config: multi-line "indent" function

This function prepends a number of spaces on to all but the first line
of a string containing multiple lines of text.

config/interpolate_funcs.go

@@ -84,6 +84,7 @@ func Funcs() map[string]ast.Function {
 		"floor":        interpolationFuncFloor(),
 		"format":       interpolationFuncFormat(),
 		"formatlist":   interpolationFuncFormatList(),
+		"indent":       interpolationFuncIndent(),
 		"index":        interpolationFuncIndex(),
 		"join":         interpolationFuncJoin(),
 		"jsonencode":   interpolationFuncJSONEncode(),
@@ -675,6 +676,21 @@ func interpolationFuncFormatList() ast.Function {
 	}
 }
 
+// interpolationFuncIndent indents a multi-line string with the
+// specified number of spaces
+func interpolationFuncIndent() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeInt, ast.TypeString},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			spaces := args[0].(int)
+			data := args[1].(string)
+			pad := strings.Repeat(" ", spaces)
+			return strings.Replace(data, "\n", "\n"+pad, -1), nil
+		},
+	}
+}
+
 // interpolationFuncIndex implements the "index" function that allows one to
 // find the index of a specific element in a list
 func interpolationFuncIndex() ast.Function {

config/interpolate_funcs_test.go

@@ -1354,6 +1354,34 @@ func TestInterpolateFuncIndex(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncIndent(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				`${indent(4, "Fleas:
+Adam
+Had'em
+
+E.E. Cummings")}`,
+				"Fleas:\n    Adam\n    Had'em\n    \n    E.E. Cummings",
+				false,
+			},
+			{
+				`${indent(4, "oneliner")}`,
+				"oneliner",
+				false,
+			},
+			{
+				`${indent(4, "#!/usr/bin/env bash
+date
+pwd")}`,
+				"#!/usr/bin/env bash\n    date\n    pwd",
+				false,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncJoin(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Vars: map[string]ast.Variable{

website/docs/configuration/interpolation.html.md

@@ -257,6 +257,12 @@ The supported built-in functions are:
       `formatlist("instance %v has private ip %v", aws_instance.foo.*.id, aws_instance.foo.*.private_ip)`.
       Passing lists with different lengths to formatlist results in an error.
 
+  * `indent(numspaces, string)` - Prepends the specified number of spaces to all but the first
+      line of the given multi-line string. May be useful when inserting a multi-line string
+      into an already-indented context. The first line is not indented, to allow for the
+      indented string to be placed after some sort of already-indented preamble.
+      Example: `"    \"items\": ${ indent(4, "[\n    \"item1\"\n]") },"`
+
   * `index(list, elem)` - Finds the index of a given element in a list.
       This function only works on flat lists.
       Example: `index(aws_instance.foo.*.tags.Name, "foo-test")`