Loko

Loko is an interactive interpreter for the Logo programming language. Logo is best known for beginner-oriented “Turtle Graphics” functionality, but internally the language is actually a dynamically-scoped member of the Lisp family. As such, Logo provides sophisticated list manipulation facilities and support for higher-order functions.

All values in Logo are either signed integers, words or lists. Words can contain alphanumeric characters as well as .,?!, and when a word is encountered it is executed. If a word is prefaced with a ', it instead represents the name of the word. If a word is prefaced with a :, it is a variable, and represents the value associated with the given name. Lists are enclosed in square brackets and none of the words within a list are executed immediately.

Logo expressions are given in prefix-notation, with the arguments to the right of the operators. Several operators (+-*/%<>=) also have an optional infix syntax. Infix expressions must always be separated from prefix expressions by parentheses.

Turtle Graphics

At startup, Loko awaits instructions in a scrolling command-line mode. The showturtle command switches to Turtle Graphics mode and hideturtle switches back. Turtle Graphics centers around manipulating the triangular object on the screen (the turtle) by instructing it to move or turn. As the turtle moves around the screen, it draws a line- there are commands for disabling the pen or changing the color it draws with. Instructions can be entered individually or together on a single line:

showturtle
forward 100
right 75 forward 50 right 30 forward 20

• showturtle enable turtle graphics.
• hideturtle disable turtle graphics.
• forward or fd move the turtle forward by a specified number of pixels.
• back or bk move the turtle backward by a specified number of pixels.
• right or rt turn the turtle right by a specified number of degrees.
• left or lt turn the turtle left by a specified number of degrees.
• clear or cs clear the screen.
• home move the turtle to the center of the screen.
• heading return the angle of the turtle in degrees.
• setheading or seth set the angle of the turtle in degrees.
• xcor return the x position of the turtle.
• ycor return the y position of the turtle.
• pos return a list containing the turtle’s coordinates.
• setx set the x position of the turtle.
• sety set the y position of the turtle.
• setpos given a list of coordinates, set the turtle’s position.
• penup or pu tell the turtle to stop drawing a line as it moves.
• pendown or pd resume drawing a line as the turtle moves.
• setcolor given list of 3 numbers between 0 and 255, set the pen’s RGB color.
• lineto given X and Y coordinates, draw a line from the turtle to an absolute position.

Defining Procedures

Procedures are defined by using the to keyword, specifying a series of arguments, and a procedure body, terminated by end. When a line beginning with to is entered at the Loko command prompt, the line editor will expand and allow the procedure to be freely edited with the keyboard and cursor keys. If return is pressed with the last line of this editor selected, the procedure will be entered and further commands can be issued. Previously entered procedures can be modified by using the edit command and specifying a word corresponding to a procedure name.

Here’s an example procedure calculating the maximum of two values:

to maximum :a :b
	if less? :a :b [ output :b ]
	output :a
end

Flow Control and I/O

Logo provides a minimal set of flow control primitives:

• if takes a boolean expression and a list. If the expression is 'true, the list will be executed.
• unless is a counterpart to if which executes the list if the expression is not 'true.
• repeat takes a number and a list. The list is executed 0 or more times based on the number.
• run takes a list and executes it. If this list is bound to arguments (see bind) it will consume additional arguments when run is invoked.
• stop exits the currently executing procedure and returns no value.
• output is a counterpoint to stop which returns an expression.

Logo also provides primitives for reading from and printing to the command console:

• print or pr takes a word, number or list and prints it. Lists are printed in a “flat” form ignoring any brackets or :' prefixes.
• printlist or pl functions like print but preserves the structure of arguments.
• readlist prompts the user to enter text, which is parsed as a Logo list (an outer set of square brackets are implied.)

Math

• negate returns the negation of a number.
• random returns a random integer from 0 up to but not including a given number.
• equal? or infix = returns 'true if both arguments are the same. Works for numbers, words or lists (which are compared recursively).
• less? or infix < returns 'true if the first number is less than the second.
• greater? or infix > returns 'true if the first number is greater than the second.
• sum or infix + returns the sum of two numbers.
• difference or infix - returns the difference of two numbers.
• product or infix * returns the product of two numbers.
• quotient or infix / returns the quotient of dividing two numbers.
• remainder or infix % returns the remainder of dividing two numbers.

Type Predicates

To distinguish between value types, Logo provides a set of predicates. Just like the comparison operators, these routines return either 'true or 'false as a word.

• num? takes a value and returns 'true if it is a number.
• word? takes a value and returns 'true if it is a word.
• list? takes a value and returns 'true if it is a list.
• empty? takes a value and returns 'true if it is an empty list ([]).

List Operations

• first returns the first element of a list
• last returns the last element of a list
• butfirst or bf returns everything after the first element of a list
• butlast or bl returns everything before the last element of a list
• size returns the number of elements in a list.
• item takes an 0-based index followed by a list and extracts an element from that list, or returns an empty list if the index is out of range.
• list takes two values and returns a list containing them.
• member takes a value and a list, and returns a subsequence of the list starting with any instance of the given value, or an empty list if the original list did not contain the value.
• flatten takes a list which may contain sublists and flattens all the elements recursively into a single list.

All list operations are functional- that is, they return a new list rather than altering an existing list in-place:

• fput takes a value and a list and adds the value to the beginning of the list.
• lput takes a value and a list and adds the value to the end of the list.

Misc

• make takes a name and a value, and gives the specified name the specified value in the global scope, or if the name is already defined in the most recent definition’s scope.
• local takes a name and a value and gives the specified name the specified value in a local scope- that is, within the current function declaration.
• bind takes two lists. The first should be a list of names, while the second is an executable list. bind attaches the first list as an arglist for the second. This is roughly equivalent to a Lisp lambda.
• args takes a list and returns the arglist associated with it or an empty list. Essentially the other half of bind.
• thing takes a name, and yields the value associated with that name. This works like : on a variable name, but can be chained to follow arbitrary degrees of indirect reference.
• words prints out a list of all globally-defined names.
• trace prints out a stack trace of the currently executing program, including all locally bound arguments and values.
• free performs garbage collection and prints out how many words of heap space are still free.

Examples

Recursively Draw a Spiral:

to spiral :x
	if equal? :x 0 [stop]
	fd :x rt 45
	spiral difference :x 1
end

Novel Control Structures:

to forever :proc
	run :proc
	forever :proc
end

to while :cond :proc
	unless run :cond [ stop ]
	run :proc
	while :cond :proc
end

A Push-Down Stack:

to push :stack :value
	make :stack fput :value thing :stack
end

to pop :stack
	local  'top      first thing :stack
	make   :stack butfirst thing :stack
	output :top
end

Recursive Tree:

to branch :a :b :depth
	fd 10
	tree difference :depth 1
	setheading :a
	setpos     :b
end

to tree :depth
	if equal? :depth 0 [stop]
	local 'dir   heading
	local 'place pos
	lt 15 branch :dir :place :depth
	rt 15 branch :dir :place :depth
end

Dragon Curve:

to x :c
	if (:c = 0) [ stop ]
	x  (:c - 1)
	right 90
	y  (:c - 1)
	forward 4
end

to y :c
	if (:c = 0) [ stop ]
	forward 4
	x  (:c - 1)
	left  90
	y  (:c - 1)
end

to dragon
	showturtle
	x 11
end

Curses:

to any :list
	output item random size :list :list
end

to noun
	output any [
		[an enraged camel]
		[an ancient philosopher]
		[a cocker spaniel]
		[the Eiffel Tower]
		[a cowardly moose]
		[the silent majority]
	]
end

to verb
	output any [
		[get inspiration from]
		[redecorate]
		[become an obsession of]
		[make a salt lick out of]
		[buy an interest in]
	]
end

to object
	output any [
		[mother in law]
		[psychoanalyst]
		[rumpus room]
		[fern]
		[garage]
		[love letters]
	]
end

to curse
	pr	fput 'May
		fput noun
		fput verb
		fput 'your
		list object '!
end