Allow setting the default value of nullable

.. to true. This is to make the schema more scrictly align with what Terraform actually permits when `nullable` is not set in a variable definition. See the updated README for details.

README.md

@@ -65,6 +65,8 @@ running Terraform.
 - `--disallow-additional-properties`: Set additionalProperties to false in the root object and nested objects
   (see [JSON Schema definition](https://json-schema.org/understanding-json-schema/reference/object#additionalproperties)).
 
+- `--nullable-all`: Change the default value for `nullable` in a Variable block to 'true'. This is to make the behaviour more closely reflect Terraform's own validation. See 'Nullable Variables' below.
+
 - `--overwrite`: Allow overwriting an existing file at the output location.
 
 - `--debug`: Print debug logs for variable retrieval and errors related to custom validation rules.
@@ -295,7 +297,10 @@ If `nullable` is true in the `variable` block, then the JSON Schema will be modi
 },
 ```
 
-This is actually a slight behaviour change from the validator used by terraform. If `nullable` is unset, then terraform treats them as `nullable` by default. I chose not to implement that behaviour here and instead am making the Terraform module author specify `nullable = true`. This is because otherwise schema definitions for simple programs would have to become a lot more verbose just to handle this case.
+This is actually a slight behaviour change from the validator used by terraform. If `nullable` is unset, then terraform treats them as `nullable` by default. I chose not to implement that default behaviour here and instead am making the Terraform module author specify `nullable = true`. This is because otherwise schema definitions for simple programs would have to become a lot more verbose just to handle this case.
+
+For behaviour more consistent with Terraform, the flag `--nullable-all` can be used to reset the default value for nullable to be true. Note: this rule only applies to variables which have not explicitly set the value of nullable themselves. See [Terraform documentation on nullable](https://developer.hashicorp.com/terraform/language/values/variables#disallowing-null-input-values
+).
 
 ### Default Handling
 

cmd/cmd.go

@@ -19,6 +19,7 @@ var (
 	allowEmpty                   bool
 	requireAll                   bool
 	outputStdOut                 bool
+	nullableAll                  bool
 	inputPath                    string
 	outputPath                   string
 )
@@ -81,6 +82,10 @@ func init() {
 			"unless an error occurs. Overrides 'debug' and 'output.",
 	)
 
+	rootCmd.Flags().BoolVar(&nullableAll, "nullable-all", false,
+		"make all variables nullable unless nullable set to false explicitly, to make behavior consistent with Terraform",
+	)
+
 	rootCmd.SetFlagErrorFunc(func(cmd *cobra.Command, err error) error {
 		_ = rootCmd.Usage()
 
@@ -151,6 +156,7 @@ func runCommand(cmd *cobra.Command, args []string) error {
 		RequireAll:                requireAll,
 		AllowAdditionalProperties: !disallowAdditionalProperties,
 		AllowEmpty:                allowEmpty,
+		NullableAll:               nullableAll,
 	})
 	if err != nil {
 		return fmt.Errorf("error creating schema: %w", err)

pkg/jsonschema/json-schema.go

@@ -14,6 +14,7 @@ type CreateSchemaOptions struct {
 	RequireAll                bool
 	AllowAdditionalProperties bool
 	AllowEmpty                bool
+	NullableAll               bool
 }
 
 func CreateSchema(path string, options CreateSchemaOptions) (map[string]any, error) {
@@ -75,8 +76,14 @@ func createNode(name string, v model.TranslatedVariable, options CreateSchemaOpt
 		return nil, fmt.Errorf("getting type constraint for %q: %w", name, err)
 	}
 
-	nullableIsTrue := v.Variable.Nullable != nil && *v.Variable.Nullable
-	node, err := getNodeFromType(name, tc, nullableIsTrue, options)
+	// The default value for nullable is the value of NullableAll. For the purpose of keeping the JSON Schema relatively
+	// clean, this is normally set to false. Setting the default value to true is consistent with Terraform behavior.
+	nullableTranslatedValue := options.NullableAll
+	if v.Variable.Nullable != nil {
+		nullableTranslatedValue = *v.Variable.Nullable
+	}
+
+	node, err := getNodeFromType(name, tc, nullableTranslatedValue, options)
 	if err != nil {
 		return nil, fmt.Errorf("%q: %w", name, err)
 	}

pkg/jsonschema/json-schema_test.go

@@ -36,6 +36,7 @@ func TestCreateSchema(t *testing.T) {
 				RequireAll:                false,
 				AllowAdditionalProperties: true,
 				AllowEmpty:                true,
+				NullableAll:               false,
 			})
 			require.NoError(t, err)