refactor: simplify criteria Expression interface

Replaced the Fields() type switch with a fields() method on the
Expression interface, eliminating the need to update a central switch
when adding new expression types. Removed the now-redundant
criteriaExpression() marker method since fields() alone suffices to
restrict the interface. Extracted a conjunction interface for the
ChildPlaylistIds() lookup used by All and Any.

model/criteria/criteria.go

@@ -9,7 +9,7 @@ import (
 )
 
 type Expression interface {
-	criteriaExpression()
+	fields() map[string]any
 }
 
 type Criteria struct {
@@ -53,7 +53,7 @@ func (c Criteria) ChildPlaylistIds() []string {
 		return nil
 	}
 
-	if parent, ok := c.Expression.(interface{ ChildPlaylistIds() (ids []string) }); ok {
+	if parent, ok := c.Expression.(conjunction); ok {
 		return parent.ChildPlaylistIds()
 	}
 

model/criteria/operators.go

@@ -2,12 +2,17 @@ package criteria
 
 import "time"
 
+// Conjunctions need to implement this interface, to allow Criteria to extract child playlist IDs recursively
+type conjunction interface {
+	ChildPlaylistIds() []string
+}
+
 type (
 	All []Expression
 	And = All
 )
 
-func (All) criteriaExpression() {}
+func (All) fields() map[string]any { return nil }
 
 func (all All) MarshalJSON() ([]byte, error) {
 	return marshalConjunction("all", all)
@@ -22,7 +27,7 @@ type (
 	Or  = Any
 )
 
-func (Any) criteriaExpression() {}
+func (Any) fields() map[string]any { return nil }
 
 func (any Any) MarshalJSON() ([]byte, error) {
 	return marshalConjunction("any", any)
@@ -35,128 +40,128 @@ func (any Any) ChildPlaylistIds() (ids []string) {
 type Is map[string]any
 type Eq = Is
 
-func (Is) criteriaExpression() {}
-
 func (is Is) MarshalJSON() ([]byte, error) {
 	return marshalExpression("is", is)
 }
 
+func (is Is) fields() map[string]any { return is }
+
 type IsNot map[string]any
 
-func (IsNot) criteriaExpression() {}
-
-func (in IsNot) MarshalJSON() ([]byte, error) {
-	return marshalExpression("isNot", in)
+func (isn IsNot) MarshalJSON() ([]byte, error) {
+	return marshalExpression("isNot", isn)
 }
 
-type Gt map[string]any
+func (isn IsNot) fields() map[string]any { return isn }
 
-func (Gt) criteriaExpression() {}
+type Gt map[string]any
 
 func (gt Gt) MarshalJSON() ([]byte, error) {
 	return marshalExpression("gt", gt)
 }
 
-type Lt map[string]any
+func (gt Gt) fields() map[string]any { return gt }
 
-func (Lt) criteriaExpression() {}
+type Lt map[string]any
 
 func (lt Lt) MarshalJSON() ([]byte, error) {
 	return marshalExpression("lt", lt)
 }
 
-type Before map[string]any
+func (lt Lt) fields() map[string]any { return lt }
 
-func (Before) criteriaExpression() {}
+type Before map[string]any
 
 func (bf Before) MarshalJSON() ([]byte, error) {
 	return marshalExpression("before", bf)
 }
 
-type After Gt
+func (bf Before) fields() map[string]any { return bf }
 
-func (After) criteriaExpression() {}
+type After Gt
 
 func (af After) MarshalJSON() ([]byte, error) {
 	return marshalExpression("after", af)
 }
 
-type Contains map[string]any
+func (af After) fields() map[string]any { return af }
 
-func (Contains) criteriaExpression() {}
+type Contains map[string]any
 
 func (ct Contains) MarshalJSON() ([]byte, error) {
 	return marshalExpression("contains", ct)
 }
 
-type NotContains map[string]any
+func (ct Contains) fields() map[string]any { return ct }
 
-func (NotContains) criteriaExpression() {}
+type NotContains map[string]any
 
 func (nct NotContains) MarshalJSON() ([]byte, error) {
 	return marshalExpression("notContains", nct)
 }
 
-type StartsWith map[string]any
+func (nct NotContains) fields() map[string]any { return nct }
 
-func (StartsWith) criteriaExpression() {}
+type StartsWith map[string]any
 
 func (sw StartsWith) MarshalJSON() ([]byte, error) {
 	return marshalExpression("startsWith", sw)
 }
 
+func (sw StartsWith) fields() map[string]any { return sw }
+
 type EndsWith map[string]any
 
-func (EndsWith) criteriaExpression() {}
-
-func (sw EndsWith) MarshalJSON() ([]byte, error) {
-	return marshalExpression("endsWith", sw)
+func (ew EndsWith) MarshalJSON() ([]byte, error) {
+	return marshalExpression("endsWith", ew)
 }
 
-type InTheRange map[string]any
+func (ew EndsWith) fields() map[string]any { return ew }
 
-func (InTheRange) criteriaExpression() {}
+type InTheRange map[string]any
 
 func (itr InTheRange) MarshalJSON() ([]byte, error) {
 	return marshalExpression("inTheRange", itr)
 }
 
-type InTheLast map[string]any
+func (itr InTheRange) fields() map[string]any { return itr }
 
-func (InTheLast) criteriaExpression() {}
+type InTheLast map[string]any
 
 func (itl InTheLast) MarshalJSON() ([]byte, error) {
 	return marshalExpression("inTheLast", itl)
 }
 
-type NotInTheLast map[string]any
+func (itl InTheLast) fields() map[string]any { return itl }
 
-func (NotInTheLast) criteriaExpression() {}
+type NotInTheLast map[string]any
 
 func (nitl NotInTheLast) MarshalJSON() ([]byte, error) {
 	return marshalExpression("notInTheLast", nitl)
 }
 
+func (nitl NotInTheLast) fields() map[string]any { return nitl }
+
 func startOfPeriod(numDays int64, from time.Time) string {
 	return from.Add(time.Duration(-24*numDays) * time.Hour).Format("2006-01-02")
 }
 
 type InPlaylist map[string]any
 
-func (InPlaylist) criteriaExpression() {}
-
 func (ipl InPlaylist) MarshalJSON() ([]byte, error) {
 	return marshalExpression("inPlaylist", ipl)
 }
 
+func (ipl InPlaylist) fields() map[string]any { return ipl }
+
 type NotInPlaylist map[string]any
 
-func (NotInPlaylist) criteriaExpression() {}
-
-func (ipl NotInPlaylist) MarshalJSON() ([]byte, error) {
-	return marshalExpression("notInPlaylist", ipl)
+func (nipl NotInPlaylist) MarshalJSON() ([]byte, error) {
+	return marshalExpression("notInPlaylist", nipl)
 }
 
+func (nipl NotInPlaylist) fields() map[string]any { return nipl }
+
 func extractPlaylistIds(inputRule any) (ids []string) {
 	var id string
 	var ok bool

model/criteria/walk.go

@@ -32,41 +32,6 @@ func Walk(expr Expression, visit Visitor) error {
 	return nil
 }
 
-// Fields returns field values for leaf expressions only.
-// Use Walk to traverse All and Any expressions before calling Fields.
 func Fields(expr Expression) map[string]any {
-	switch e := expr.(type) {
-	case Is:
-		return map[string]any(e)
-	case IsNot:
-		return map[string]any(e)
-	case Gt:
-		return map[string]any(e)
-	case Lt:
-		return map[string]any(e)
-	case Before:
-		return map[string]any(e)
-	case After:
-		return map[string]any(Gt(e))
-	case Contains:
-		return map[string]any(e)
-	case NotContains:
-		return map[string]any(e)
-	case StartsWith:
-		return map[string]any(e)
-	case EndsWith:
-		return map[string]any(e)
-	case InTheRange:
-		return map[string]any(e)
-	case InTheLast:
-		return map[string]any(e)
-	case NotInTheLast:
-		return map[string]any(e)
-	case InPlaylist:
-		return map[string]any(e)
-	case NotInPlaylist:
-		return map[string]any(e)
-	default:
-		return nil
-	}
+	return expr.fields()
 }

model/criteria/walk_test.go

@@ -9,7 +9,7 @@ import (
 
 type unknownExpression struct{}
 
-func (unknownExpression) criteriaExpression() {}
+func (unknownExpression) fields() map[string]any { return nil }
 
 var _ = Describe("Walk", func() {
 	It("visits the expression tree depth-first", func() {