Decouple parse & resolve steps (#148)

This separates variables resolution (the resolver) from the parser.
It also adds an ast.Walk() method to generically walk a program's AST.

internal/ast/ast.go

@@ -13,11 +13,17 @@ import (
 // Program is an entire AWK program.
 type Program struct {
 	Begin     []Stmts
-	Actions   []Action
+	Actions   []*Action
 	End       []Stmts
-	Functions []Function
-	Scalars   map[string]int
-	Arrays    map[string]int
+	Functions []*Function
+}
+
+// ResolvedProgram is a parsed AWK program + additional data prepared by resolve step
+// needed for subsequent interpretation
+type ResolvedProgram struct {
+	Program
+	Scalars map[string]int
+	Arrays  map[string]int
 }
 
 // String returns an indented, pretty-printed version of the parsed
@@ -75,8 +81,54 @@ func (a *Action) String() string {
 	return strings.Join(patterns, ", ") + sep + stmtsStr
 }
 
+// Node is an interface to be satisfied by all AST elements.
+// We need it to be able to work with AST in a generic way, like in ast.Walk().
+type Node interface {
+	node()
+}
+
+// All these types implement the Node interface.
+func (p *Program) node()        {}
+func (a *Action) node()         {}
+func (f *Function) node()       {}
+func (e *FieldExpr) node()      {}
+func (e *NamedFieldExpr) node() {}
+func (e *UnaryExpr) node()      {}
+func (e *BinaryExpr) node()     {}
+func (e *ArrayExpr) node()      {}
+func (e *InExpr) node()         {}
+func (e *CondExpr) node()       {}
+func (e *NumExpr) node()        {}
+func (e *StrExpr) node()        {}
+func (e *RegExpr) node()        {}
+func (e *VarExpr) node()        {}
+func (e *IndexExpr) node()      {}
+func (e *AssignExpr) node()     {}
+func (e *AugAssignExpr) node()  {}
+func (e *IncrExpr) node()       {}
+func (e *CallExpr) node()       {}
+func (e *UserCallExpr) node()   {}
+func (e *MultiExpr) node()      {}
+func (e *GetlineExpr) node()    {}
+func (s *PrintStmt) node()      {}
+func (s *PrintfStmt) node()     {}
+func (s *ExprStmt) node()       {}
+func (s *IfStmt) node()         {}
+func (s *ForStmt) node()        {}
+func (s *ForInStmt) node()      {}
+func (s *WhileStmt) node()      {}
+func (s *DoWhileStmt) node()    {}
+func (s *BreakStmt) node()      {}
+func (s *ContinueStmt) node()   {}
+func (s *NextStmt) node()       {}
+func (s *ExitStmt) node()       {}
+func (s *DeleteStmt) node()     {}
+func (s *ReturnStmt) node()     {}
+func (s *BlockStmt) node()      {}
+
 // Expr is the abstract syntax tree for any AWK expression.
 type Expr interface {
+	Node
 	expr()
 	String() string
 }
@@ -154,6 +206,7 @@ type ArrayExpr struct {
 	Scope VarScope
 	Index int
 	Name  string
+	Pos   Position
 }
 
 func (e *ArrayExpr) String() string {
@@ -221,6 +274,9 @@ func (e *RegExpr) String() string {
 	return "/" + escaped + "/"
 }
 
+// meaning it will be set during resolve step
+const resolvedLater = -1
+
 type VarScope int
 
 const (
@@ -236,6 +292,7 @@ type VarExpr struct {
 	Scope VarScope
 	Index int
 	Name  string
+	Pos   Position
 }
 
 func (e *VarExpr) String() string {
@@ -315,6 +372,7 @@ type UserCallExpr struct {
 	Index  int
 	Name   string
 	Args   []Expr
+	Pos    Position
 }
 
 func (e *UserCallExpr) String() string {
@@ -375,6 +433,7 @@ func IsLValue(expr Expr) bool {
 
 // Stmt is the abstract syntax tree for any AWK statement.
 type Stmt interface {
+	Node
 	stmt()
 	String() string
 }
@@ -585,6 +644,7 @@ type Function struct {
 	Params []string
 	Arrays []bool
 	Body   Stmts
+	Pos    Position
 }
 
 func (f *Function) String() string {
@@ -598,3 +658,38 @@ func trimParens(s string) string {
 	}
 	return s
 }
+
+// VarRef is a constructor for *VarExpr
+func VarRef(name string, pos Position) *VarExpr {
+	return &VarExpr{resolvedLater, resolvedLater, name, pos}
+}
+
+// ArrayRef is a constructor for *ArrayExpr
+func ArrayRef(name string, pos Position) *ArrayExpr {
+	return &ArrayExpr{resolvedLater, resolvedLater, name, pos}
+}
+
+// UserCall is a constructor for *UserCallExpr
+func UserCall(name string, args []Expr, pos Position) *UserCallExpr {
+	return &UserCallExpr{false, resolvedLater, name, args, pos}
+}
+
+// PositionError represents an error bound to specific position in source.
+type PositionError struct {
+	// Source line/column position where the error occurred.
+	Position Position
+	// Error message.
+	Message string
+}
+
+// PosErrorf like fmt.Errorf, but with an explicit position.
+func PosErrorf(pos Position, format string, args ...interface{}) error {
+	message := fmt.Sprintf(format, args...)
+	return &PositionError{pos, message}
+}
+
+// Error returns a formatted version of the error, including the line
+// and column numbers.
+func (e *PositionError) Error() string {
+	return fmt.Sprintf("parse error at %d:%d: %s", e.Position.Line, e.Position.Column, e.Message)
+}

internal/ast/walk.go

@@ -0,0 +1,180 @@
+package ast
+
+import "fmt"
+
+// A Visitor's Visit method is invoked for each node encountered by Walk.
+// If the result visitor w is not nil, Walk visits each of the children
+// of node with the visitor w, followed by a call of w.Visit(nil).
+type Visitor interface {
+	Visit(node Node) (w Visitor)
+}
+
+// WalkExprList walks a visitor over a list of expression AST nodes
+func WalkExprList(v Visitor, exprs []Expr) {
+	for _, expr := range exprs {
+		Walk(v, expr)
+	}
+}
+
+// WalkStmtList walks a visitor over a list of statement AST nodes
+func WalkStmtList(v Visitor, stmts []Stmt) {
+	for _, stmt := range stmts {
+		Walk(v, stmt)
+	}
+}
+
+// Walk traverses an AST in depth-first order: It starts by calling
+// v.Visit(node); if node is nil, it does nothing. If the visitor w returned by
+// v.Visit(node) is not nil, Walk is invoked recursively with visitor
+// w for each of the non-nil children of node, followed by a call of
+// w.Visit(nil).
+//
+func Walk(v Visitor, node Node) {
+	if node == nil {
+		return
+	}
+	if v = v.Visit(node); v == nil {
+		return
+	}
+
+	// walk children
+	// (the order of the cases matches the order
+	// of the corresponding node types in ast.go)
+	switch n := node.(type) {
+
+	// expressions
+	case *FieldExpr:
+		Walk(v, n.Index)
+
+	case *NamedFieldExpr:
+		Walk(v, n.Field)
+
+	case *UnaryExpr:
+		Walk(v, n.Value)
+
+	case *BinaryExpr:
+		Walk(v, n.Left)
+		Walk(v, n.Right)
+
+	case *ArrayExpr: // leaf
+	case *InExpr:
+		WalkExprList(v, n.Index)
+		Walk(v, n.Array)
+
+	case *CondExpr:
+		Walk(v, n.Cond)
+		Walk(v, n.True)
+		Walk(v, n.False)
+
+	case *NumExpr: // leaf
+	case *StrExpr: // leaf
+	case *RegExpr: // leaf
+	case *VarExpr: // leaf
+	case *IndexExpr:
+		Walk(v, n.Array)
+		WalkExprList(v, n.Index)
+
+	case *AssignExpr:
+		Walk(v, n.Left)
+		Walk(v, n.Right)
+
+	case *AugAssignExpr:
+		Walk(v, n.Left)
+		Walk(v, n.Right)
+
+	case *IncrExpr:
+		Walk(v, n.Expr)
+
+	case *CallExpr:
+		WalkExprList(v, n.Args)
+
+	case *UserCallExpr:
+		WalkExprList(v, n.Args)
+
+	case *MultiExpr:
+		WalkExprList(v, n.Exprs)
+
+	case *GetlineExpr:
+		Walk(v, n.Command)
+		Walk(v, n.Target)
+		Walk(v, n.File)
+
+	// statements
+	case *PrintStmt:
+		WalkExprList(v, n.Args)
+		Walk(v, n.Dest)
+
+	case *PrintfStmt:
+		WalkExprList(v, n.Args)
+		Walk(v, n.Dest)
+
+	case *ExprStmt:
+		Walk(v, n.Expr)
+
+	case *IfStmt:
+		Walk(v, n.Cond)
+		WalkStmtList(v, n.Body)
+		WalkStmtList(v, n.Else)
+
+	case *ForStmt:
+		Walk(v, n.Pre)
+		Walk(v, n.Cond)
+		Walk(v, n.Post)
+		WalkStmtList(v, n.Body)
+
+	case *ForInStmt:
+		Walk(v, n.Var)
+		Walk(v, n.Array)
+		WalkStmtList(v, n.Body)
+
+	case *WhileStmt:
+		Walk(v, n.Cond)
+		WalkStmtList(v, n.Body)
+
+	case *DoWhileStmt:
+		WalkStmtList(v, n.Body)
+		Walk(v, n.Cond)
+
+	case *BreakStmt: // leaf
+	case *ContinueStmt: // leaf
+	case *NextStmt: // leaf
+	case *ExitStmt:
+		Walk(v, n.Status)
+
+	case *DeleteStmt:
+		Walk(v, n.Array)
+		WalkExprList(v, n.Index)
+
+	case *ReturnStmt:
+		Walk(v, n.Value)
+
+	case *BlockStmt:
+		WalkStmtList(v, n.Body)
+
+	case *Program:
+		for _, stmts := range n.Begin {
+			WalkStmtList(v, stmts)
+		}
+		for _, action := range n.Actions {
+			Walk(v, action)
+		}
+		for _, function := range n.Functions {
+			Walk(v, function)
+		}
+		for _, stmts := range n.End {
+			WalkStmtList(v, stmts)
+		}
+
+	case *Action:
+		WalkExprList(v, n.Pattern)
+		WalkStmtList(v, n.Stmts)
+
+	case *Function:
+		WalkStmtList(v, n.Body)
+
+	default:
+		panic(fmt.Sprintf("ast.Walk: unexpected node type %T", n))
+	}
+
+	v.Visit(nil)
+}

internal/compiler/compiler.go

@@ -54,7 +54,7 @@ func (e *compileError) Error() string {
 }
 
 // Compile compiles an AST (parsed program) into virtual machine instructions.
-func Compile(prog *ast.Program) (compiledProg *Program, err error) {
+func Compile(prog *ast.ResolvedProgram) (compiledProg *Program, err error) {
 	defer func() {
 		// The compiler uses panic with a *compileError to signal compile
 		// errors internally, and they're caught here. This avoids the