golox/interpreter/interpreter.go

package interpreter

import (
	"fmt"
	"golox/ast"
	"golox/errors"
	"golox/token"
	"strings"
)

// Interpreter interprets the AST.
type Interpreter struct {
	errLogger errors.Logger
	env       *environment
}

// New creates a new Interpreter.
func New(el errors.Logger) *Interpreter {
	return &Interpreter{el, newEnvironment(nil)}
}

// Interpret interprets the AST.
func (i *Interpreter) Interpret(stmts []ast.Stmt) {
	defer i.afterPanic()

	for _, stmt := range stmts {
		i.execute(stmt)
	}
}

// VisitErrorStmt visits an error statement.
func (i *Interpreter) VisitErrorStmt(es *ast.ErrorStmt) any {
	panic(es.Value)
}

// VisitExpressionStmt visits an expression.
func (i *Interpreter) VisitExpressionStmt(es *ast.ExpressionStmt) any {
	i.evaluate(es.Expression)
	return nil
}

// VisitIfStmt visits an if statement.
func (i *Interpreter) VisitIfStmt(is *ast.IfStmt) any {
	if isTruthy(i.evaluate(is.Condition)) {
		i.execute(is.ThenBranch)
	} else if is.ElseBranch != nil {
		i.execute(is.ElseBranch)
	}

	return nil
}

// VisitPrintStmt visits a print statement.
func (i *Interpreter) VisitPrintStmt(ps *ast.PrintStmt) any {
	value := i.evaluate(ps.Expression)
	fmt.Println(stringify(value))
	return nil
}

// VisitWhileStmt visits a while statement.
func (i *Interpreter) VisitWhileStmt(ws *ast.WhileStmt) any {
	for isTruthy(i.evaluate(ws.Condition)) {
		i.execute(ws.Body)
	}

	return nil
}

// VisitVarStmt visits a var statement.
func (i *Interpreter) VisitVarStmt(vs *ast.VarStmt) any {
	var value any
	if vs.Initializer != nil {
		value = i.evaluate(vs.Initializer)
	}

	i.env.define(vs.Name.Lexeme, value)
	return nil
}

// VisitBlockStmt visits a block statement.
func (i *Interpreter) VisitBlockStmt(bs *ast.BlockStmt) any {
	i.executeBlock(bs.Statements, newEnvironment(i.env))
	return nil
}

// Interpret interprets the AST.
func (i *Interpreter) InterpretExpr(expr ast.Expr) string {
	defer i.afterPanic()

	value := i.evaluate(expr)
	return stringify(value)
}

// VisitErrorExpr visits an ErrorExpr.
func (i *Interpreter) VisitErrorExpr(e *ast.ErrorExpr) any {
	panic(e.Value)
}

// VisitAssignExpr visits an AssignExpr.
func (i *Interpreter) VisitAssignExpr(a *ast.AssignExpr) any {
	value := i.evaluate(a.Value)
	i.env.assign(a.Name.Lexeme, value)
	return value
}

// VisitLiteralExpr visits a LiteralExpr.
func (i *Interpreter) VisitLiteralExpr(l *ast.LiteralExpr) any {
	return l.Value
}

// VisitLogicalExpr visits a LogicalExpr.
func (i *Interpreter) VisitLogicalExpr(l *ast.LogicalExpr) any {
	left := i.evaluate(l.Left)

	if l.Operator.Type == token.OR {
		if isTruthy(left) {
			return left
		}
	} else {
		if !isTruthy(left) {
			return left
		}
	}

	return i.evaluate(l.Right)
}

// VisitGroupingExpr visits a GroupingExpr.
func (i *Interpreter) VisitGroupingExpr(g *ast.GroupingExpr) any {
	return i.evaluate(g.Expression)
}

// VisitUnaryExpr visits a UnaryExpr.
func (i *Interpreter) VisitUnaryExpr(u *ast.UnaryExpr) any {
	right := i.evaluate(u.Right)

	switch u.Operator.Type {
	case token.MINUS:
		checkNumberOperands(u.Operator, right)
		return -right.(float64)
	case token.BANG:
		return !isTruthy(right)
	}

	return nil
}

// VisitBinaryExpr visits a BinaryExpr.
func (i *Interpreter) VisitBinaryExpr(b *ast.BinaryExpr) any {
	left := i.evaluate(b.Left)
	right := i.evaluate(b.Right)

	switch b.Operator.Type {
	case token.MINUS:
		checkNumberOperands(b.Operator, left, right)
		return left.(float64) - right.(float64)
	case token.SLASH:
		checkNumberOperands(b.Operator, left, right)
		denominator := right.(float64)
		if denominator == 0.0 {
			panic(fmt.Sprintf("Division by zero [line %d]", b.Operator.Line))
		}
		return left.(float64) / denominator
	case token.STAR:
		checkNumberOperands(b.Operator, left, right)
		return left.(float64) * right.(float64)
	case token.PLUS:
		if l, ok := left.(float64); ok {
			if r, ok := right.(float64); ok {
				return l + r
			}
		}

		if l, ok := left.(string); ok {
			if r, ok := right.(string); ok {
				return l + r
			}
		}

		panic(fmt.Sprintf("Operands must be two numbers or two strings [line %d]", b.Operator.Line))
	case token.GREATER:
		checkNumberOperands(b.Operator, left, right)
		return left.(float64) > right.(float64)
	case token.GREATER_EQUAL:
		checkNumberOperands(b.Operator, left, right)
		return left.(float64) >= right.(float64)
	case token.LESS:
		checkNumberOperands(b.Operator, left, right)
		return left.(float64) < right.(float64)
	case token.LESS_EQUAL:
		checkNumberOperands(b.Operator, left, right)
		return left.(float64) <= right.(float64)
	case token.BANG_EQUAL:
		return !isEqual(left, right)
	case token.EQUAL_EQUAL:
		return isEqual(left, right)
	}

	panic(fmt.Sprintf("Unknown binary operator '%s' [line %d]", b.Operator.Lexeme, b.Operator.Line))
}

// VisitVariableExpr visits a VariableExpr.
func (i *Interpreter) VisitVariableExpr(v *ast.VariableExpr) any {
	return i.env.get(v.Name.Lexeme)
}

// checkNumberOperands checks if the operands are numbers.
func checkNumberOperands(operator token.Token, operands ...any) {
	for _, operand := range operands {
		if _, ok := operand.(float64); !ok {
			panic(fmt.Sprintf("Operands of operator '%s' must be numbers [line %d]", operator.Lexeme, operator.Line))
		}
	}
}

// isTruthy checks if a value is truthy.
func isTruthy(v any) bool {
	if v == nil {
		return false
	}

	if b, ok := v.(bool); ok {
		return b
	}

	return true
}

// isEqual checks if two values are equal.
func isEqual(a, b any) bool {
	if a == nil && b == nil {
		return true
	}

	if a == nil {
		return false
	}

	return a == b
}

// evaluate evaluates an expression.
func (i *Interpreter) evaluate(e ast.Expr) any {
	return e.Accept(i)
}

// execute executes a statement.
func (i *Interpreter) execute(s ast.Stmt) {
	s.Accept(i)
}

// executeBlock executes a block of statements.
func (i *Interpreter) executeBlock(stmts []ast.Stmt, env *environment) {
	previous := i.env
	defer func() {
		i.env = previous
	}()

	i.env = env

	for _, stmt := range stmts {
		i.execute(stmt)
	}
}

// stringify returns a string representation of a value.
func stringify(v any) string {
	if v == nil {
		return "nil"
	}

	if b, ok := v.(bool); ok {
		if b {
			return "true"
		}

		return "false"
	}

	s := fmt.Sprintf("%v", v)

	if f, ok := v.(float64); ok {
		if strings.HasSuffix(s, ".0") {
			return fmt.Sprintf("%d", int(f))
		}

		return fmt.Sprintf("%g", f)
	}

	return s
}

// afterPanic handles a panic.
func (i *Interpreter) afterPanic() {
	if r := recover(); r != nil {
		i.errLogger.RuntimeError(r.(string))
	}
}