Re-organize helper functions

ast/pos.go

@@ -4,6 +4,51 @@ import (
 	"github.com/cloudspannerecosystem/memefish/token"
 )
 
+// ================================================================================
+//
+// Helper functions for Pos(), End()
+// These functions are intended for use within this file only.
+//
+// ================================================================================
+
+// lastNode returns last element of Node slice.
+// This function corresponds to NodeSliceVar[$] in ast.go.
+func lastNode[T Node](s []T) T {
+	return s[len(s)-1]
+}
+
+// firstValidEnd returns the first valid Pos() in argument.
+// "valid" means the node is not nil and Pos().Invalid() is not true.
+// This function corresponds to "(n0 ?? n1 ?? ...).End()"
+func firstValidEnd(ns ...Node) token.Pos {
+	for _, n := range ns {
+		if n != nil && !n.End().Invalid() {
+			return n.End()
+		}
+	}
+	return token.InvalidPos
+}
+
+// firstPos returns the Pos() of the first node.
+// If argument is an empty slice, this function returns token.InvalidPos.
+// This function corresponds to NodeSliceVar[0].pos in ast.go.
+func firstPos[T Node](s []T) token.Pos {
+	if len(s) == 0 {
+		return token.InvalidPos
+	}
+	return s[0].Pos()
+}
+
+// lastEnd returns the End() of the last node.
+// If argument is an empty slice, this function returns token.InvalidPos.
+// This function corresponds to NodeSliceVar[$].end in ast.go.
+func lastEnd[T Node](s []T) token.Pos {
+	if len(s) == 0 {
+		return token.InvalidPos
+	}
+	return lastNode(s).End()
+}
+
 // ================================================================================
 //
 // SELECT
@@ -39,7 +84,7 @@ func (c *CTE) End() token.Pos { return c.Rparen + 1 }
 func (s *Select) Pos() token.Pos { return s.Select }
 
 func (s *Select) End() token.Pos {
-	return firstValidEnd(s.Limit, s.OrderBy, s.Having, s.GroupBy, s.Where, s.From, lastElem(s.Results))
+	return firstValidEnd(s.Limit, s.OrderBy, s.Having, s.GroupBy, s.Where, s.From, lastNode(s.Results))
 }
 
 func (c *CompoundQuery) Pos() token.Pos {

ast/sql.go

@@ -2,8 +2,59 @@ package ast
 
 import (
 	"github.com/cloudspannerecosystem/memefish/token"
+	"strings"
 )
 
+// ================================================================================
+//
+// Helper functions for SQL()
+// These functions are intended for use within this file only.
+//
+// ================================================================================
+
+// sqlOpt outputs:
+//
+//	when node != nil: left + node.SQL() + right
+//	else            : empty string
+//
+// This function corresponds to sqlOpt in ast.go
+func sqlOpt[T interface {
+	Node
+	comparable
+}](left string, node T, right string) string {
+	var zero T
+	if node == zero {
+		return ""
+	}
+	return left + node.SQL() + right
+}
+
+// strOpt outputs:
+//
+//	when pred == true: s
+//	else            : empty string
+//
+// This function corresponds to {{if pred}}s{{end}} in ast.go
+func strOpt(pred bool, s string) string {
+	if pred {
+		return s
+	}
+	return ""
+}
+
+// sqlJoin outputs joined string of SQL() of all elems by sep.
+// This function corresponds to sqlJoin in ast.go
+func sqlJoin[T Node](elems []T, sep string) string {
+	var b strings.Builder
+	for i, r := range elems {
+		if i > 0 {
+			b.WriteString(sep)
+		}
+		b.WriteString(r.SQL())
+	}
+	return b.String()
+}
+
 type prec int
 
 const (