config: Add map() interpolation function

* `map(key, value, ...)` - Returns a map consisting of the key/value pairs
  specified as arguments. Every odd argument must be a string key, and every
  even argument must have the same type as the other values specified.
  Duplicate keys are not allowed. Examples:
  * `map("hello", "world")`
  * `map("us-east", list("a", "b", "c"), "us-west", list("b", "c", "d"))`

config/interpolate_funcs.go

@@ -71,6 +71,7 @@ func Funcs() map[string]ast.Function {
 		"length":       interpolationFuncLength(),
 		"list":         interpolationFuncList(),
 		"lower":        interpolationFuncLower(),
+		"map":          interpolationFuncMap(),
 		"md5":          interpolationFuncMd5(),
 		"uuid":         interpolationFuncUUID(),
 		"replace":      interpolationFuncReplace(),
@@ -123,6 +124,50 @@ func interpolationFuncList() ast.Function {
 	}
 }
 
+// interpolationFuncMap creates a map from the parameters passed
+// to it.
+func interpolationFuncMap() ast.Function {
+	return ast.Function{
+		ArgTypes:     []ast.Type{},
+		ReturnType:   ast.TypeMap,
+		Variadic:     true,
+		VariadicType: ast.TypeAny,
+		Callback: func(args []interface{}) (interface{}, error) {
+			outputMap := make(map[string]ast.Variable)
+
+			if len(args)%2 != 0 {
+				return nil, fmt.Errorf("requires an even number of arguments, got %d", len(args))
+			}
+
+			var firstType *ast.Type
+			for i := 0; i < len(args); i += 2 {
+				key, ok := args[i].(string)
+				if !ok {
+					return nil, fmt.Errorf("argument %d represents a key, so it must be a string", i+1)
+				}
+				val := args[i+1]
+				variable, err := hil.InterfaceToVariable(val)
+				if err != nil {
+					return nil, err
+				}
+				// Enforce map type homogeneity
+				if firstType == nil {
+					firstType = &variable.Type
+				} else if variable.Type != *firstType {
+					return nil, fmt.Errorf("all map values must have the same type, got %s then %s", firstType.Printable(), variable.Type.Printable())
+				}
+				// Check for duplicate keys
+				if _, ok := outputMap[key]; ok {
+					return nil, fmt.Errorf("argument %d is a duplicate key: %q", i+1, key)
+				}
+				outputMap[key] = variable
+			}
+
+			return outputMap, nil
+		},
+	}
+}
+
 // interpolationFuncCompact strips a list of multi-variable values
 // (e.g. as returned by "split") of any empty strings.
 func interpolationFuncCompact() ast.Function {

config/interpolate_funcs_test.go

@@ -113,6 +113,81 @@ func TestInterpolateFuncList(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncMap(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			// empty input returns empty map
+			{
+				`${map()}`,
+				map[string]interface{}{},
+				false,
+			},
+
+			// odd args is error
+			{
+				`${map("odd")}`,
+				nil,
+				true,
+			},
+
+			// two args returns map w/ one k/v
+			{
+				`${map("hello", "world")}`,
+				map[string]interface{}{"hello": "world"},
+				false,
+			},
+
+			// four args get two k/v
+			{
+				`${map("hello", "world", "what's", "up?")}`,
+				map[string]interface{}{"hello": "world", "what's": "up?"},
+				false,
+			},
+
+			// map of lists is okay
+			{
+				`${map("hello", list("world"), "what's", list("up?"))}`,
+				map[string]interface{}{
+					"hello":  []interface{}{"world"},
+					"what's": []interface{}{"up?"},
+				},
+				false,
+			},
+
+			// map of maps is okay
+			{
+				`${map("hello", map("there", "world"), "what's", map("really", "up?"))}`,
+				map[string]interface{}{
+					"hello":  map[string]interface{}{"there": "world"},
+					"what's": map[string]interface{}{"really": "up?"},
+				},
+				false,
+			},
+
+			// keys have to be strings
+			{
+				`${map(list("listkey"), "val")}`,
+				nil,
+				true,
+			},
+
+			// types have to match
+			{
+				`${map("some", "strings", "also", list("lists"))}`,
+				nil,
+				true,
+			},
+
+			// duplicate keys are an error
+			{
+				`${map("key", "val", "key", "again")}`,
+				nil,
+				true,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncCompact(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{

website/source/docs/configuration/interpolation.html.md

@@ -181,6 +181,13 @@ The supported built-in functions are:
 
   * `lower(string)` - Returns a copy of the string with all Unicode letters mapped to their lower case.
 
+  * `map(key, value, ...)` - Returns a map consisting of the key/value pairs
+    specified as arguments. Every odd argument must be a string key, and every
+    even argument must have the same type as the other values specified.
+    Duplicate keys are not allowed. Examples:
+    * `map("hello", "world")`
+    * `map("us-east", list("a", "b", "c"), "us-west", list("b", "c", "d"))`
+
   * `md5(string)` - Returns a (conventional) hexadecimal representation of the
     MD5 hash of the given string.