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  2. logo
    1. Introduction
    2. Usage:
    3. Sample Interaction
    4. Defined Logo Primitives
      1. Math
      2. Infix
      3. Predicates
      4. Logical Connectives
      5. Symbolic Processing
      6. Control
      7. State
      8. I/O
      9. Property Lists
      10. Special Form
    5. Author
    6. License

Introduction

This Scheme code is a small but serious implementation of Logo. It is inspired by Brian Harvey's UCB Logo. No attempt at optimization has been made. There are many optimizations that are easily made; however, the implementation is not properly tail recursive and adding that optimization would be non-trivial. In general, making a dynamically scoped language properly tail recursive is tricky, and even experts have goofed. See Darius Bacon's note for a discussion of how to implement both dynamic scope and proper tail recursion.

While the syntax and semantics are very similar to UCB Logo, there are some small differences. For example, no line continuation indicators are necessary; carriage returns can be used to format the input. To cause the input to be evaluated, press return on a blank line. Also, the infix operations are not self delimiting. Thus 2+3 does not evaluate to a number. Finally, there is no special mode for entering procedures. They can be typed in on one line or many using either the To special form or the Define command.

Usage:

 
[procedure] (go-logo)

Starts an interactive Logo session.

Sample Interaction

 
Welcome to Arthur Nunes-Harwitt's Classic Logo
?to downup :word
  print :word
  if equalp count :word 1 [stop]
  downup bl :word
  print :word
end

?downup "Hello

Hello
Hell
Hel
He
H
He
Hel
Hell
Hello
?print 2+3i + 4

6+3i
?print count "Hello

5
?to fact :n
  if :n = 0 [op 1]
  op :n * fact :n - 1
end

?print fact 5

120
?to fib :n
  if :n < 2 [op 1]
  op (fib :n - 1) + (fib :n - 2)
end

?print fib 4

5
?print fib 5

8
?print fib 6

13
?make "x 0

?print :x

0
?repeat 720 [make "x :x + 1]

?print :x

720

Defined Logo Primitives

Math
Sum Difference Minus Product Quotient
Power Remainder Int Round Abs
Sqrt Exp ln Log10 Sin
Cos ArcTan RadSin RadCos RadArcTan
Float Numerator Denominator RealPart ImagPart

Infix
< = > +
- * / ^

Predicates
GreaterP LessP EqualP IntegerP
FloatP RatioP ComplexP ListP
WordP NumberP EmptyP MemberP
ProcedureP PrimitiveP DefinedP NameP

Logical Connectives
Not And Or

Symbolic Processing
First Butfirst BF Fput
Sentence SE List Last
Butlast BL Lput Count
Item Ascii Char Word

Control
If IfElse Repeat
Output OP Stop
Catch Throw Run

State
Make Define Text

I/O
Print Show Type Readlist RL

Property Lists
GProp RemProp PProp PList

Special Form

To name var* command* end

Author

Arthur Nunes-Harwitt

License

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/2.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.