feat: support optional multi select variables (#136)

examples/variable-types/recipe.yml

@@ -36,6 +36,18 @@ vars:
       - option_1
       - option_2
       - option_3
+
+  - name: OPTIONAL_MULTI_SELECT_VAR
+    description: |
+      User chooses multiple values or no values at all from the predefined values in `options` property.
+
+      Defined by: non-empty `options` property, `optional: true` and `multi: true`.
+    multi: true
+    optional: true
+    options:
+      - option_1
+      - option_2
+      - option_3
 
   - name: TABLE_VAR
     description: |

pkg/recipe/variable.go

@@ -104,7 +104,7 @@ func (v *Variable) Validate() error {
 	if len(v.Options) > 0 {
 		if len(v.Columns) > 0 {
 			return errors.New("`options` and `columns` properties can not be defined at the same time")
-		} else if v.Optional {
+		} else if v.Optional && !v.Multi {
 			return errors.New("select variables can not be optional")
 		}
 	}
@@ -184,7 +184,7 @@ func (val VariableValues) Validate() error {
 		case string, bool, MultiSelectValue, TableValue:
 			break
 		default:
-			return fmt.Errorf("unsupported variable value type")
+			return fmt.Errorf("unsupported variable value type (%T)", v)
 		}
 	}
 
@@ -350,6 +350,17 @@ func (vv *VariableValues) UnmarshalYAML(unmarshal func(interface{}) error) error
 					Rows:    rows,
 				}
 			}
+		// Multiselect values without a value are an empty []interface{} and we want to cast them into MultiSelectValues
+		case []interface{}:
+			multiV := make(MultiSelectValue, len(v))
+			for i, interfaceV := range v {
+				str, ok := interfaceV.(string)
+				if !ok {
+					return fmt.Errorf("failed to read value to multi select value: %v", v)
+				}
+				multiV[i] = str
+			}
+			(*vv)[name] = multiV
 		default:
 			(*vv)[name] = v
 		}

pkg/ui/survey/survey_test.go

@@ -73,6 +73,16 @@ func TestPromptUserForValues(t *testing.T) {
 			},
 			input: "↓ ↓ \r",
 		},
+		{
+			name: "optional_multi_select_variable",
+			variables: []recipe.Variable{
+				{Name: "VAR_1", Options: []string{"a", "b", "c"}, Multi: true, Optional: true},
+			},
+			expectedValues: recipe.VariableValues{
+				"VAR_1": recipe.MultiSelectValue{},
+			},
+			input: "\r",
+		},
 		{
 			name: "table_variable_with_arrows",
 			variables: []recipe.Variable{