- `attrsWith { elemType = t; }` is equivalent to `attrsOf t`
    - `attrsWith { lazy = true; elemType = t; }` is equivalent to `lazyAttrsOf t`
    - `attrsWith { placeholder = "id"; elemType = t; }`

      Displays the option as `foo.<id>` in the manual.


`types.uniq` *`t`*

:   Ensures that type *`t`* cannot be merged. It is used to ensure option
    definitions are provided only once.

`types.unique` `{ message = m }` *`t`*

:   Ensures that type *`t`* cannot be merged. Prints the message *`m`*, after
    the line `The option <option path> is defined multiple times.` and before
    a list of definition locations.

`types.coercedTo` *`from f to`*

:   Type *`to`* or type *`from`* which will be coerced to type *`to`* using
    function *`f`* which takes an argument of type *`from`* and return a
    value of type *`to`*. Can be used to preserve backwards compatibility
    of an option if its type was changed.

## Submodule {#section-option-types-submodule}

`submodule` is a very powerful type that defines a set of sub-options
that are handled like a separate module.

It takes a parameter *`o`*, that should be a set, or a function returning
a set with an `options` key defining the sub-options. Submodule option
definitions are type-checked accordingly to the `options` declarations.
Of course, you can nest submodule option definitions for even higher
modularity.

The option set can be defined directly
([Example: Directly defined submodule](#ex-submodule-direct)) or as reference
([Example: Submodule defined as a reference](#ex-submodule-reference)).

Note that even if your submodule’s options all have a default value,
you will still need to provide a default value (e.g. an empty attribute set)
if you want to allow users to leave it undefined.

::: {#ex-submodule-direct .example}
### Directly defined submodule
```nix
{
  options.mod = mkOption {
    description = "submodule example";
    type =
      with types;
      submodule {
        options = {
          foo = mkOption { type = int; };
          bar = mkOption { type = str; };
        };
      };
  };
}
```
:::

::: {#ex-submodule-reference .example}
### Submodule defined as a reference
```nix
let
  modOptions = {
    options = {
      foo = mkOption { type = int; };
      bar = mkOption { type = int; };
    };
  };
in
{
  options.mod = mkOption {
    description = "submodule example";
    type = with types; submodule modOptions;
  };
}
```
:::

The `submodule` type is especially interesting when used with composed
types like `attrsOf` or `listOf`. When composed with `listOf`
([Example: Declaration of a list of submodules](#ex-submodule-listof-declaration)), `submodule` allows
multiple definitions of the submodule option set
([Example: Definition of a list of submodules](#ex-submodule-listof-definition)).

::: {#ex-submodule-listof-declaration .example}
### Declaration of a list of submodules
```nix
{
  options.mod = mkOption {
    description = "submodule example";
    type =
      with types;
      listOf (submodule {
        options = {
          foo = mkOption { type = int; };
          bar = mkOption { type = str; };
        };
      });
  };
}
```
:::

::: {#ex-submodule-listof-definition .example}
### Definition of a list of submodules
```nix
{
  config.mod = [
    {
      foo = 1;
      bar = "one";
    }
    {
      foo = 2;
      bar = "two";
    }
  ];
}
```
:::

When composed with `attrsOf`
([Example: Declaration of attribute sets of submodules](#ex-submodule-attrsof-declaration)), `submodule` allows
multiple named definitions of the submodule option set
([Example: Definition of attribute sets of submodules](#ex-submodule-attrsof-definition)).

::: {#ex-submodule-attrsof-declaration .example}
### Declaration of attribute sets of submodules
```nix
{
  options.mod = mkOption {
    description = "submodule example";
    type =
      with types;
      attrsOf (submodule {
        options = {
          foo = mkOption { type = int; };
          bar = mkOption { type = str; };