Add support for variable environments and declaration; Add nullable and non-nullable types

cobalt/lang/Cobalt.java

@@ -22,6 +22,7 @@ public class Cobalt {
         } else if (args.length == 1) {
             runFile(args[0]);
         } else {
+            System.out.println("Cobalt v0.1.0");
             runPrompt();
         }
     }

cobalt/lang/Environment.java

@@ -0,0 +1,54 @@
+package cobalt.lang;
+
+import java.util.HashMap;
+import java.util.Map;
+
+class Environment {
+    final Environment enclosing;
+    private final Map<String, Object> values = new HashMap<>();
+
+
+    Environment() {
+        enclosing = null;
+    }
+
+    Environment(Environment enclosing) {
+        this.enclosing = enclosing;
+    }
+
+
+    Object get(Token name) {
+        if (values.containsKey(name.lexeme)) {
+            return values.get(name.lexeme);
+        }
+
+        if (enclosing != null) return enclosing.get(name);
+
+        throw new RuntimeError(name,
+            "Undefined reference to variable: '" + name.lexeme + "'.");
+    }
+
+    void define(Token name, Object value) {
+        if (!values.containsKey(name.lexeme)) {
+            values.put(name.lexeme, value);
+        } else {
+            throw new RuntimeError(name,
+                "Undefined reference to variable: '" + name.lexeme + "'.");
+        }
+    }
+
+    void assign(Token name, Object value) {
+        if (values.containsKey(name.lexeme)) {
+            values.put(name.lexeme, value); 
+            return;
+        }
+
+        if (enclosing != null) {
+            enclosing.assign(name, value);
+            return;
+        }
+
+        throw new RuntimeError(name,
+            "Undefined variable '" + name.lexeme + "'.");
+    }
+}

cobalt/lang/Expr.java

@@ -8,6 +8,8 @@ abstract class Expr {
         R visitGroupingExpr(Grouping expr);
         R visitLiteralExpr(Literal expr);
         R visitUnaryExpr(Unary expr);
+        R visitVarExpr(Variable expr);
+        R visitAssignExpr(Assign expr);
     }
 
 
@@ -68,5 +70,33 @@ abstract class Expr {
         final Expr right;
     }
 
+    static class Variable extends Expr {
+        Variable(Token name, boolean nullable) {
+            this.name = name;
+            this.nullable = nullable;
+        }
+
+        <R> R accept(Visitor<R> visitor) {
+            return visitor.visitVarExpr(this);
+        }
+
+        final Token name;
+        final boolean nullable;
+    }
+
+    static class Assign extends Expr {
+        Assign(Token name, Expr value) {
+            this.name = name;
+            this.value = value;
+        }
+
+        <R> R accept(Visitor<R> visitor) {
+            return visitor.visitAssignExpr(this);
+        }
+
+        final Token name;
+        final Expr value;
+    }
+
     abstract <R> R accept(Visitor<R> visitor);
 }

cobalt/lang/Interpreter.java

@@ -1,14 +1,11 @@
-//
-//  Continue at Section 7.2.3
-//  Page 100
-//
-
 package cobalt.lang;
 
 import java.util.List;
 
 class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
 
+    private Environment environment = new Environment();
+
     void interpret(List<Stmt> statements) {
         try {
             for (Stmt statement : statements) {
@@ -38,6 +35,19 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return null;
     }
 
+    @Override
+    public Object visitVarExpr(Expr.Variable expr) {
+        // TODO: Fix analogous variable issues
+        return environment.get(expr.name);
+        // Object var_value = environment.get(expr.name);
+        // if (var_value != null) {
+        //     return environment.get(expr.name);
+        // } else {
+        //     throw new RuntimeError(expr.name,
+        //         "Reference to undefined variable: '" + expr.name + "'.");
+        // }
+    }
+
     private boolean isTruthy(Object object) {
         if (object == null)
             return false;
@@ -59,6 +69,26 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         stmt.accept(this);
     }
 
+    private void executeBlock(List<Stmt> statements,
+                              Environment environment) {
+        Environment previous = this.environment;
+        try {
+            this.environment = environment;
+
+            for (Stmt statement : statements) {
+                execute(statement);
+            }
+        } finally {
+            this.environment = previous;
+        }
+    }
+
+    @Override
+    public Void visitBlockStmt(Stmt.Block stmt) {
+        executeBlock(stmt.statements, new Environment(environment));
+        return null;
+    }
+
     @Override
     public Void visitExpressionStmt(Stmt.Expression stmt) {
         evaluate(stmt.expression);
@@ -72,6 +102,28 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
         return null;
     }
 
+    @Override
+    public Void visitVarStmt(Stmt.Var stmt) {
+        Object value = null;
+        if (stmt.initializer != null) {
+            value = evaluate(stmt.initializer);
+        }
+
+        if (!stmt.nullable && value == null) {
+            throw new RuntimeError(stmt.name, "Assignment of 'nil' to non-nullable type");
+        } else {
+            environment.define(stmt.name, value);
+            return null;
+        }
+    }
+
+    @Override
+    public Object visitAssignExpr(Expr.Assign expr) {
+        Object value = evaluate(expr.value);
+        environment.assign(expr.name, value);
+        return value;
+    }
+
     @Override
     public Object visitBinaryExpr(Expr.Binary expr) {
         Object left = evaluate(expr.left);
@@ -100,8 +152,6 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
                 checkNumberOperand(expr.operator, right);
                 return (double) left - (double) right;
             case PLUS:
-                System.out.println("Left: " + left.getClass());
-                System.out.println("Right: " + right.getClass());
                 if (left instanceof Double && right instanceof Double) {
                     return (double) left + (double) right;
                 } else if (left instanceof String && right instanceof String) {
@@ -110,8 +160,12 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
                     return (String) left + (Double) right;
                 } else if (left instanceof Double && right instanceof String) {
                     return (Double) left + (String) right;
+                } else if ((left == null) && right instanceof String) {
+                    return "nil" + (String) right;
+                } else if (left instanceof String && (right == null)) {
+                    return (String) left + "nil";
                 }
-                throw new RuntimeError(expr.operator, "Operands must be of type number or string");
+                throw new RuntimeError(expr.operator, "Invalid operands: '" + left + "' and '" + right + "'");
             case SLASH:
                 checkNumberOperands(expr.operator, left, right);
                 checkNumberOperand(expr.operator, right);

cobalt/lang/Parser.java

@@ -1,6 +1,9 @@
 package cobalt.lang;
 
 import java.util.List;
+
+import cobalt.lang.Expr.Variable;
+
 import java.util.ArrayList;
 import static cobalt.lang.TokenType.*;
 
@@ -18,7 +21,7 @@ class Parser {
     List<Stmt> parse() {
         List<Stmt> statements = new ArrayList<>();
         while (!isAtEnd()) {
-            statements.add(statement());
+            statements.add(declaration());
         }
 
         return statements;
@@ -26,12 +29,24 @@ class Parser {
 
 
     private Expr expression() {
-        return equality();
+        return assignment();
+    }
+
+
+    private Stmt declaration() {
+        try {
+            if (match(VAR)) return varDeclaration();
+            return statement();
+        } catch (ParseError error) {
+            synchronize();
+            return null;
+        }
     }
 
     
     private Stmt statement() {
         if (match(PRINT)) return printStatement();
+        if (match(LEFT_BRACE)) return new Stmt.Block(block());
 
         return expressionStatement();
     }
@@ -39,17 +54,72 @@ class Parser {
 
     private Stmt printStatement() {
         Expr value = expression();
-        consume(SEMICOLON, "Expect ';' after value.");
+        consume(SEMICOLON, "Expected ';' after value.");
         return new Stmt.Print(value);
     }
 
+
+    private Stmt varDeclaration() {
+        Token name = consume(IDENTIFIER, "Expected variable name.");
+
+        Expr initializer = null;
+        boolean nullable = false;
+        
+        if (match(NULLABLE)) {
+            nullable = true;
+        }
+
+        if (match(EQUAL)) {
+            initializer = expression();
+        }
+
+        consume(SEMICOLON, "Expected ';' after variable declaration.");
+
+        return new Stmt.Var(name, nullable, initializer);
+    }
+
+
     private Stmt expressionStatement() {
         Expr expr = expression();
-        consume(SEMICOLON, "Expect ';' after value.");
+        consume(SEMICOLON, "Expected ';' after value.");
         return new Stmt.Expression(expr);
     }
 
 
+    private List<Stmt> block() {
+        List<Stmt> statements = new ArrayList<>();
+
+        while (!check(RIGHT_BRACE) && !isAtEnd()) {
+            statements.add(declaration());
+        }
+
+        consume(RIGHT_BRACE, "Expected '}' after block.");
+        return statements;
+    }
+
+
+    private Expr assignment() {
+        Expr expr = equality();
+
+        if (match(EQUAL)) {
+            Token equals = previous();
+            Expr value = assignment();
+
+            if (expr instanceof Expr.Variable) {
+                Variable var = (Expr.Variable) expr;
+                Token name = var.name;
+                if (!var.nullable && value == null) {
+                    error(equals, "Invalid assignment of 'nil' on non-nullable type");
+                } else {
+                    return new Expr.Assign(name, value);
+                }
+            }
+            error(equals, "Invalid assignment on non-variable type");
+        }
+
+        return expr;
+    }
+
     // Parser definition for handling equalities
     //
     // equality     ->  comparison ( ( "!=" | "==" ) comparison )* ;
@@ -139,6 +209,14 @@ class Parser {
             return new Expr.Literal(previous().literal);
         }
 
+        if (match(IDENTIFIER)) {
+            if (match(NULLABLE)) {
+                return new Expr.Variable(previous(), true);
+            } else {
+                return new Expr.Variable(previous(), false);
+            }
+        }
+
         if (match(LEFT_PAREN)) {
             Expr expr = expression();
             consume(RIGHT_PAREN, "Expect ')' after expression.");

cobalt/lang/Scanner.java

@@ -89,15 +89,16 @@ class Scanner {
             case '>':
                 addToken(match('=') ? GREATER_EQUAL : GREATER);
                 break;
-            case '/':
+            case '/': addToken(SLASH); break;
-                if (match('/')) {
+            case '?': addToken(NULLABLE); break;
-                    // A comment goes until the end of the line
+    
-                    while (peek() != '\n' && !isAtEnd()) advance();
+            // Comments
-                } else {
+            case '#': 
-                    addToken(SLASH);
+                // A comment goes until the end of the line
-                }
+                while (peek() != '\n' && !isAtEnd()) advance();
                 break;
     
+
             // Whitespace and new lines
             case ' ':
             case '\r':

cobalt/lang/Stmt.java

@@ -4,22 +4,24 @@ import java.util.List;
 
 abstract class Stmt {
     interface Visitor<R> {
-        //R visitBlockStmt(Block stmt);
+        R visitBlockStmt(Block stmt);
         R visitExpressionStmt(Expression stmt);
         R visitPrintStmt(Print stmt);
-        //R visitVarStmt(Var stmt);
+        R visitVarStmt(Var stmt);
     }
-    // static class Block extends Stmt {
-    //     Block(List<Stmt> statements) {
-    //         this.statements = statements;
-    //     }
 
-    //     <R> R accept(Visitor<R> visitor) {
+    static class Block extends Stmt {
-    //         return visitor.visitBlockStmt(this);
+        Block(List<Stmt> statements) {
-    //     }
+            this.statements = statements;
+        }
+
+        <R> R accept(Visitor<R> visitor) {
+            return visitor.visitBlockStmt(this);
+        }
+
+        final List<Stmt> statements;
+    }
 
-    //     final List<Stmt> statements;
-    // }
     static class Expression extends Stmt {
         Expression(Expr expression) {
             this.expression = expression;
@@ -42,19 +44,22 @@ abstract class Stmt {
 
         final Expr expression;
     }
-    // static class Var extends Stmt {
-    //     Var(Token name, Expr initializer) {
-    //         this.name = name;
-    //         this.initializer = initializer;
-    //     }
     
-    //     <R> R accept(Visitor<R> visitor) {
+    static class Var extends Stmt {
-    //         return visitor.visitVarStmt(this);
+        Var(Token name, boolean nullable, Expr initializer) {
-    //     }
+            this.name = name;
+            this.nullable = nullable;
+            this.initializer = initializer;
+        }
 
-    //     final Token name;
+        <R> R accept(Visitor<R> visitor) {
-    //     final Expr initializer;
+            return visitor.visitVarStmt(this);
-    // }
+        }
+
+        final Token name;
+        final boolean nullable;
+        final Expr initializer;
+    }
 
     abstract <R> R accept(Visitor<R> visitor);
 }

cobalt/lang/TokenType.java

@@ -7,7 +7,7 @@ enum TokenType {
 
     // One or two character tokens.
     BANG, BANG_EQUAL, EQUAL, EQUAL_EQUAL,
-    GREATER, GREATER_EQUAL, LESS, LESS_EQUAL,
+    GREATER, GREATER_EQUAL, LESS, LESS_EQUAL, NULLABLE,
 
     // Literals
     IDENTIFIER, STRING, NUMBER,

cobalt/tool/GenerateAst.java

@@ -18,6 +18,7 @@ public class GenerateAst {
             "Binary   : Expr left, Token operator, Expr right",
             "Grouping : Expr expression",
             "Literal  : Object value",
+            "Variable : Token Name",
             "Unary    : Token operator, Expr right",
             "Variable : Token name"
         ));

examples/test.cobalt

@@ -1,5 +1,25 @@
+# Statement tests
 print "These are statement tests for Cobalt!";
 print 10 + 12;
 print 13 + (14 - 7) * 3;
 print "Hello, " + "World!";
 print "String " + "Multiplication " * 3;
+
+# Variable declaration
+var a = 10;
+print ("a is " + a);
+
+# Nullable variable declaration
+var b? = nil;
+print ("b is " + b);
+
+# Examples of structured block statements with scoping
+{
+    print "we are now in a nested scope!";
+
+    # Don't really like this behaviour, as I think it should throw a runtime error
+    # Instead it pulls a's state of 10 in the upper environment block
+    # Still trying to resolve this...
+    var a = a + 5;
+    print ("a is " + a);
+}