golox/resolver/resolver.go

package resolver

import (
	"golox/ast"
	"golox/errors"
	"golox/interpreter"
	"golox/token"
)

// FuncType represents the type of a function.
type FuncType int

const (
	// NoneFuncType represents a function with no type.
	NoneFuncType FuncType = iota
	// FunctionFuncType represents a function.
	FunctionFuncType
	// MethodFuncType represents a method.
	MethodFuncType
	// InitializerFuncType represents an initializer.
	InitializerFuncType
)

// Resolver implements variable resolution for the AST.
type Resolver struct {
	errLogger   errors.Logger
	interpreter *interpreter.Interpreter
	scopes      []map[string]bool
	currFunc    FuncType
}

// New creates a new Resolver.
func New(i *interpreter.Interpreter, el errors.Logger) *Resolver {
	return &Resolver{el, i, []map[string]bool{}, NoneFuncType}
}

// VisitBlockStmt visits a block statement.
func (r *Resolver) VisitBlockStmt(bs *ast.BlockStmt) any {
	r.beginScope()
	r.Resolve(bs.Statements)
	r.endScope()
	return nil
}

// Resolve resolves the variables for each statements in stmts.
func (r *Resolver) Resolve(stmts []ast.Stmt) {
	for _, stmt := range stmts {
		r.resolveStmt(stmt)
	}
}

// resolveStmt resolves a statement.
func (r *Resolver) resolveStmt(stmt ast.Stmt) {
	stmt.Accept(r)
}

// resolveExpr resolves an expression.
func (r *Resolver) resolveExpr(expr ast.Expr) {
	expr.Accept(r)
}

// beginScope begins a new scope.
func (r *Resolver) beginScope() {
	r.scopes = append(r.scopes, map[string]bool{})
}

// endScope ends the current scope.
func (r *Resolver) endScope() {
	r.scopes = r.scopes[:len(r.scopes)-1]
}

// visitVarStmt visits a variable statement.
func (r *Resolver) VisitVarStmt(vs *ast.VarStmt) any {
	r.declare(&vs.Name)
	if vs.Initializer != nil {
		r.resolveExpr(vs.Initializer)
	}
	r.define(&vs.Name)

	return nil
}

// declare declares a variable in the current scope.
func (r *Resolver) declare(name *token.Token) {
	if len(r.scopes) == 0 {
		return
	}

	scope := r.scopes[len(r.scopes)-1]
	if _, ok := scope[name.Lexeme]; ok {
		r.errLogger.ErrorAtToken(*name, "Variable with this name already declared in this scope.")
	}
	scope[name.Lexeme] = false
}

// define defines a variable in the current scope.
func (r *Resolver) define(name *token.Token) {
	if len(r.scopes) == 0 {
		return
	}

	r.scopes[len(r.scopes)-1][name.Lexeme] = true
}

// VisitVariableExpr visits a variable expression.
func (r *Resolver) VisitVariableExpr(ve *ast.VariableExpr) any {
	if len(r.scopes) > 0 {
		if _, ok := r.scopes[len(r.scopes)-1][ve.Name.Lexeme]; ok && !r.scopes[len(r.scopes)-1][ve.Name.Lexeme] {
			r.errLogger.ErrorAtToken(ve.Name, "Cannot read local variable in its own initializer.")
		}
	}

	r.resolveLocal(ve, &ve.Name)

	return nil
}

// resolveLocal resolves a local variable.
func (r *Resolver) resolveLocal(expr ast.Expr, name *token.Token) {
	for i := len(r.scopes) - 1; i >= 0; i-- {
		if _, ok := r.scopes[i][name.Lexeme]; ok {
			r.interpreter.Resolve(expr, len(r.scopes)-1-i)
			return
		}
	}
}

// VisitAssignExpr visits an assignment expression.
func (r *Resolver) VisitAssignExpr(ae *ast.AssignExpr) any {
	r.resolveExpr(ae.Value)
	r.resolveLocal(ae, &ae.Name)

	return nil
}

// VisitFunctionStmt visits a function statement.
func (r *Resolver) VisitFunctionStmt(fs *ast.FunctionStmt) any {
	r.declare(&fs.Name)
	r.define(&fs.Name)

	r.resolveFunction(fs, FunctionFuncType)

	return nil
}

// resolveFunction resolves a function.
func (r *Resolver) resolveFunction(fs *ast.FunctionStmt, t FuncType) {
	enclosingFunc := r.currFunc
	r.currFunc = t

	r.beginScope()
	for _, param := range fs.Params {
		r.declare(&param)
		r.define(&param)
	}

	r.Resolve(fs.Body)
	r.endScope()

	r.currFunc = enclosingFunc
}

// VisitExpressionStmt visits an expression statement.
func (r *Resolver) VisitExpressionStmt(es *ast.ExpressionStmt) any {
	r.resolveExpr(es.Expression)
	return nil
}

// VisitIfStmt visits an if statement.
func (r *Resolver) VisitIfStmt(is *ast.IfStmt) any {
	r.resolveExpr(is.Condition)
	r.resolveStmt(is.ThenBranch)
	if is.ElseBranch != nil {
		r.resolveStmt(is.ElseBranch)
	}

	return nil
}

// VisitPrintStmt visits a print statement.
func (r *Resolver) VisitPrintStmt(ps *ast.PrintStmt) any {
	r.resolveExpr(ps.Expression)

	return nil
}

// VisitReturnStmt visits a return statement.
func (r *Resolver) VisitReturnStmt(rs *ast.ReturnStmt) any {
	if r.currFunc == NoneFuncType {
		r.errLogger.ErrorAtToken(rs.Keyword, "Cannot return from top-level code.")
	}

	if rs.Value != nil {
		r.resolveExpr(rs.Value)
	}

	return nil
}

// VisitWhileStmt visits a while statement.
func (r *Resolver) VisitWhileStmt(ws *ast.WhileStmt) any {
	r.resolveExpr(ws.Condition)
	r.resolveStmt(ws.Body)

	return nil
}

// VisitErrorStmt visits an error statement.
func (r *Resolver) VisitErrorStmt(es *ast.ErrorStmt) any {
	return nil
}

// VisitBinaryExpr visits a binary expression.
func (r *Resolver) VisitBinaryExpr(be *ast.BinaryExpr) any {
	r.resolveExpr(be.Left)
	r.resolveExpr(be.Right)

	return nil
}

// VisitCallExpr visits a call expression.
func (r *Resolver) VisitCallExpr(ce *ast.CallExpr) any {
	r.resolveExpr(ce.Callee)
	for _, arg := range ce.Arguments {
		r.resolveExpr(arg)
	}

	return nil
}

// VisitGroupingExpr visits a grouping expression.
func (r *Resolver) VisitGroupingExpr(ge *ast.GroupingExpr) any {
	r.resolveExpr(ge.Expression)

	return nil
}

// VisitLiteralExpr visits a literal expression.
func (r *Resolver) VisitLiteralExpr(le *ast.LiteralExpr) any {
	return nil
}

// VisitLogicalExpr visits a logical expression.
func (r *Resolver) VisitLogicalExpr(le *ast.LogicalExpr) any {
	r.resolveExpr(le.Left)
	r.resolveExpr(le.Right)

	return nil
}

// VisitUnaryExpr visits a unary expression.
func (r *Resolver) VisitUnaryExpr(ue *ast.UnaryExpr) any {
	r.resolveExpr(ue.Right)

	return nil
}

// VisitErrorExpr visits an error statement.
func (r *Resolver) VisitErrorExpr(ee *ast.ErrorExpr) any {
	return nil
}