hush

Removes the sprite’s speech bubble, if any.

syntax
hide
syntax
show

Hides or shows the sprite.

syntax
send ‹expr› to ‹expr›
send ‹expr› to everyone

Sends a message to a specific sprite or to all sprites in the land. A sprite can respond to the message using an on ‹string› message clause.

syntax
watch ‹expr›

Switches the Scratchy view to the specified land.

syntax
forever { ‹statement›* }

Repeats the ‹statement›s forever.

syntax
while ‹expr› { ‹statement›* }

Repeats the ‹statement›s as long as ‹expr› produces true.

syntax
if ‹expr› { ‹statement›* }

Performs the ‹statement›s only if ‹expr› produces true.

syntax
is
syntax
to
syntax
by
syntax
key
syntax
message
syntax
everyone

Keywords that are combined with many others.