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creating:expressions [2025/04/06 19:58] – Summary ahelwercreating:expressions [2025/04/14 17:57] (current) – Finished first draft of expression interpretation tutorial ahelwer
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-====== Handling Constant TLA⁺ Expressions ======+======= Handling Constant TLA⁺ Expressions =======
  
 This tutorial page covers the next three chapters in //Crafting Interpreters//: This tutorial page covers the next three chapters in //Crafting Interpreters//:
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 Just primitive literal values stuck together, not too much more advanced than a simple calculator app. Just primitive literal values stuck together, not too much more advanced than a simple calculator app.
 This will give us a skeleton on which to hang the rest of the language. This will give us a skeleton on which to hang the rest of the language.
 +
 +Each section in this tutorial corresponds to one or more sections of //Crafting Interpreters// (henceforth referred to as "the book").
 +First read the section of the book, then read the corresponding commentary & modifications given by this tutorial.
 +
 +====== Chapter 5: Representing Code ======
 +
 +[[https://craftinginterpreters.com/representing-code.html|Chapter 5]] focuses more on concepts than code, so this section will not have many TLA⁺-specific modifications.
 +However, since this tutorial is intended to be self-contained code-wise, all necessary code is reproduced.
 +
 +===== Section 5.1: Context-Free Grammars =====
 +
 +Everything before [[https://craftinginterpreters.com/representing-code.html#a-grammar-for-lox-expressions|Section 5.1.3: A Grammar for Lox expressions]] applies equally to TLA⁺.
 +The ambiguous first-draft grammar for Lox expressions can be adapted to TLA⁺:
 +<code eiffel>
 +expression     → literal
 +               | unary
 +               | binary
 +               | ternary
 +               | variadic
 +               | grouping ;
 +
 +literal        → NUMBER | "TRUE" | "FALSE" ;
 +grouping       → "(" expression ")" ;
 +unary          → (( "ENABLED" | "-" ) expression ) | ( expression "'" ) ;
 +binary         → expression operator expression ;
 +ternary        → "IF" expression "THEN" expression "ELSE" expression;
 +variadic       → "{" ( expression ( "," expression )* )? "}"
 +operator       → "=" | "+" | "-" | ".." | "/\" | "\/" | "<"  | "\in" ;
 +</code>
 +There are a few interesting differences.
 +The ''unary'' rule now captures both prefix //and// suffix operators, which both only accept a single parameter.
 +The ''ternary'' rule matches the ''IF''/''THEN''/''ELSE'' operator, with the three parameters being the predicate, the true branch, and the false branch.
 +The operators are changed to the set of operators defined in our TLA⁺ implementation.
 +These are all the expressions we can use without introducing the concept of identifiers referring to something else.
 +
 +There is also the ''variadic'' rule matching finite set literals like ''{1, 2, 3}'' or the empty set ''{}''.
 +This one is so named because it's where we'll put operators accepting varying numbers of parameters.
 +It's kind of weird to think of the finite set literal ''{1, 2, 3}'' as an operator, but it is!
 +The only difference between ''{1, 2, 3}'' and an operator like ''constructSet(1, 2, 3)'' is syntactic sugar.
 +This is the perspective of a language implementer.
 +Later on we will extend the definition of ''variadic'' to include vertically-aligned conjunction & disjunction lists.
 +
 +===== Section 5.2: Implementing Syntax Trees =====
 +
 +While some programmers might have an aversion to generating code, the approach taken in the book is actually very convenient - as you will discover if you take some time to prototype & play around with different class representations of the parse tree!
 +In [[https://craftinginterpreters.com/representing-code.html#metaprogramming-the-trees|Section 5.2.2: Metaprogramming the trees]], the main differences in the ''GenerateAst'' class reflect our modification of the Lox grammar in the previous section:
 +
 +<code java [highlight_lines_extra="19,20,21"]>
 +package tool;
 +
 +import java.io.IOException;
 +import java.io.PrintWriter;
 +import java.util.Arrays;
 +import java.util.List;
 +
 +public class GenerateAst {
 +  public static void main(String[] args) throws IOException {
 +    if (args.length != 1) {
 +      System.err.println("Usage: generate_ast <output directory>");
 +      System.exit(64);
 +    }
 +    String outputDir = args[0];
 +    defineAst(outputDir, "Expr", Arrays.asList(
 +      "Binary   : Expr left, Token operator, Expr right",
 +      "Grouping : Expr expression",
 +      "Literal  : Object value",
 +      "Unary    : Token operator, Expr expr",
 +      "Ternary  : Token operator, Expr first, Expr second, Expr third",
 +      "Variadic : Token operator, List<Expr> parameters"
 +    ));
 +  }
 +}
 +</code>
 +
 +In the ''defineAst'' function, we only need to modify the output code so that it uses the package ''.tla'' instead of ''.lox''; add this after the ''main'' method of ''GenerateAst'':
 +
 +<code java [highlight_lines_extra="7"]>
 +  private static void defineAst(
 +      String outputDir, String baseName, List<String> types)
 +      throws IOException {
 +    String path = outputDir + "/" + baseName + ".java";
 +    PrintWriter writer = new PrintWriter(path, "UTF-8");
 +
 +    writer.println("package tla;");
 +    writer.println();
 +    writer.println("import java.util.List;");
 +    writer.println();
 +    writer.println("abstract class " + baseName + " {");
 +
 +
 +    // The AST classes.
 +    for (String type : types) {
 +      String className = type.split(":")[0].trim();
 +      String fields = type.split(":")[1].trim();
 +      defineType(writer, baseName, className, fields);
 +    }
 +    writer.println("}");
 +    writer.close();
 +  }
 +</code>
 +
 +Finally, the ''defineType'' method - added after the ''defineAst'' method - is unchanged:
 +
 +<code java>
 +  private static void defineType(
 +      PrintWriter writer, String baseName,
 +      String className, String fieldList) {
 +    writer.println("  static class " + className + " extends " +
 +        baseName + " {");
 +
 +    // Constructor.
 +    writer.println("    " + className + "(" + fieldList + ") {");
 +
 +    // Store parameters in fields.
 +    String[] fields = fieldList.split(", ");
 +    for (String field : fields) {
 +      String name = field.split(" ")[1];
 +      writer.println("      this." + name + " = " + name + ";");
 +    }
 +
 +    writer.println("    }");
 +
 +    // Fields.
 +    writer.println();
 +    for (String field : fields) {
 +      writer.println("    final " + field + ";");
 +    }
 +
 +    writer.println("  }");
 +  }
 +</code>
 +
 +
 +===== Section 5.3: Working with Trees =====
 +
 +[[https://craftinginterpreters.com/representing-code.html#working-with-trees|Section 5.3]] introduces the Visitor pattern.
 +No TLA⁺-specific differences are necessary when modifying ''GenerateAst'' to support it.
 +Insert this line in ''defineAst()'':
 +<code java [highlight_lines_extra="3"]>
 +    writer.println("abstract class " + baseName + " {");
 +
 +    defineVisitor(writer, baseName, types);
 +
 +    // The AST classes.
 +</code>
 +
 +This calls the ''defineVisitor'' function which writes the visitor interface, defined as follows:
 +<code java>
 +  private static void defineVisitor(
 +      PrintWriter writer, String baseName, List<String> types) {
 +    writer.println("  interface Visitor<R> {");
 +
 +    for (String type : types) {
 +      String typeName = type.split(":")[0].trim();
 +      writer.println("    R visit" + typeName + baseName + "(" +
 +          typeName + " " + baseName.toLowerCase() + ");");
 +    }
 +
 +    writer.println("  }");
 +  }
 +</code>
 +
 +Again insert some lines in ''defineAst()'' to create the abstract ''accept'' method:
 +<code java [highlight_lines_extra="4,5,6"]>
 +      defineType(writer, baseName, className, fields);
 +    }
 +
 +    // The base accept() method.
 +    writer.println();
 +    writer.println("  abstract <R> R accept(Visitor<R> visitor);");
 +
 +    writer.println("}");
 +</code>
 +
 +Finally, insert some lines in ''defineType'' to add a types-specific ''accept'' method in each output class:
 +<code java [highlight_lines_extra="3,4,5,6,7,8,9"]>
 +    writer.println("    }");
 +
 +    // Visitor pattern.
 +    writer.println();
 +    writer.println("    @Override");
 +    writer.println("    <R> R accept(Visitor<R> visitor) {");
 +    writer.println("      return visitor.visit" +
 +        className + baseName + "(this);");
 +    writer.println("    }");
 +
 +    // Fields.
 +</code>
 +Our generated syntax tree node types now support the visitor pattern!
 +
 +===== Section 5.4: A (Not Very) Pretty Printer =====
 +
 +[[https://craftinginterpreters.com/representing-code.html#a-not-very-pretty-printer|Section 5.4]] provides an implementation of a visitor called ''AstPrinter'' that prints out the parse tree.
 +There are a few TLA⁺-specific modifications, starting of course with the package name:
 +
 +<code java [highlight_lines_extra="1"]>
 +package tla;
 +
 +class AstPrinter implements Expr.Visitor<String> {
 +  String print(Expr expr) {
 +    return expr.accept(this);
 +  }
 +}
 +</code>
 +
 +We also have some modifications and additions to the ''visit'' methods of the ''AstPrinter'' class, again reflecting our modified TLA⁺-specific grammar:
 +
 +<code java [highlight_lines_extra="20,23,24,25,26,27,29,30,31,32"]>
 +  @Override
 +  public String visitBinaryExpr(Expr.Binary expr) {
 +    return parenthesize(expr.operator.lexeme,
 +                        expr.left, expr.right);
 +  }
 +
 +  @Override
 +  public String visitGroupingExpr(Expr.Grouping expr) {
 +    return parenthesize("group", expr.expression);
 +  }
 +
 +  @Override
 +  public String visitLiteralExpr(Expr.Literal expr) {
 +    if (expr.value == null) return "nil";
 +    return expr.value.toString();
 +  }
 +
 +  @Override
 +  public String visitUnaryExpr(Expr.Unary expr) {
 +    return parenthesize(expr.operator.lexeme, expr.expr);
 +  }
 +
 +  @Override
 +  public string visitTernaryExpr(Expr.Ternary expr) {
 +    return parenthesize(expr.operator.lexeme, expr.first,
 +                        expr.second, expr.third);
 +  }
 +
 +  @Override
 +  public string visitVariadicExpr(Expr.Variadic expr) {
 +    return parenthesize(expr.operator.lexeme,
 +                        expr.parameters.toArray(Expr[]::new));
 +  }
 +</code>
 +
 +The ''parenthesize'' method is unchanged from the book and should be inserted after the ''visit'' methods:
 +
 +<code java>
 +  private String parenthesize(String name, Expr... exprs) {
 +    StringBuilder builder = new StringBuilder();
 +
 +    builder.append("(").append(name);
 +    for (Expr expr : exprs) {
 +      builder.append(" ");
 +      builder.append(expr.accept(this));
 +    }
 +    builder.append(")");
 +
 +    return builder.toString();
 +  }
 +</code>
 +
 +It isn't necessary to define a ''main'' method in the ''AstPrinter'' class, but if you'd like to try it out you are free to copy the one given in the book.
 +
 +====== Chapter 6: Parsing Expressions ======
 +
 +In [[https://craftinginterpreters.com/parsing-expressions.html|Chapter 6]], we finally build a parse tree out of our tokens!
 +
 +===== Section 6.1: Ambiguity and the Parsing Game =====
 +
 +First, as in [[https://craftinginterpreters.com/parsing-expressions.html#ambiguity-and-the-parsing-game|section 6.1 of the book]], we have to disambiguate our grammar.
 +The way that precedence works in TLA⁺ is different from Lox, and indeed different from most other languages!
 +One side-quote from this section of the book reads:
 +
 +>While not common these days, some languages specify that certain pairs of operators have no relative precedence. That makes it a syntax error to mix those operators in an expression without using explicit grouping.
 +
 +>Likewise, some operators are *non-associative*. That means it’s an error to use that operator more than once in a sequence. For example, Perl’s range operator isn’t associative, so a .. b is OK, but a .. b .. c is an error.
 +
 +TLA⁺ has both of these features!
 +Instead of operators occupying a slot in some hierarchy of precedence, each operator has a precedence //range//.
 +When the precedence ranges of two operators overlap it is a parse error.
 +For example, the ''ENABLED'' prefix operator has precedence range 4-15, and the prime operator has precedence range 15-15; thus, the following expression has a precedence conflict and should not parse:
 +<code>
 +ENABLED x'
 +</code>
 +Users must add parentheses to indicate their desired grouping in the expression.
 +Similarly, some operators like ''='' are not associative so ''a = b = c'' should be a parse error.
 +Both of these factors combine to make our operator parsing code quite a bit different from that given in the book.
 +Worry not, it can still be made terse and understandable!
 +
 +===== Section 6.2: Recursive Descent Parsing =====
 +
 +[[https://craftinginterpreters.com/parsing-expressions.html#recursive-descent-parsing|Section 6.2]] is where we start writing our parser in the recursive descent style.
 +Recursive descent can seem a bit magical even if you're used to reasoning about recursive functions!
 +Unfortunately TLA⁺ requires us to push this magic even further.
 +We'll take it step by step.
 +
 +We start with the same basic ''Parser.java'' file as in the book, renaming ''lox'' to ''tla'' in the package name as usual.
 +We also add an import for ''ArrayList'', which we will later make use of when parsing variadic operators:
 +
 +<code java [highlight_lines_extra="1,4,6"]>
 +package tla;
 +
 +import java.util.List;
 +import java.util.ArrayList;
 +
 +import static tla.TokenType.*;
 +
 +class Parser {
 +  private final List<Token> tokens;
 +  private int current = 0;
 +
 +  Parser(List<Token> tokens) {
 +    this.tokens = tokens;
 +  }
 +}
 +</code>
 +
 +Now we hit our first major difference.
 +In Lox, precedence is given by a small hierarchy of named rules like ''equality'', ''comparison'', ''term'', etc.
 +TLA⁺ is more complicated than that.
 +The full language has around 100 operators spanning precedences 1 to 15!
 +If we wanted to match the book's style we'd have to write a tower of recursive functions like:
 +<code java>
 +private Expr operatorExpressionPrec1() { ... }
 +private Expr operatorExpressionPrec2() { ... }
 +private Expr operatorExpressionPrec3() { ... }
 +...
 +private Expr operatorExpressionPrec15() { ... }
 +</code>
 +where each ''operatorExpressionPrecN()'' function parses all the prefix, infix, and postfix operators of precedence ''N'', and calls ''operatorExpressionPrecN+1()''.
 +Life is too short for this.
 +Instead, we'll adopt a technique alluded to in the text:
 +>If you wanted to do some clever Java 8, you could create a helper method for parsing a left-associative series of binary operators given a list of token types, and an operand method handle to simplify this redundant code.
 +
 +Here's the skeleton of our operator parsing function; the trick is to make the precedence a parameter to the function instead of a component of the name.
 +Add this code after the ''Parser()'' constructor:
 +<code java>
 +  private Expr expression() {
 +    return operatorExpression(1);
 +  }
 +
 +  private Expr operatorExpression(int prec) {
 +    if (prec == 16) return primary();
 +
 +    Expr expr = operatorExpression(prec + 1);
 +
 +    return expr;
 +  }
 +</code>
 +
 +Before filling out ''operatorExpression()'', we'll add the helper methods; these form an incredibly well-designed parsing API and are unchanged from the book:
 +
 +<code java>
 +  private boolean match(TokenType... types) {
 +    for (TokenType type : types) {
 +      if (check(type)) {
 +        advance();
 +        return true;
 +      }
 +    }
 +
 +    return false;
 +  }
 +
 +  private boolean check(TokenType type) {
 +    if (isAtEnd()) return false;
 +    return peek().type == type;
 +  }
 +
 +  private Token advance() {
 +    if (!isAtEnd()) current++;
 +    return previous();
 +  }
 +
 +  private boolean isAtEnd() {
 +    return peek().type == EOF;
 +  }
 +
 +  private Token peek() {
 +    return tokens.get(current);
 +  }
 +
 +  private Token previous() {
 +    return tokens.get(current - 1);
 +  }
 +</code>
 +
 +Now we have to define a table of operators with their details.
 +For this, create a new file ''Operator.java'' containing a class recording operator fix type, token type, associativity, and precedence range:
 +
 +<code java>
 +package tla;
 +
 +enum Fix {
 +  PREFIX, INFIX, POSTFIX
 +}
 +
 +class Operator {
 +  final Fix fix;
 +  final TokenType token;
 +  final boolean assoc;
 +  final int lowPrec;
 +  final int highPrec;
 +
 +  public Operator(Fix fix, TokenType token, boolean assoc,
 +                  int lowPrec, int highPrec) {
 +    this.fix = fix;
 +    this.token = token;
 +    this.assoc = assoc;
 +    this.lowPrec = lowPrec;
 +    this.highPrec = highPrec;
 +  }
 +}
 +</code>
 +
 +For convenience, import the ''Fix'' enum values in ''Parser.java'' so they can be referenced directly:
 +
 +<code java [highlight_lines_extra="7"]>
 +package tla;
 +
 +import java.util.List;
 +import java.util.ArrayList;
 +
 +import static tla.TokenType.*;
 +import static tla.Fix.*;
 +
 +class Parser {
 +</code>
 +
 +
 +You can find operator attributes on page 271 of //[[https://lamport.azurewebsites.net/tla/book.html|Specifying Systems]]// by Leslie Lamport, or [[https://github.com/tlaplus/tlaplus/blob/13e5a39b5368a6da4906b8ed1c2c1114d2e7de15/tlatools/org.lamport.tlatools/src/tla2sany/parser/Operators.java#L130-L234|this TLA⁺ tools source file]].
 +We use a small subset of these operators.
 +Record their attributes in a table in ''Parser.java'', below ''operatorExpression()'':
 +
 +<code java>
 +  private static final Operator[] operators = new Operator[] {
 +    new Operator(PREFIX,  NOT,        true,   4,  4 ),
 +    new Operator(PREFIX,  ENABLED,    false,  4,  15),
 +    new Operator(PREFIX,  MINUS,      true,   12, 12),
 +    new Operator(INFIX,   AND,        true,   3,  3 ),
 +    new Operator(INFIX,   OR,         true,   3,  3 ),
 +    new Operator(INFIX,   IN,         false,  5,  5 ),
 +    new Operator(INFIX,   EQUAL,      false,  5,  5 ),
 +    new Operator(INFIX,   LESS_THAN,  false,  5,  5 ),
 +    new Operator(INFIX,   DOT_DOT,    false,  9,  9 ),
 +    new Operator(INFIX,   PLUS,       true,   10, 10),
 +    new Operator(INFIX,   MINUS,      true,   11, 11),
 +    new Operator(POSTFIX, PRIME,      false,  15, 15),
 +  };
 +</code>
 +
 +Here's something a bit odd.
 +In TLA⁺, the infix minus operator (subtraction) has higher precedence at 11-11 than the infix plus operator (addition) at 10-10!
 +In grade school you probably learned acronyms like PEMDAS or BEDMAS to remember the order of operations in arithmetic.
 +Really, you learned parsing rules!
 +Now when writing our own parsing algorithm we come to understand that the order of these operations is not inscribed in the bedrock of mathematical truth but instead is a simple convention of mathematical notation.
 +TLA⁺ subverts this by parsing the expression ''a + b - c'' as ''a + (b - c)'' instead of the PEMDAS-style ''(a + b) - c''.
 +While this design decision is unusual, it is [[https://groups.google.com/g/tlaplus/c/Bt1fl8GvizE/m/ohYTfQWiCgAJ|unlikely to cause any problems]].
 +
 +We need one more helper method before we can start working on ''operatorExpression()'': a superpowered ''match()'' method specific to operators, which will try to match any operators of a given fix type & (low) precedence.
 +Add this code above ''match()'':
 +<code java>
 +  private Operator matchOp(Fix fix, int prec) {
 +    for (Operator op : operators) {
 +      if (op.fix == fix && op.lowPrec == prec) {
 +        if (match(op.token)) return op;
 +      }
 +    }
 +
 +    return null;
 +  }
 +</code>
 +
 +Okay, we're all set for the main event!
 +Here is how we modify ''operatorExpression()'' to parse infix operators.
 +You can see a strong resemblance to the infix operator parsing code given in the book (new lines highlighted):
 +<code java [highlight_lines_extra="4,7,8,9,10,11"]>
 +  private Expr operatorExpression(int prec) {
 +    if (prec == 16) return primary();
 +
 +    Operator op;
 +
 +    Expr expr = operatorExpression(prec + 1);
 +    while ((op = matchOp(INFIX, prec)) != null) {
 +      Token operator = previous();
 +      Expr right = operatorExpression(op.highPrec + 1);
 +      expr = new Expr.Binary(expr, operator, right);
 +    }
 +
 +    return expr;
 +  }
 +</code>
 +
 +We use Java's combined conditional-assignment atop the ''while'' loop to make our code more terse, both checking whether any operators were matched and getting details of the matched operator if so.
 +The interior of the ''while'' loop is largely identical to infix parsing logic from the book, except for when we recurse to a higher precedence level: in the book the code recurses one level above the matched operator's precedence, but here we go one level above the //upper bound// of the matched operator's precedence.
 +It takes a bit of thinking to understand why this works for parsing expressions according to the precedence rules we want, but it does.
 +Take a minute to ponder it.
 +If you still don't get it, wait until we have a fully-functional expression parser then play around with this line to see how it changes the behavior.
 +
 +Our code implements precedence ranges, but assumes all infix operators are associative.
 +We need to modify the loop to return immediately if the infix operator is not associative:
 +<code java [highlight_lines_extra="5"]>
 +    while ((op = matchOp(INFIX, prec)) != null) {
 +      Token operator = previous();
 +      Expr right = operatorExpression(op.highPrec + 1);
 +      expr = new Expr.Binary(expr, operator, right);
 +      if (!op.assoc) return expr;
 +    }
 +</code>
 +
 +And that's how we parse infix operators!
 +That was the most complicated case, so let's move on to prefix operators.
 +Again our code resembles the prefix operator parsing logic given in the book.
 +Add this snippet near the top of ''operatorExpression()'':
 +
 +<code java [highlight_lines_extra="5,6,7,8,9,10"]>
 +  private Expr operatorExpression(int prec) {
 +    if (prec == 16) return primary();
 +
 +    Operator op;
 +    if ((op = matchOp(PREFIX, prec)) != null) {
 +      Token opToken = previous();
 +      Expr expr = operatorExpression(
 +                    op.assoc ? op.lowPrec : op.highPrec + 1);
 +      return new Expr.Unary(opToken, expr);
 +    }
 +
 +    Expr expr = operatorExpression(prec + 1);
 +</code>
 +
 +Of note is the expression ''op.assoc ? op.lowPrec : op.highPrec + 1'' controlling the precedence level at which we recurse.
 +To understand this expression, it is helpful to consider an example.
 +The negative prefix operator ''-1'' is associative, so we should be able to parse the expression ''--1'' as ''-(-1)''.
 +In order to do that after consuming the first ''-'' we have to recurse into ''operatorExpression()'' //at the same precedence level//.
 +Then we will consume the second ''-'', then again recurse and ultimately consume ''1'' in our yet-to-be-defined ''primary'' method.
 +In contrast, the prefix operator ''ENABLED'' is not associative and has range 4-15.
 +In that case we want to reject the expression ''ENABLED ENABLED TRUE'', so we recurse at a level higher than ''ENABLED'''s upper precedence bound.
 +Thus the second ''ENABLED'' will not be matched and we will report a parse error using methods defined in chapter section 6.3.
 +
 +All that remains is to parse postfix operators.
 +These are not covered in the book, but they pretty much resemble infix operators without matching an expression on the right-hand side of the operator.
 +Add the highlighted code below the ''INFIX'' logic in ''operatorExpression()'':
 +
 +<code java [highlight_lines_extra="3,4,5,6,7"]>
 +    }
 +
 +    while ((op = matchOp(POSTFIX, prec)) != null) {
 +      Token opToken = previous();
 +      expr = new Expr.Unary(opToken, expr);
 +      if (!op.assoc) break;
 +    }
 +
 +    return expr
 +  }
 +</code>
 +
 +And that completes our operator parsing logic!
 +All that remains is our ''primary()'' method.
 +It is fairly similar to that given in the book, with some omissions since our minimal TLA⁺ subset does not contain strings or null values.
 +Add this code after ''operatorExpression()'':
 +
 +<code java>
 +  private Expr primary() {
 +    if (match(FALSE)) return new Expr.Literal(false);
 +    if (match(TRUE)) return new Expr.Literal(true);
 +
 +    if (match(NUMBER)) {
 +      return new Expr.Literal(previous().literal);
 +    }
 +
 +    if (match(LEFT_PAREN)) {
 +      Expr expr = expression();
 +      consume(RIGHT_PAREN, "Expect ')' after expression.");
 +      return new Expr.Grouping(expr);
 +    }
 +  }
 +</code>
 +
 +We have two final additions to ''primary()'', the ''IF''/''THEN''/''ELSE'' operator and the set construction operator.
 +Here's how we parse ''IF''/''THEN''/''ELSE''; add the highlighted code to the bottom of ''primary()'':
 +
 +<code java [highlight_lines_extra="4,5,6,7,8,9,10,11,12"]>
 +      return new Expr.Grouping(expr);
 +    }
 +
 +    if (match(IF)) {
 +      Token operator = previous();
 +      Expr condition = expression();
 +      consume(THEN, "'THEN' required after 'IF' expression.");
 +      Expr yes = expression();
 +      consume(ELSE, "'ELSE' required after 'THEN' expression.");
 +      Expr no = expression();
 +      return new Expr.Ternary(operator, condition, yes, no);
 +    }
 +  }
 +</code>
 +
 +And here's how we parse the set construction operator.
 +Again add the highlighted code to the bottom of ''primary()'':
 +
 +<code java [highlight_lines_extra="4,5,6,7,8,9,10,11,12,13,14"]>
 +      return new Expr.Ternary(operator, condition, yes, no);
 +    }
 +
 +    if (match(LEFT_BRACE)) {
 +      Token operator = previous();
 +      List<Expr> elements = new ArrayList<Expr>();
 +      if (RIGHT_BRACE != peek().type) {
 +        do {
 +          elements.add(expression());
 +        } while (match(COMMA));
 +      }
 +      consume(RIGHT_BRACE, "'}' is required to terminate finite set literal.");
 +      return new Expr.Variadic(operator, elements);
 +    }
 +  }
 +</code>
 +
 +This will handle expressions like ''{1,2,3}'' and (crucially) ''{}''.
 +
 +The ''consume()'' operator is defined in the next section, along with error reporting generally.
 +
 +===== Section 6.3: Syntax Errors =====
 +
 +We don't need to modify most of the error reporting code given in [[https://craftinginterpreters.com/parsing-expressions.html#syntax-errors|Section 6.3]].
 +Here it is; add the ''consume()'' method after ''match()'' in the ''Parser'' class:
 +
 +<code java>
 +  private Token consume(TokenType type, String message) {
 +    if (check(type)) return advance();
 +
 +    throw error(peek(), message);
 +  }
 +</code>
 +
 +Then add the ''error()'' method after ''previous()'':
 +
 +<code java>
 +  private ParseError error(Token token, String message) {
 +    Lox.error(token, message);
 +    return new ParseError();
 +  }
 +</code>
 +
 +In the ''TlaPlus'' class, add the ''error()'' method after ''report()'':
 +
 +<code java>
 +  static void error(Token token, String message) {
 +    if (token.type == TokenType.EOF) {
 +      report(token.line, " at end", message);
 +    } else {
 +      report(token.line, " at '" + token.lexeme + "'", message);
 +    }
 +  }
 +</code>
 +
 +Add the ''ParseError'' class definition at the top of the ''Parser'' class:
 +
 +<code java [highlight_lines_extra="2"]>
 +class Parser {
 +  private static class ParseError extends RuntimeException {}
 +
 +  private final List<Token> tokens;
 +</code>
 +
 +One piece of code we //do// need to modify is the ''synchronization()'' method.
 +TLA⁺ doesn't have statements separated by semicolons, so our best choice of synchronization point is the ''EQUAL_EQUAL'' symbol denoting an operator definition; add this in the ''Parser'' class after ''error()'':
 +
 +<code java>
 +  private void synchronize() {
 +    advance();
 +
 +    while(!isAtEnd()) {
 +      if (previous().type == EQUAL_EQUAL) return;
 +
 +      advance();
 +    }
 +  }
 +</code>
 +
 +Fully-featured TLA⁺ parsers generally look ahead to identify the start of a new operator definition (not trivial!) and insert a synthetic zero-width token for the parser to consume.
 +For an example, see SANY's very complicated ''[[https://github.com/tlaplus/tlaplus/blob/13e5a39b5368a6da4906b8ed1c2c1114d2e7de15/tlatools/org.lamport.tlatools/javacc/tla%2B.jj#L229-L392|belchDEF()]]'' method.
 +We won't be doing that here, but it's a good technique to know about; looking ahead then inserting synthetic tokens to disambiguate syntax is a common method when parsing complicated & ambiguous languages like TLA⁺.
 +
 +===== Section 6.4: Wiring up the Parser =====
 +
 +In [[https://craftinginterpreters.com/parsing-expressions.html#wiring-up-the-parser|section 6.4]] we finally arrive at the point where you can parse TLA⁺ expressions in your REPL.
 +In the ''run()'' method of the ''TlaPlus'' class, delete the code printing out the scanned tokens (or not, it can be informative to keep around) and replace it with this:
 +
 +<code java [highlight_lines_extra="3,4,6,7,9"]>
 +    List<Token> tokens = scanner.scanTokens();
 +
 +    Parser parser = new Parser(tokens);
 +    Expr expression = parser.parse();
 +
 +    // Stop if there was a syntax error.
 +    if (hadError) return;
 +
 +    System.out.println(new AstPrinter().print(expression));
 +  }
 +</code>
 +
 +Fire up the interpreter and parse a TLA⁺ expression!
 +<code>
 +> {1 + 2, IF TRUE THEN 3 ELSE 4, {}}
 +({ (+ 1 2) (IF true 3 4) ({))
 +</code>
 +
 +====== Chapter 7: Evaluating Expressions ======
 +
 +Now that we can parse expressions, let's evaluate them!
 +Here we follow the material in [[https://craftinginterpreters.com/evaluating-expressions.html|Chapter 7]] of //Crafting Interpreters//.
 +It's an exciting chapter, so let's jump in.
 +
 +===== Section 7.1: Representing Values =====
 +
 +In [[https://craftinginterpreters.com/evaluating-expressions.html#representing-values|section 7.1]] we define a mapping from language values to internal Java values.
 +Our mapping for TLA⁺ is fairly similar to Lox, although we use ''Integer'' instead of ''Double'' for number values and also have a set type.
 +TLA⁺ thankfully lacks ''nil'' types.
 +
 +^ TLA⁺ type      ^ Java representation       ^
 +| Any TLA⁺ value | ''Object''                |
 +| Boolean        | ''Boolean''               |
 +| number         | ''Integer''               |
 +| set            | ''java.util.Set<Object>'' |
 +
 +Java does a huge amount of work for us with the capabilities of the ''Object'' type, as we'll see.
 +
 +===== Section 7.2: Representing Values =====
 +
 +[[https://craftinginterpreters.com/evaluating-expressions.html#evaluating-expressions|Section 7.2]] is where we start laying down interpreter code.
 +Create a new file ''Interpreter.java'' with the highlighted lines changed or added compared with the book:
 +
 +<code java [highlight_lines_extra="1,3,4"]>
 +package tla;
 +
 +import java.util.Set;
 +import java.util.HashSet;
 +
 +class Interpreter implements Expr.Visitor<Object> {
 +}
 +</code>
 +
 +If your IDE supports it, get it to automatically add all necessary stub methods to implement the ''Expr.Visitor<Object>'' interface; we'll be filling these in, defining the interpretation of every expression type!
 +
 +Let's start with defining the value of literals.
 +Our code is completely unchanged from the book; add this ''visitLiteralExpr()'' method in the ''Interpreter'' class:
 +
 +<code java>
 +  @Override
 +  public Object visitLiteralExpr(Expr.Literal expr) {
 +    return expr.value;
 +  }
 +</code>
 +
 +Our code for evaluating grouping (parentheses) is similarly unchanged from the book:
 +
 +<code java>
 +  @Override
 +  public Object visitGroupingExpr(Expr.Grouping expr) {
 +    return evaluate(expr.expression);
 +  }
 +</code>
 +
 +This uses the ''evaluate()'' helper method; add it to the ''Interpreter'' class:
 +
 +<code java>
 +  private Object evaluate(Expr expr) {
 +    return expr.accept(this);
 +  }
 +</code>
 +
 +==== Evaluating Unary Operators ====
 +
 +Our first real difference is in the ''visitUnaryExpr()'' method.
 +Recall that since our ''Expr.Unary'' type represents both prefix and suffix operators, we don't call its parameter ''right'' as in the book.
 +Here's how we define the negative prefix operator, casting the parameter to an ''int'' instead of a ''double'' (highlighted lines differ from book):
 +
 +<code java [highlight_lines_extra="3,7"]>
 +  @Override
 +  public Object visitUnaryExpr(Expr.Unary expr) {
 +    Object operand = evaluate(expr.expr);
 +
 +    switch (expr.operator.type) {
 +      case MINUS:
 +        return -(int)operand;
 +    }
 +
 +    // Unreachable.
 +    return null;
 +  }
 +</code>
 +
 +Note that the negative prefix operator is not actually a built-in operator of TLA⁺.
 +It is defined in the ''Integers'' standard module, and TLA⁺ users need to import that module to access it.
 +However, because our minimal TLA⁺ language subset lacks module importing, for convenience we just define some common arithmetic operators as built-in.
 +You can find more information about the built-in TLA⁺ operators in chapter 16 of //[[https://lamport.azurewebsites.net/tla/book.html|Specifying Systems]]// by Leslie Lamport, or [[https://github.com/tlaplus/tlaplus/blob/13e5a39b5368a6da4906b8ed1c2c1114d2e7de15/tlatools/org.lamport.tlatools/src/tla2sany/semantic/BuiltInOperators.java#L80-L153|this SANY source file]].
 +
 +Our logical-not prefix operator is denoted by the ''NOT'' token type instead of ''BANG'' as in Lox.
 +We also have no use for the ''isTruthy()'' helper method, since TLA⁺ is quite strict: only Boolean values can be given to Boolean operators!
 +Add this code to the ''visitUnaryExpr()'' method:
 +
 +<code java [highlight_lines_extra="2,3"]>
 +    switch (expr.operator.type) {
 +      case NOT:
 +        return !(boolean)operand;
 +      case MINUS:
 +</code>
 +
 +We still have two more unary operators to define: ''ENABLED'', and the prime operator.
 +Both are trivial in this domain but will become much more complicated later on.
 +For constant expressions, ''ENABLED'' is true if and only if the expression itself is true.
 +Add the highlighted code to the ''visitUnaryExpr()'' method:
 +
 +<code java [highlight_lines_extra="2,3"]>
 +    switch (expr.operator.type) {
 +      case ENABLED:
 +        return (boolean)operand;
 +      case NOT:
 +</code>
 +
 +Priming a constant expression has no effect on the value of that expression, so just return the operand's value:
 +
 +<code java [highlight_lines_extra="2,3"]>
 +    switch (expr.operator.type) {
 +      case PRIME:
 +        return operand;
 +      case ENABLED:
 +</code>
 +
 +==== Evaluating Binary Operators ====
 +
 +Unlike Lox, addition in TLA⁺ is only defined between two numbers (at least in its standard ''Integers'' module definition).
 +Here's how we define our basic binary arithmetic & comparison operators; add a ''visitBinaryExpr()'' method in the ''Interpreter'' class:
 +
 +<code java [highlight_lines_extra="8,9,10,11,12,13,14"]>
 +  @Override
 +  public Object visitBinaryExpr(Expr.Binary expr) {
 +    Object left = evaluate(expr.left);
 +    Object right = evaluate(expr.right);
 +
 +    switch (expr.operator.type) {
 +      case MINUS:
 +        return (int)left - (int)right;
 +      case PLUS:
 +        return (int)left + (int)right;
 +      case LESS_THAN:
 +        return (int)left < (int)right;
 +      case EQUAL:
 +        return left.equals(right);
 +    }
 +
 +    // Unreachable.
 +    return null;
 +  }
 +</code>
 +
 +Note that since we don't have to worry about ''null'' values, we don't need the ''isEqual()'' helper method from the book and can use Java's ''Object.equals()'' method directly.
 +However, this actually gives us different equality semantics from TLC.
 +In TLC, evaluating things like ''123 = TRUE'' results in a runtime error instead of evaluating to ''false'' as will be the case here.
 +In the interest of simplicity we are fine with keeping these looser semantics.
 +See the challenges at the end of this tutorial page if you're interested in more closely matching the semantics of TLC.
 +
 +Let's take our first foray into sets by defining the set membership operator in ''visitBinaryExpr()'':
 +
 +<code java [highlight_lines_extra="2,3"]>
 +    switch (expr.operator.type) {
 +      case IN:
 +        return ((Set<?>)right).contains(left);
 +      case MINUS:
 +</code>
 +
 +The question mark is some unusual Java syntax you probably haven't seen before, called a [[https://docs.oracle.com/javase/tutorial/java/generics/wildcards.html|wildcard]].
 +It has to do with //covariance//, which you can read about at the end of the chapter in the book.
 +Think of it as casting the right operand to //some kind// of set, so we can access the ''Set.contains()'' method.
 +It is tempting to cast it to ''Set<Object>'' (as we only ever create ''Set<Object>'' instances) but this produces a compiler warning.
 +
 +Similar to equality, we get looser semantics here than with TLC.
 +TLC will emit a runtime error when evaluating the expression ''TRUE \in {1, 2, 3}'', while our interpreter will simple evaluate it to ''false''.
 +Again this is acceptable.
 +
 +Now for something more complicated.
 +Here's the set construction helper operator ''..'', also defined in ''visitBinaryExpr()''; this constructs a set containing all numbers between the left and right parameters, inclusive:
 +
 +<code java [highlight_lines_extra="2,3,4,5,6,7,8,9,10,11"]>
 +    switch (expr.operator.type) {
 +      case DOT_DOT:
 +        Set<Object> set = new HashSet<Object>();
 +        int lower = (int)left;
 +        int higher = (int)right;
 +        if (lower <= higher) {
 +          for (int i = lower; i <= higher; i++) {
 +            set.add(i);
 +          }
 +        }
 +        return set;
 +      case IN:
 +</code>
 +
 +So expressions like ''1 .. 3'' produce the set ''{1, 2, 3}'' while ''3 .. 2'' produces the empty set ''{}''.
 +We use ''Set<Object>'', so our sets can hold any value you can imagine including nested sets.
 +It's incredible how much we get for free here; Java sets even implement value-based de-duplication and equality comparison for us for arbitrarily-nested sets!
 +
 +The only binary operators remaining are logical ''AND'' & ''OR'', and they differ in a subtle but important way.
 +We actually want ''AND'' to implement //short-circuit evaluation//, so an expression like ''FALSE /\ TRUE'' evaluates to ''false'' without even looking at the right hand operand.
 +Add the highlighted lines near the top of the ''visitBinaryExpr()'' method:
 +
 +<code java [highlight_lines_extra="3,4,5,6,7,8,9,10"]>
 +  public Object visitBinaryExpr(Expr.Binary expr) {
 +    Object left = evaluate(expr.left);
 +    switch (expr.operator.type) {
 +      case AND:
 +        if (!(boolean)left) return false;
 +        Object right = evaluate(expr.right);
 +        return (boolean)right;
 +      default:
 +        break;
 +    }
 +
 +</code>
 +
 +In an odd contrast, our ''OR'' binary operator is //not// short-circuited.
 +We add it in with the rest of the operators in ''visitBinaryExpr()'':
 +
 +<code java [highlight_lines_extra="3,4"]>
 +    Object right = evaluate(expr.right);
 +    switch (expr.operator.type) {
 +      case OR:
 +        return (boolean)left || (boolean)right;
 +      case DOT_DOT:
 +</code>
 +
 +Why are these different?
 +It has to do with how they are used in TLA⁺ formulas.
 +Actions are usually defined as a conjunction of expressions serving as guards, where later expressions could result in a runtime error if evaluated in some cases.
 +For example, consider a variable ''x'' that can either be a sentinel value ''None'' or a set.
 +People often write expressions like ''x /= None /\ 1 \in x''.
 +It would be a runtime error to evaluate ''1 \in x'' if ''x = None''.
 +It is thus useful to use conjunction as a guard stopping expression interpretation if the first operand is not true.
 +In contrast, disjunction is used in TLA⁺ to express nondeterminism and so both branches of the disjunct need to be evaluated to see whether their expressions satisfy alternative next states.
 +You can read [[https://groups.google.com/g/tlaplus/c/U6tOJ4dsjVM/m/1zXWHrZbBwAJ|this]] thread on the TLA⁺ mailing list for more information.
 +
 +==== Evaluating Other Operators ====
 +
 +Our ''IF''/''THEN''/''ELSE'' and set constructor operators are all that remain.
 +Add this ''visitTernaryExpr()'' method to the ''Interpreter'' class:
 +
 +<code java>
 +  @Override
 +  public Object visitTernaryExpr(Ternary expr) {
 +    switch (expr.operator.type) {
 +      case IF:
 +        Object conditional = evaluate(expr.first);
 +        return (boolean)conditional ?
 +            evaluate(expr.second) : evaluate(expr.third);
 +      default:
 +        // Unreachable.
 +        return null;
 +    }
 +  }
 +</code>
 +
 +''IF''/''THEN''/''ELSE'' has a straightforward translation to Java's ''?''/'':'' mixfix operator.
 +Note our definition is also short-circuiting.
 +
 +Finally, here's how we define the set construction operator; add this ''visitVariadicExpr()'' method in the ''Interpreter'' class:
 +
 +<code java>
 +  @Override
 +  public Object visitVariadicExpr(Expr.Variadic expr) {
 +    switch (expr.operator.type) {
 +      case LEFT_BRACE:
 +        Set<Object> set = new HashSet<Object>();
 +        for (Expr parameter : expr.parameters) {
 +          set.add(evaluate(parameter));
 +        }
 +        return set;
 +      default:
 +        // Unreachable.
 +        return null;
 +    }
 +  }
 +</code>
 +
 +===== Section 7.3: Runtime Errors =====
 +
 +In [[https://craftinginterpreters.com/evaluating-expressions.html#runtime-errors|section 7.3]] we add some error detection & reporting to our interpreter code.
 +
 +Here's our variant of the ''checkNumberOperand()'' and ''checkNumberOperands()'' methods given in the book, using ''Integer'' instead of ''Double''; put these in the ''Interpreter'' class:
 +
 +<code java [highlight_lines_extra="2,8"]>
 +  private void checkNumberOperand(Token operator, Object operand) {
 +    if (operand instanceof Integer) return;
 +    throw new RuntimeError(operator, "Operand must be a number.");
 +  }
 +
 +  private void checkNumberOperands(Token operator,
 +                                   Object left, Object right) {
 +    if (left instanceof Integer && right instanceof Integer) return;
 +
 +    throw new RuntimeError(operator, "Operands must be numbers.");
 +  }
 +</code>
 +
 +Create a new ''RuntimeError.java'' file.
 +Contents are identical to that give in the book except for the package name:
 +
 +<code java [highlight_lines_extra="1"]>
 +package tla;
 +
 +class RuntimeError extends RuntimeException {
 +  final Token token;
 +
 +  RuntimeError(Token token, String message) {
 +    super(message);
 +    this.token = token;
 +  }
 +}
 +</code>
 +
 +We also need some helpers for boolean operator parameters since TLA⁺ is more strict about that; add these to the ''Interpreter'' class:
 +
 +<code java>
 +  private void checkBooleanOperand(Token operator, Object operand) {
 +    if (operand instanceof Boolean) return;
 +    throw new RuntimeError(operator, "Operand must be a boolean.");
 +  }
 +
 +  private void checkBooleanOperands(Token operator, Object left, Object right) {
 +    if (left instanceof Boolean && right instanceof Boolean) return;
 +    throw new RuntimeError(operator, "Operands must be booleans.");
 +  }
 +</code>
 +
 +Finally, we need a helper for checking whether an operand is a set:
 +
 +<code java>
 +  private void checkSetOperand(Token operator, Object operand) {
 +    if (operand instanceof Set<?>) return;
 +    throw new RuntimeError(operator, "Operand must be a set.");
 +  }
 +</code>
 +
 +Now add these checks to our interpreter code.
 +Here's negative:
 +<code java [highlight_lines_extra="2"]>
 +      case MINUS:
 +        checkNumberOperand(expr.operator, operand);
 +        return -(int)operand;
 +</code>
 +Logical not:
 +<code java [highlight_lines_extra="2"]>
 +      case NOT:
 +        checkBooleanOperand(expr.operator, operand);
 +        return !(boolean)operand;
 +</code>
 +Enabled:
 +<code java [highlight_lines_extra="2"]>
 +      case ENABLED:
 +        checkBooleanOperand(expr.operator, operand);
 +        return (boolean)operand;
 +</code>
 +Subtraction:
 +<code java [highlight_lines_extra="2"]>
 +      case MINUS:
 +        checkNumberOperands(expr.operator, left, right);
 +        return (int)left - (int)right;
 +</code>
 +Addition:
 +<code java [highlight_lines_extra="2"]>
 +      case PLUS:
 +        checkNumberOperands(expr.operator, left, right);
 +        return (int)left + (int)right;
 +</code>
 +Less than:
 +<code java [highlight_lines_extra="2"]>
 +      case LESS_THAN:
 +        checkNumberOperands(expr.operator, left, right);
 +        return (int)left < (int)right;
 +</code>
 +The ''..'' range set constructor:
 +<code java [highlight_lines_extra="2"]>
 +      case DOT_DOT:
 +        checkNumberOperands(expr.operator, left, right);
 +        Set<Object> set = new HashSet<Object>();
 +</code>
 +Set membership:
 +<code java [highlight_lines_extra="2"]>
 +      case IN:
 +        checkSetOperand(expr.operator, right);
 +        return ((Set<?>)right).contains(left);
 +</code>
 +Disjunction:
 +<code java [highlight_lines_extra="2"]>
 +      case OR:
 +        checkBooleanOperands(expr.operator, left, right);
 +        return (boolean)left || (boolean)right;
 +</code>
 +Conjunction:
 +<code java [highlight_lines_extra="2,5"]>
 +      case AND:
 +        checkBooleanOperand(expr.operator, left);
 +        if (!(boolean)left) return false;
 +        Object right = evaluate(expr.right);
 +        checkBooleanOperand(expr.operator, right);
 +        return (boolean)right;
 +</code>
 +Finally, ''IF''/''THEN''/''ELSE'':
 +<code java [highlight_lines_extra="3"]>
 +      case IF:
 +        Object conditional = evaluate(expr.first);
 +        checkBooleanOperand(expr.operator, conditional);
 +        return (boolean)conditional ?
 +            evaluate(expr.second) : evaluate(expr.third);
 +</code>
 +
 +===== Section 7.4: Hooking Up the Interpreter =====
 +
 +We're very close!
 +In [[https://craftinginterpreters.com/evaluating-expressions.html#hooking-up-the-interpreter|section 7.4]] we put in the finishing touches to get to a real running TLA⁺ REPL!
 +First, add this public ''interpret()'' method to the ''Interpreter'' class; highlighted line shows a difference from the book:
 +<code java [highlight_lines_extra="6"]>
 +  void interpret(Expr expression) { 
 +    try {
 +      Object value = evaluate(expression);
 +      System.out.println(stringify(value));
 +    } catch (RuntimeError error) {
 +      TlaPlus.runtimeError(error);
 +    }
 +  }
 +</code>
 +
 +In contrast to the book, our ''stringify()'' method is very simple; add this to the ''Interpreter'' class:
 +
 +<code java>
 +  private String stringify(Object object) {
 +    return object.toString();
 +  }
 +</code>
 +
 +Add a ''runtimeError()'' method to your main ''TlaPlus'' class after ''error()'':
 +
 +<code java>
 +  static void runtimeError(RuntimeError error) {
 +    System.err.println(error.getMessage() +
 +        "\n[line " + error.token.line + "]");
 +    hadRuntimeError = true;
 +  }
 +</code>
 +
 +Then add a static ''hadRuntimeError'' field to the ''TlaPlus'' class:
 +
 +<code java [highlight_lines_extra="2"]>
 +  static boolean hadError = false;
 +  static boolean hadRuntimeError = false;
 +
 +  public static void main(String[] args) throws IOException {
 +</code>
 +
 +And exit from the ''runFile()'' method with a particular error code if a runtime error occurs:
 +
 +<code java [highlight_lines_extra="5"]>
 +    run(new String(bytes, StandardCharsets.UTF_8));
 +
 +    // Indicate an error in the exit code.
 +    if (hadError) System.exit(65);
 +    if (hadRuntimeError) System.exit(70);
 +  }
 +</code>
 +
 +Finally, create a static ''Interpreter'' instance at the top of the ''TlaPlus'' class:
 +
 +<code java [highlight_lines_extra="2"]>
 +public class TlaPlus {
 +  private static final Interpreter interpreter = new Interpreter();
 +  static boolean hadError = false;
 +</code>
 +
 +Then finish it off by actually interpreting the expression given by the user; add this in the ''run()'' method in the ''TlaPlus'' class:
 +
 +<code java [highlight_lines_extra="4"]>
 +    // Stop if there was a syntax error.
 +    if (hadError) return;
 +
 +    interpreter.interpret(expression);
 +  }
 +</code>
 +
 +Voila!
 +Your program will now interpret any constant TLA⁺ expression you provide it!
 +Try it out:
 +<code>
 +> {0 .. 2, IF TRUE THEN 1 ELSE 2, {}}
 +[[], 1, [0, 1, 2]]
 +</code>
 +
 +If you got out of sync, you can find a snapshot of the expected state of the code in [[https://github.com/tlaplus-community/tlaplus-creator/tree/main/3-expressions|this repo directory]].
 +Next tutorial: [[https://docs.tlapl.us/creating:statements|Handle TLA⁺ Statements]]!
 +
 +====== Challenges ======
 +
 +Here are some optional challenges to flesh out your TLA⁺ interpreter, roughly ranked from simplest to most difficult.
 +You should save a copy of your code before attempting these.
 +  - TLC evaluates cross-type equality comparison as a runtime error. For example, ''123 = TRUE'' evaluates to ''false'' in our interpreter but will throw an exception in TLC. Modify your interpreter to match the behavior of TLC.
 +  - TLC requires set elements to all be of the same type. Trying to construct the set ''{1, 2, TRUE}'' results in a TLC runtime error. Similarly, TLC only allows nested sets if //all// elements are similarly nested and of the same type: ''{1, {2}}'' is not allowed. Modify your interpreter to match the behavior of TLC. Use the TLC REPL to check which set constructions are allowed or disallowed. Checking whether an element of the wrong type is part of a set using the ''\in'' operator also should result in a runtime error.
 +
 +[[https://docs.tlapl.us/creating:scanning|< Previous Page]] | [[https://docs.tlapl.us/creating:statements|Next Page >]]
 +
  • creating/expressions.1743969525.txt.gz
  • Last modified: 2025/04/06 19:58
  • by ahelwer