Python has this module called turtle. LOGO is a programming language, to draw with turtle graphics. This repo implements a compiler and runtime for LOGO language using python and its turtle module.

The syntax may not be completely compatible with the original, as I have added certain features while removed certain original features. This is just a fun project for the Turtle Graphics with REPL for the UI.

# This is a comment, comments should be in fresh line only
# RT is for right turn, 90 is the angle to turn right
RT 90
# FD is for forward, 100 is length to move forward
FD 100

# The REPEAT construct, is for looping
# The number 4, instructs to loop 4 times
REPEAT 4 [
  RT 90
  FD 100
]

# The TO construct, is to define procedures
TO SQUARE [
  REPEAT 4 [
    RT 90
    FD 100
  ]
]

# In the above example, the proceduce defines 
# how to draw a square. But this useless if
# we want to draw a square of some size

# :FOO is a variable, while 100 is a literal
# There are no data types, expect FLOAT or INT

# To draw a square of some size, say :SIDE
TO SQUARE :SIDE [
  REPEAT 4 [
    RT 90
    FD :SIDE
  ]
]

# Inside the REPEAT construct, at each loop iteration
# The is an inbuild variable called :REPCOUNT that
# will be available and automatically updated each iteration.

REPEAT 6 [
  RT :REPCOUNT * 60
]

# The above code is same as 
RT 1 * 60
RT 2 * 60
RT 3 * 60
RT 4 * 60
RT 5 * 60
RT 6 * 60

# For simplicity, there are is no IF-THEN-ELSE construct
# There are neither variable assignments, nor booleans

# But, there are expressions which can be placed
# as arguments in place of literals and variables
# Expressions can contain variables and literals, 
# which will be dynamically evaluated.

# Example for expressions, in argument
FD SQRT(100 * :SIDE + :SIDE)

# note the variable such as :SIDE here should be used
# only inside as Procedure, It is best to enclose expressions
# in brackets and also, enclose brackets to be clear about
# the order of evaluation

All the basic arithmetic operators such as +, -, *, /, %, ^ are supported. % is modulus and ^ exponentiation. Some inbuild math function are SQRT(a), MIN(a, b), MAX(a, b), RAND(a, b).

Steps to use this repo

• Download or clone this repo
• Install cmd2 python module with python -m pip install cmd2.
• To run the program, do cd /path/to/root/of/this/project and type python logo/runtime/repl.py

The prompt pops up logo[{count}]:>

There is a sample code in samples/polygon.logo.

• To load and compile this code, in the REPL execute load samples/polygon.logo
• Type ast to see the compiled code
• Type Hexagon 100, this draws hexagon of size 100
• Type RESET or CLEAR or HOME, to recenter, clear or move turtle to home position
• Type REPEAT 5 [ FD 100; RT (360 / 5) ] to draw a pentagon
• Type up arrow to see prev commands.
• Type SPEED 0 to set speed to fastest, or 1 to 10 to be progressively faster.
• Type exit or quit to quit.

Don't forget to try FLOWER :PETALS :SIZE to draw a flower, actually you seed to substitute values, for examples FLOWER 30 100 or FLOWER 90 100.

And note, the language is case insensitive, but capitals are prefered. So, FLOWER or Flower or flower are same. But the load, hepl, quit and exit should be in lower case. They are REPL command, and not executed by the runtime.

For some adventure, try modifying the code in the sample file polygon.logo and reload it. To reload, just type load samples/polygon.logo, same as loading.

Try flower 30 100

Try repeat 5 [ polygon 100 5; rt 72 ]

Happy Turtling ...

• Add support for IF-THEN-ELSE construct
• Add Boolean data type
• Add Boolean expressions and relational expressions
• Add Assignments of expressions to variables
• Add Recursion of Procedures