Better support for writing functions in go

kyaml/fn/framework/example/Dockerfile

@@ -0,0 +1,12 @@
+# Copyright 2019 The Kubernetes Authors.
+# SPDX-License-Identifier: Apache-2.0
+
+FROM golang:1.13-stretch
+ENV CGO_ENABLED=0
+WORKDIR /go/src/
+COPY . .
+RUN go build -v -o /usr/local/bin/function ./
+
+FROM alpine:latest
+COPY --from=0 /usr/local/bin/function /usr/local/bin/function
+CMD ["function"]

kyaml/fn/framework/example/doc.go

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+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package main contains an example using the the framework.
+//
+// To generate the Dockerfile for the function image run:
+//
+//   $ go run ./main.go .
+package main

kyaml/fn/framework/example/main.go

@@ -0,0 +1,29 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package main
+
+import (
+	"os"
+
+	"sigs.k8s.io/kustomize/kyaml/fn/framework"
+	"sigs.k8s.io/kustomize/kyaml/yaml"
+)
+
+func main() {
+	var value string
+	cmd := framework.Command(nil, func(items []*yaml.RNode) ([]*yaml.RNode, error) {
+		for i := range items {
+			// set the annotation on each resource item
+			if err := items[i].PipeE(yaml.SetAnnotation("value", value)); err != nil {
+				return nil, err
+			}
+		}
+		return items, nil
+	})
+	cmd.Flags().StringVar(&value, "value", "", "annotation value")
+
+	if err := cmd.Execute(); err != nil {
+		os.Exit(1)
+	}
+}