feat(cel): add fold support

.golangci.yml

@@ -6,7 +6,6 @@ linters:
   enable:
     - asciicheck
     - bodyclose
-    - goconst
     - gocritic
     - goheader
     - gomodguard
@@ -22,9 +21,6 @@ linters:
   settings:
     dupl:
       threshold: 100
-    goconst:
-      min-len: 2
-      min-occurrences: 4
     gocritic:
       disabled-checks:
         - singleCaseSwitch

CEL.md

@@ -342,8 +342,13 @@ Combines all elements of a collection using a binary function.
 
 // For maps:
 {"a": "apple", "b": "banana"}.fold(k, v, acc, acc + v)   // "applebanana"
+
+// Build a map from a list of key/value objects:
+dyn(tags).fold(tag, acc, merge(acc, {tag.key: tag.value}))
 ```
 
+When folding a variable declared as `any`, wrap it with `dyn(...)` so CEL can use it as a comprehension range. The `merge(left, right)` helper returns a map containing all keys from both maps; values from `right` replace values from `left` on duplicate keys.
+
 ### has
 
 Tests whether a field is available in a message or map.

cel.go

@@ -38,6 +38,7 @@ func GetCelEnv(environment map[string]any) []cel.EnvOption {
 	opts = append(opts, typeAdapters...)
 	opts = append(opts, getGoTemplateCelFunction())
 	opts = append(opts, getDebugCelFunction())
+	opts = append(opts, getFoldCelLibrary())
 
 	// Load input as variables
 	for k := range environment {

cel_fold.go

@@ -0,0 +1,235 @@
+package gomplate
+
+import (
+	"fmt"
+	"sort"
+
+	"github.com/google/cel-go/cel"
+	"github.com/google/cel-go/common/ast"
+	"github.com/google/cel-go/common/operators"
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/common/types/traits"
+)
+
+const (
+	foldInitListFn       = "cel.@foldInitList"
+	foldInitMapFn        = "cel.@foldInitMap"
+	foldSortedMapEntries = "cel.@foldSortedMapEntries"
+)
+
+func getFoldCelLibrary() cel.EnvOption {
+	return cel.Lib(&foldCelLibrary{})
+}
+
+type foldCelLibrary struct{}
+
+func (l *foldCelLibrary) LibraryName() string {
+	return "gomplate.fold"
+}
+
+func (l *foldCelLibrary) CompileOptions() []cel.EnvOption {
+	return []cel.EnvOption{
+		cel.Macros(
+			cel.ReceiverMacro("fold", 3, foldListMacro,
+				cel.MacroDocs("Folds a list using an element variable, an accumulator variable, and a step expression."),
+				cel.MacroExamples("[1, 2, 3].fold(e, acc, acc + e) // 6")),
+			cel.ReceiverMacro("fold", 4, foldMapMacro,
+				cel.MacroDocs("Folds a map using key/value variables, an accumulator variable, and a step expression."),
+				cel.MacroExamples(`{"a": "apple", "b": "banana"}.fold(k, v, acc, acc + v) // "applebanana"`)),
+		),
+		cel.Function(foldInitListFn,
+			cel.Overload("fold_init_list_dyn", []*cel.Type{cel.DynType}, cel.DynType,
+				cel.UnaryBinding(func(collection ref.Val) ref.Val {
+					return foldInitialValue(collection, false)
+				})),
+		),
+		cel.Function(foldInitMapFn,
+			cel.Overload("fold_init_map_dyn", []*cel.Type{cel.DynType}, cel.DynType,
+				cel.UnaryBinding(func(collection ref.Val) ref.Val {
+					return foldInitialValue(collection, true)
+				})),
+		),
+		cel.Function(foldSortedMapEntries,
+			cel.Overload("fold_sorted_map_entries_dyn", []*cel.Type{cel.DynType}, cel.ListType(cel.ListType(cel.DynType)),
+				cel.UnaryBinding(sortedMapEntries)),
+		),
+		cel.Function("merge",
+			cel.Overload("merge_map_map", []*cel.Type{cel.MapType(cel.DynType, cel.DynType), cel.MapType(cel.DynType, cel.DynType)}, cel.MapType(cel.DynType, cel.DynType),
+				cel.BinaryBinding(mergeMaps)),
+		),
+	}
+}
+
+func (*foldCelLibrary) ProgramOptions() []cel.ProgramOption {
+	return nil
+}
+
+func foldListMacro(mef cel.MacroExprFactory, target ast.Expr, args []ast.Expr) (ast.Expr, *cel.Error) {
+	iterVar, err := extractFoldIdent(mef, args[0])
+	if err != nil {
+		return nil, err
+	}
+	accuVar, err := extractFoldIdent(mef, args[1])
+	if err != nil {
+		return nil, err
+	}
+	if iterVar == accuVar {
+		return nil, mef.NewError(args[1].ID(), fmt.Sprintf("duplicate variable name: %s", accuVar))
+	}
+
+	return mef.NewComprehension(
+		target,
+		iterVar,
+		accuVar,
+		mef.NewCall(foldInitListFn, mef.Copy(target)),
+		mef.NewLiteral(types.True),
+		args[2],
+		mef.NewIdent(accuVar),
+	), nil
+}
+
+func foldMapMacro(mef cel.MacroExprFactory, target ast.Expr, args []ast.Expr) (ast.Expr, *cel.Error) {
+	keyVar, err := extractFoldIdent(mef, args[0])
+	if err != nil {
+		return nil, err
+	}
+	valVar, err := extractFoldIdent(mef, args[1])
+	if err != nil {
+		return nil, err
+	}
+	accuVar, err := extractFoldIdent(mef, args[2])
+	if err != nil {
+		return nil, err
+	}
+	if keyVar == valVar || keyVar == accuVar || valVar == accuVar {
+		return nil, mef.NewError(args[2].ID(), "fold variable names must be unique")
+	}
+
+	entryVar := "__fold_entry__"
+	if entryVar == keyVar || entryVar == valVar || entryVar == accuVar {
+		entryVar = "__fold_entry2__"
+	}
+	entry := mef.NewIdent(entryVar)
+	key := mef.NewCall(operators.Index, entry, mef.NewLiteral(types.IntZero))
+	value := mef.NewCall(operators.Index, mef.NewIdent(entryVar), mef.NewLiteral(types.Int(1)))
+	step := mef.NewComprehensionTwoVar(
+		mef.NewMap(mef.NewMapEntry(key, value, false)),
+		keyVar,
+		valVar,
+		accuVar,
+		mef.NewIdent(accuVar),
+		mef.NewLiteral(types.True),
+		args[3],
+		mef.NewIdent(accuVar),
+	)
+
+	return mef.NewComprehension(
+		mef.NewCall(foldSortedMapEntries, mef.Copy(target)),
+		entryVar,
+		accuVar,
+		mef.NewCall(foldInitMapFn, mef.Copy(target)),
+		mef.NewLiteral(types.True),
+		step,
+		mef.NewIdent(accuVar),
+	), nil
+}
+
+func extractFoldIdent(mef cel.MacroExprFactory, expr ast.Expr) (string, *cel.Error) {
+	if expr.Kind() != ast.IdentKind {
+		return "", mef.NewError(expr.ID(), "argument must be a simple name")
+	}
+	return expr.AsIdent(), nil
+}
+
+func foldInitialValue(collection ref.Val, mapValues bool) ref.Val {
+	var first ref.Val
+	if mapValues {
+		m, ok := collection.(traits.Mapper)
+		if !ok {
+			return types.NewErr("fold target is not a map")
+		}
+		it := m.Iterator()
+		if it.HasNext() != types.True {
+			return types.DefaultTypeAdapter.NativeToValue(nil)
+		}
+		first = m.Get(it.Next())
+	} else {
+		l, ok := collection.(traits.Lister)
+		if !ok {
+			return types.NewErr("fold target is not a list")
+		}
+		if l.Size().Equal(types.IntZero) == types.True {
+			return types.DefaultTypeAdapter.NativeToValue(nil)
+		}
+		first = l.Get(types.IntZero)
+	}
+	return zeroValueForFold(first)
+}
+
+func sortedMapEntries(collection ref.Val) ref.Val {
+	m, ok := collection.(traits.Mapper)
+	if !ok {
+		return types.NewErr("fold target is not a map")
+	}
+
+	keys := []ref.Val{}
+	for it := m.Iterator(); it.HasNext() == types.True; {
+		keys = append(keys, it.Next())
+	}
+	sort.SliceStable(keys, func(i, j int) bool {
+		return fmt.Sprint(keys[i].Value()) < fmt.Sprint(keys[j].Value())
+	})
+
+	entries := make([]ref.Val, 0, len(keys))
+	for _, key := range keys {
+		entries = append(entries, types.NewRefValList(types.DefaultTypeAdapter, []ref.Val{key, m.Get(key)}))
+	}
+	return types.NewRefValList(types.DefaultTypeAdapter, entries)
+}
+
+func mergeMaps(lhs, rhs ref.Val) ref.Val {
+	left, ok := lhs.(traits.Mapper)
+	if !ok {
+		return types.NewErr("left operand is not a map")
+	}
+	right, ok := rhs.(traits.Mapper)
+	if !ok {
+		return types.NewErr("right operand is not a map")
+	}
+
+	out := map[ref.Val]ref.Val{}
+	for it := left.Iterator(); it.HasNext() == types.True; {
+		key := it.Next()
+		out[key] = left.Get(key)
+	}
+	for it := right.Iterator(); it.HasNext() == types.True; {
+		key := it.Next()
+		out[key] = right.Get(key)
+	}
+	return types.NewRefValMap(types.DefaultTypeAdapter, out)
+}
+
+func zeroValueForFold(v ref.Val) ref.Val {
+	if types.IsError(v) {
+		return v
+	}
+	switch v.(type) {
+	case types.Int:
+		return types.IntZero
+	case types.Uint:
+		return types.Uint(0)
+	case types.Double:
+		return types.Double(0)
+	case types.String:
+		return types.String("")
+	case types.Bytes:
+		return types.Bytes([]byte{})
+	case traits.Mapper:
+		return types.NewRefValMap(types.DefaultTypeAdapter, map[ref.Val]ref.Val{})
+	case traits.Lister:
+		return types.NewRefValList(types.DefaultTypeAdapter, []ref.Val{})
+	default:
+		return types.DefaultTypeAdapter.NativeToValue(nil)
+	}
+}

tests/cel_test.go

@@ -304,6 +304,24 @@ func TestCelExtensions(t *testing.T) {
 	})
 }
 
+func TestCelFold(t *testing.T) {
+	runTests(t, []Test{
+		{nil, `[1, 2, 3].fold(e, acc, acc + e)`, "6"},
+		{nil, `{"a": "apple", "b": "banana"}.fold(k, v, acc, acc + v)`, "applebanana"},
+	})
+
+	out, err := gomplate.RunExpression(map[string]any{
+		"tags": []map[string]any{
+			{"key": "application", "value": "orders"},
+			{"key": "environment", "value": "prod"},
+		},
+	}, gomplate.Template{
+		Expression: `dyn(tags).fold(tag, acc, merge(acc, {tag.key: tag.value})).toJSON()`,
+	})
+	assert.NoError(t, err)
+	assert.JSONEq(t, `{"application":"orders","environment":"prod"}`, out.(string))
+}
+
 func TestCelEncode(t *testing.T) {
 	tests := []Test{
 		{map[string]interface{}{"hello": "hello world ?"}, "urlencode(hello)", `hello+world+%3F`},