[pkg/ottl] Add the When OTTL converter

.chloggen/add-when-function.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. receiver/filelog)
+component: pkg/ottl
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add the `When` OTTL converter for selecting a value based on a lambda condition.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [49356]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

pkg/ottl/e2e/e2e_test.go

@@ -1627,6 +1627,7 @@ func Test_e2e_converters(t *testing.T) {
 }
 
 func Test_e2e_ottl_features(t *testing.T) {
+	ottltest.SetFeatureGateForTest(t, metadata.OttlFunctionsEnableLambdaFeatureGate, true)
 	tests := []struct {
 		name      string
 		statement string
@@ -1888,6 +1889,24 @@ func Test_e2e_ottl_features(t *testing.T) {
 				tCtx.GetLogRecord().Attributes().PutStr("test", "pass")
 			},
 		},
+		{
+			statement: `set(attributes["test"], When(() => attributes["int_value"] > 0, "positive", "negative"))`,
+			want: func(tCtx *ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "negative")
+			},
+		},
+		{
+			statement: `set(attributes["test"], When(() => IsMap(attributes["foo"]), attributes["foo"]["bar"], "fail"))`,
+			want: func(tCtx *ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "pass")
+			},
+		},
+		{
+			statement: `set(attributes["test"], When(() => IsMap(attributes["foo"]), When(() => attributes["foo"]["bar"] == "pass", "pass", "fail"), "fail"))`,
+			want: func(tCtx *ottllog.TransformContext) {
+				tCtx.GetLogRecord().Attributes().PutStr("test", "pass")
+			},
+		},
 	}
 
 	for _, tt := range tests {

pkg/ottl/ottlfuncs/README.md

@@ -588,6 +588,7 @@ Available Converters:
 - [UUIDv7](#UUIDv7)
 - [Values](#values)
 - [Weekday](#weekday)
+- [When](#when)
 - [XXH3](#xxh3)
 - [XXH128](#xxh128)
 - [Year](#year)
@@ -2806,6 +2807,35 @@ Examples:
 
 - `Weekday(Now())`
 
+### When
+
+> [!IMPORTANT]
+> This function is alpha and may change in future releases. It requires the [`ottl.functions.enableLambda`](../documentation.md#feature-gates) feature gate to be enabled.
+
+`When(condition, trueValue, falseValue)`
+
+The `When` converter returns `trueValue` when `condition` evaluates to true, otherwise it returns `falseValue`.
+
+`condition` is a lambda expression with no parameters that returns a `boolean`.
+
+`trueValue` and `falseValue` are OTTL expressions or literal values.
+
+If `condition` does not return a `boolean`, it returns an error.
+
+Examples:
+
+Select a value based on a type check:
+
+- `When(() => IsMap(log.attributes), "map", "not map")`
+
+Select a value based on a comparison:
+
+- `When(() => attributes["int_value"] > 0, "positive", "negative")`
+
+Store the result:
+
+- `set(log.attributes["result"], When(() => IsMap(log.attributes), "yes", "no"))`
+
 ### XXH3
 
 `XXH3(value)`

pkg/ottl/ottlfuncs/func_when.go

@@ -0,0 +1,67 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package ottlfuncs // import "github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl/ottlfuncs"
+
+import (
+	"context"
+	"errors"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl/ottlfuncs/internal/funcutil"
+)
+
+type WhenArguments[K any] struct {
+	Condition  ottl.LambdaExpression[K]
+	TrueValue  ottl.Getter[K]
+	FalseValue ottl.Getter[K]
+}
+
+func NewWhenFactory[K any]() ottl.Factory[K] {
+	return ottl.NewFactory("When", &WhenArguments[K]{}, createWhenFunction[K])
+}
+
+func createWhenFunction[K any](_ ottl.FunctionContext, oArgs ottl.Arguments) (ottl.ExprFunc[K], error) {
+	args, ok := oArgs.(*WhenArguments[K])
+	if !ok {
+		return nil, errors.New("WhenFactory args must be of type *WhenArguments[K]")
+	}
+	return whenFunction(&args.Condition, args.TrueValue, args.FalseValue), nil
+}
+
+func whenFunction[K any](condition *ottl.LambdaExpression[K], trueValueGetter, falseValueGetter ottl.Getter[K]) ottl.ExprFunc[K] {
+	var trueValue any
+	var falseValue any
+	if tv, ok := ottl.GetLiteralValue(trueValueGetter); ok {
+		trueValue = tv
+	}
+	if fv, ok := ottl.GetLiteralValue(falseValueGetter); ok {
+		falseValue = fv
+	}
+
+	return func(ctx context.Context, tCtx K) (any, error) {
+		lb, err := condition.Activate(ctx, 0)
+		if err != nil {
+			return nil, err
+		}
+		defer lb.Close()
+
+		match, err := funcutil.EvaluateLambdaActivation[K, bool](tCtx, lb)
+		if err != nil {
+			return nil, err
+		}
+
+		if match {
+			if trueValue != nil {
+				return trueValue, nil
+			}
+			return trueValueGetter.Get(ctx, tCtx)
+		}
+
+		if falseValue != nil {
+			return falseValue, nil
+		}
+
+		return falseValueGetter.Get(ctx, tCtx)
+	}
+}

pkg/ottl/ottlfuncs/func_when_test.go

@@ -0,0 +1,151 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package ottlfuncs
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+)
+
+func Test_when(t *testing.T) {
+	asLiteralGetter := func(v any) ottl.Getter[any] {
+		getter, err := ottl.NewTestingLiteralGetter[any](true, ottl.StandardGetSetter[any]{
+			Getter: func(context.Context, any) (any, error) {
+				return v, nil
+			},
+		})
+		require.NoError(t, err)
+		return getter
+	}
+
+	tests := []struct {
+		name       string
+		condition  *ottl.LambdaExpression[any]
+		trueValue  ottl.Getter[any]
+		falseValue ottl.Getter[any]
+		want       any
+	}{
+		{
+			name: "condition true with literal values",
+			condition: ottl.NewTestingLambdaExpression[any]([]string{}, func(_ context.Context, _ any, _ func(string) any) (any, error) {
+				return true, nil
+			}),
+			trueValue:  asLiteralGetter("true"),
+			falseValue: asLiteralGetter("false"),
+			want:       "true",
+		},
+		{
+			name: "condition false with literal values",
+			condition: ottl.NewTestingLambdaExpression[any]([]string{}, func(_ context.Context, _ any, _ func(string) any) (any, error) {
+				return false, nil
+			}),
+			trueValue:  asLiteralGetter("true"),
+			falseValue: asLiteralGetter("false"),
+			want:       "false",
+		},
+		{
+			name: "condition true with dynamic values",
+			condition: ottl.NewTestingLambdaExpression[any]([]string{}, func(_ context.Context, _ any, _ func(string) any) (any, error) {
+				return true, nil
+			}),
+			trueValue: &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+				return int64(1), nil
+			}},
+			falseValue: &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+				return int64(0), nil
+			}},
+			want: int64(1),
+		},
+		{
+			name: "condition false with dynamic values",
+			condition: ottl.NewTestingLambdaExpression[any]([]string{}, func(_ context.Context, _ any, _ func(string) any) (any, error) {
+				return false, nil
+			}),
+			trueValue: &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+				return int64(1), nil
+			}},
+			falseValue: &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+				return int64(0), nil
+			}},
+			want: int64(0),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			exprFunc := whenFunction(tt.condition, tt.trueValue, tt.falseValue)
+			got, err := exprFunc(t.Context(), nil)
+			require.NoError(t, err)
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}
+
+func Test_when_unused_branch(t *testing.T) {
+	condition := ottl.NewTestingLambdaExpression[any]([]string{}, func(_ context.Context, _ any, _ func(string) any) (any, error) {
+		return true, nil
+	})
+	trueValue := &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+		return "true", nil
+	}}
+	falseValue := &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+		return nil, errors.New("should not be reached")
+	}}
+
+	exprFunc := whenFunction(condition, trueValue, falseValue)
+	got, err := exprFunc(t.Context(), nil)
+	require.NoError(t, err)
+	assert.Equal(t, "true", got)
+}
+
+func Test_when_lambda_type_error(t *testing.T) {
+	condition := ottl.NewTestingLambdaExpression[any]([]string{}, func(_ context.Context, _ any, _ func(string) any) (any, error) {
+		return "not a bool", nil
+	})
+	trueValue := &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+		return "true", nil
+	}}
+	falseValue := &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+		return "false", nil
+	}}
+
+	exprFunc := whenFunction(condition, trueValue, falseValue)
+	_, err := exprFunc(t.Context(), nil)
+	require.Error(t, err)
+	assert.ErrorContains(t, err, "lambda expression must return a value of type bool")
+}
+
+func Test_createWhenFunction(t *testing.T) {
+	fCtx := ottl.FunctionContext{}
+	condition := ottl.NewTestingLambdaExpression[any]([]string{}, func(_ context.Context, _ any, _ func(string) any) (any, error) {
+		return true, nil
+	})
+	trueValue := &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+		return "true", nil
+	}}
+	falseValue := &ottl.StandardGetSetter[any]{Getter: func(context.Context, any) (any, error) {
+		return "false", nil
+	}}
+
+	t.Run("valid args", func(t *testing.T) {
+		fn, err := createWhenFunction[any](fCtx, &WhenArguments[any]{
+			Condition:  *condition,
+			TrueValue:  trueValue,
+			FalseValue: falseValue,
+		})
+		require.NoError(t, err)
+		require.NotNil(t, fn)
+	})
+
+	t.Run("invalid args type", func(t *testing.T) {
+		_, err := createWhenFunction[any](fCtx, &struct{}{})
+		assert.EqualError(t, err, "WhenFactory args must be of type *WhenArguments[K]")
+	})
+}

pkg/ottl/ottlfuncs/functions.go

@@ -124,6 +124,7 @@ func converters[K any]() []ottl.Factory[K] {
 		NewURLFactory[K](),
 		NewValuesFactory[K](),
 		NewWeekdayFactory[K](),
+		NewWhenFactory[K](),
 		NewUserAgentFactory[K](),
 		NewAppendFactory[K](),
 		NewDeleteIndexFactory[K](),