Add test.Required function

Signed-off-by: Dave Henderson <[email protected]>

docs-src/content/functions/test.yml

@@ -37,3 +37,43 @@ funcs:
         template: <arg>:1:3: executing "<arg>" at <fail>: error calling fail: template generation failed
         $ gomplate -i '{{ test.Fail "something is wrong!" }}'
         template: <arg>:1:7: executing "<arg>" at <test.Fail>: error calling Fail: template generation failed: something is wrong!
+  - name: test.Required
+    alias: required
+    description: |
+      Passes through the given value, if it's non-empty, non-zero, and non-`nil`.
+      Otherwise, exits and prints a given error message so the user can adjust as
+      necessary.
+
+      This is particularly useful for cases where templates require user-provided
+      data (such as datasources or environment variables), and rendering can not
+      continue correctly.
+
+      This was inspired by [Helm's `required` function](https://github.com/kubernetes/helm/blob/master/docs/charts_tips_and_tricks.md#know-your-template-functions),
+      but has slightly different behaviour. Notably, gomplate will always fail in
+      cases where a referenced _key_ is missing, and this function will have no
+      effect.
+    pipeline: true
+    arguments:
+      - name: message
+        required: false
+        description: The optional message to provide when the required value is not provided
+      - name: value
+        required: true
+        description: The required value
+    examples:
+      - |
+        $ FOO=foobar gomplate -i '{{ getenv "FOO" | required "Missing FOO environment variable!" }}'
+        foobar
+        $ FOO= gomplate -i '{{ getenv "FOO" | required "Missing FOO environment variable!" }}'
+        error: Missing FOO environment variable!
+      - |
+        $ cat <<EOF> config.yaml
+        defined: a value
+        empty: ""
+        EOF
+        $ gomplate -d config=config.yaml -i '{{ (ds "config").defined | required "The `config` datasource must have a value defined for `defined`" }}'
+        a value
+        $ gomplate -d config=config.yaml -i '{{ (ds "config").empty | required "The `config` datasource must have a value defined for `empty`" }}'
+        template: <arg>:1:25: executing "<arg>" at <required "The `confi...>: error calling required: The `config` datasource must have a value defined for `empty`
+        $ gomplate -d config=config.yaml -i '{{ (ds "config").bogus | required "The `config` datasource must have a value defined for `bogus`" }}'
+        template: <arg>:1:7: executing "<arg>" at <"config">: map has no entry for key "bogus"

docs/content/functions/test.md

@@ -69,3 +69,57 @@ template: <arg>:1:3: executing "<arg>" at <fail>: error calling fail: template g
 $ gomplate -i '{{ test.Fail "something is wrong!" }}'
 template: <arg>:1:7: executing "<arg>" at <test.Fail>: error calling Fail: template generation failed: something is wrong!
 ```
+
+## `test.Required`
+
+**Alias:** `required`
+
+Passes through the given value, if it's non-empty, non-zero, and non-`nil`.
+Otherwise, exits and prints a given error message so the user can adjust as
+necessary.
+
+This is particularly useful for cases where templates require user-provided
+data (such as datasources or environment variables), and rendering can not
+continue correctly.
+
+This was inspired by [Helm's `required` function](https://github.com/kubernetes/helm/blob/master/docs/charts_tips_and_tricks.md#know-your-template-functions),
+but has slightly different behaviour. Notably, gomplate will always fail in
+cases where a referenced _key_ is missing, and this function will have no
+effect.
+
+### Usage
+```go
+test.Required [message] value 
+```
+
+```go
+value | test.Required [message]  
+```
+
+### Arguments
+
+| name | description |
+|------|-------------|
+| `message` | _(optional)_ The optional message to provide when the required value is not provided |
+| `value` | _(required)_ The required value |
+
+### Examples
+
+```console
+$ FOO=foobar gomplate -i '{{ getenv "FOO" | required "Missing FOO environment variable!" }}'
+foobar
+$ FOO= gomplate -i '{{ getenv "FOO" | required "Missing FOO environment variable!" }}'
+error: Missing FOO environment variable!
+```
+```console
+$ cat <<EOF> config.yaml
+defined: a value
+empty: ""
+EOF
+$ gomplate -d config=config.yaml -i '{{ (ds "config").defined | required "The `config` datasource must have a value defined for `defined`" }}'
+a value
+$ gomplate -d config=config.yaml -i '{{ (ds "config").empty | required "The `config` datasource must have a value defined for `empty`" }}'
+template: <arg>:1:25: executing "<arg>" at <required "The `confi...>: error calling required: The `config` datasource must have a value defined for `empty`
+$ gomplate -d config=config.yaml -i '{{ (ds "config").bogus | required "The `config` datasource must have a value defined for `bogus`" }}'
+template: <arg>:1:7: executing "<arg>" at <"config">: map has no entry for key "bogus"
+```

funcs/test.go

@@ -26,6 +26,7 @@ func AddTestFuncs(f map[string]interface{}) {
 
 	f["assert"] = TestNS().Assert
 	f["fail"] = TestNS().Fail
+	f["required"] = TestNS().Required
 }
 
 // TestFuncs -
@@ -59,3 +60,19 @@ func (f *TestFuncs) Fail(args ...interface{}) (string, error) {
 		return "", errors.Errorf("wrong number of args: want 0 or 1, got %d", len(args))
 	}
 }
+
+// Required -
+func (f *TestFuncs) Required(args ...interface{}) (interface{}, error) {
+	switch len(args) {
+	case 1:
+		return test.Required("", args[0])
+	case 2:
+		message, ok := args[0].(string)
+		if !ok {
+			return nil, errors.Errorf("at <1>: expected string; found %T", args[0])
+		}
+		return test.Required(message, args[1])
+	default:
+		return "", errors.Errorf("wrong number of args: want 1 or 2, got %d", len(args))
+	}
+}

test/test.go

@@ -2,6 +2,7 @@ package test
 
 import (
 	"github.com/pkg/errors"
+	"reflect"
 )
 
 // Assert -
@@ -22,3 +23,15 @@ func Fail(message string) error {
 	}
 	return errors.New("template generation failed")
 }
+
+// Required -
+func Required(message string, value interface{}) (interface{}, error) {
+	if message == "" {
+		message = "can not render template: a required value not was not set"
+	}
+
+	if value == nil || reflect.DeepEqual(value, reflect.Zero(reflect.TypeOf(value)).Interface()) {
+		return nil, errors.New(message)
+	}
+	return value, nil
+}