How go/ast represents syntax tree

ast.go

// source: https://github.com/golang/go/blob/d54b60a2b2470ea42559b58995e86ff20dd70471/src/go/ast/ast.go

type Node interface {
	Pos() token.Pos
	End() token.Pos
}

type Expr interface {
	Node
	exprNode()
}

type Stmt interface {
	Node
	stmtNode()
}

// Pos and End implementations for expression/type nodes.
func (x *BadExpr) Pos() token.Pos  { return x.From }
func (x *Ident) Pos() token.Pos    { return x.NamePos }
// ...

// exprNode() ensures that only expression/type nodes can be
// assigned to an Expr.
//
func (*BadExpr) exprNode()        {}
func (*Ident) exprNode()          {}
// ...

// stmtNode() ensures that only statement nodes can be
// assigned to a Stmt.
//
func (*BadStmt) stmtNode()        {}
func (*DeclStmt) stmtNode()       {}
// ...