Add fail function

docs/flow_control.md

@@ -0,0 +1,11 @@
+# Flow Control Functions
+
+## fail
+
+Unconditionally returns an empty `string` and an `error` with the specified
+text. This is useful in scenarios where other conditionals have determined that
+template rendering should fail.
+
+```
+fail "Please accept the end user license agreement"
+```

docs/index.md

@@ -13,6 +13,7 @@ The Sprig library provides over 70 template functions for Go's template language
 - [Dictionaries and Dict Functions](dicts.html): `dict`, `hasKey`, `pluck`, etc.
 - [Type Conversion Functions](conversion.html): `atoi`, `int64`, `toString`, etc.
 - [File Path Functions](paths.html): `base`, `dir`, `ext`, `clean`, `isAbs`
+- [Flow Control Functions](flow_control.html): `fail`
 - Advanced Functions
   - [UUID Functions](uuid.html): `uuidv4`
   - [OS Functions](os.html): `env`, `expandenv`

flow_control_test.go

@@ -0,0 +1,16 @@
+package sprig
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFail(t *testing.T) {
+	const msg = "This is an error!"
+	tpl := fmt.Sprintf(`{{fail "%s"}}`, msg)
+	_, err := runRaw(tpl, nil)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), msg)
+}

functions.go

@@ -1,6 +1,7 @@
 package sprig
 
 import (
+	"errors"
 	"html/template"
 	"os"
 	"path"
@@ -254,4 +255,7 @@ var genericMap = map[string]interface{}{
 	// SemVer:
 	"semver":        semver,
 	"semverCompare": semverCompare,
+
+	// Flow Control:
+	"fail": func(msg string) (string, error) { return "", errors.New(msg) },
 }