Basic AST to represent expressions, statement, identifiers

I now understand how it works.

git-svn-id: https://svn.tlawal.org/svn/monkey@12 f6afcba9-9ef1-4bdd-9b72-7484f5705bac

ast/ast.go

@@ -0,0 +1,52 @@
+package ast
+
+import "monkey/token"
+
+type Node interface {
+	TokeLiteral() string
+}
+
+type Statement interface {
+	Node
+	statement_node()
+}
+
+type Expression interface {
+	Node
+	expression_node()
+}
+
+type Program struct {
+	Statements []Statement
+}
+
+type Identifier struct {
+	Token token.Token // the token.IDENT token
+	Value string
+}
+
+type LetStatement struct {
+	Token token.Token // the token.LET token
+	Name  *Identifier
+	Value Expression
+}
+
+func (ls *LetStatement) statement_node() {}
+
+func (ls *LetStatement) TokenLiteral() string {
+	return ls.Token.Literal
+}
+
+func (i *Identifier) expression_node() {}
+
+func (i *Identifier) TokenLiteral() string {
+	return i.Token.Literal
+}
+
+func (p *Program) TokenLiteral() string {
+	if len(p.Statements) > 0 {
+		return p.Statements[0].TokenLiteral()
+	} else {
+		return ""
+	}
+}