net.sourceforge.plantuml.jasic
Class Jasic

java.lang.Object
  net.sourceforge.plantuml.jasic.Jasic

public class Jasic
extends Object

This defines a single class that contains an entire interpreter for a language very similar to the original BASIC. Everything is here (albeit in very simplified form): tokenizing, parsing, and interpretation. The file is organized in phases, with each appearing roughly in the order that they occur when a program is run. You should be able to read this top-down to walk through the entire process of loading and running a program. Jasic language syntax --------------------- Comments start with ' and proceed to the end of the line: print "hi there" ' this is a comment Numbers and strings are supported. Strings should be in "double quotes", and only positive integers can be parsed (though numbers are double internally). Variables are identified by name which must start with a letter and can contain letters or numbers. Case is significant for names and keywords. Each statement is on its own line. Optionally, a line may have a label before the statement. A label is a name that ends with a colon: foo: The following statements are supported: = Evaluates the expression and assigns the result to the given named variable. All variables are globally scoped. pi = (314159 / 10000) print Evaluates the expression and prints the result. print "hello, " + "world" input Reads in a line of input from the user and stores it in the variable with the given name. input guess goto Jumps to the statement after the label with the given name. goto loop if then Evaluates the expression. If it evaluates to a non-zero number, then jumps to the statement after the given label. if a < b then dosomething The following expressions are supported: = Evaluates to 1 if the two expressions are equal, 0 otherwise. + If the left-hand expression is a number, then adds the two expressions, otherwise concatenates the two strings. - * / < > You can figure it out. A name in an expression simply returns the value of the variable with that name. If the variable was never set, it defaults to 0. All binary operators have the same precedence. Sorry, I had to cut corners somewhere. To keep things simple, I've omitted some stuff or hacked things a bit. When possible, I'll leave a "HACK" note there explaining what and why. If you make your own interpreter, you'll want to address those.

Author:

Bob Nystrom

Nested Class Summary

class	Jasic.AssignStatement An assignment statement evaluates an expression and stores the result in a variable.
static interface	Jasic.Expression Base interface for an expression.
class	Jasic.GotoStatement A "goto" statement jumps execution to another place in the program.
class	Jasic.IfThenStatement An if then statement jumps execution to another place in the program, but only if an expression evaluates to something other than 0.
class	Jasic.InputStatement An "input" statement reads input from the user and stores it in a variable.
class	Jasic.NumberValue A numeric value.
class	Jasic.OperatorExpression An operator expression evaluates two expressions and then performs some arithmetic operation on the results.
class	Jasic.PrintStatement A "print" statement evaluates an expression, converts the result to a string, and displays it to the user.
static interface	Jasic.Statement Base interface for a Jasic statement.
class	Jasic.StringValue A string value.
static interface	Jasic.Value This is the base interface for a value.
class	Jasic.VariableExpression A variable expression evaluates to the current value stored in that variable.

Constructor Summary

Jasic()
Constructs a new Jasic instance.

Method Summary

void	interpret(String source) This is where the magic happens.
static void	main(String[] args) Runs the interpreter as a command-line app.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Jasic

public Jasic()

Constructs a new Jasic instance. The instance stores the global state of the interpreter such as the values of all of the variables and the current statement.

Method Detail

main

public static void main(String[] args)

Runs the interpreter as a command-line app. Takes one argument: a path to a script file to load and run. The script should contain one statement per line.

Parameters:

args - Command-line arguments.

interpret

public void interpret(String source)

This is where the magic happens. This runs the code through the parsing pipeline to generate the AST. Then it executes each statement. It keeps track of the current line in a member variable that the statement objects have access to. This lets "goto" and "if then" do flow control by simply setting the index of the current statement. In an interpreter that didn't mix the interpretation logic in with the AST node classes, this would be doing a lot more work.

Parameters:

source - A string containing the source code of a .jas script to interpret.