# String context

> **Note**
>
> This is an advanced topic.
> The Nix language is designed to be used without the programmer consciously dealing with string contexts or even knowing what they are.

A string in the Nix language is not just a sequence of characters like strings in other languages.
It is actually a pair of a sequence of characters and a *string context*.
The string context is an (unordered) set of *string context elements*.

The purpose of string contexts is to collect non-string values attached to strings via
[string concatenation](./operators.md#string-concatenation),
[string interpolation](./string-interpolation.md),
and similar operations.
The idea is that a user can reference other files when creating text files through Nix expressions, without manually keeping track of the exact paths.
Nix will ensure that the all referenced files are accessible – that all [store paths](@docroot@/glossary.md#gloss-store-path) are [valid](@docroot@/glossary.md#gloss-validity).

> **Note**
>
> String contexts are *not* explicitly manipulated in idiomatic Nix language code.

String context elements come in different forms:

- [deriving path]{#string-context-element-derived-path}

  A string context element of this type is a [deriving path](@docroot@/glossary.md#gloss-deriving-path).
  They can be either of type [constant](#string-context-constant) or [output](#string-context-output), which correspond to the types of deriving paths.

  - [Constant string context elements]{#string-context-constant}

    > **Example**
    >
    > [`builtins.storePath`] creates a string with a single constant string context element:
    >
    > ```nix
    > builtins.getContext (builtins.storePath "/nix/store/wkhdf9jinag5750mqlax6z2zbwhqb76n-hello-2.10")
    > ```
    > evaluates to
    > ```nix
    > {
    >   "/nix/store/wkhdf9jinag5750mqlax6z2zbwhqb76n-hello-2.10" = {
    >     path = true;
    >   };
    > }
    > ```


  - [Output string context elements]{#string-context-output}

    > **Example**
    >
    > The behavior of string contexts are best demonstrated with a built-in function that is still experimental: [`builtins.outputOf`].
    > This example will *not* work with stable Nix!
    >
    > ```nix
    > builtins.getContext
    >   (builtins.outputOf
    >     (builtins.storePath "/nix/store/fvchh9cvcr7kdla6n860hshchsba305w-hello-2.12.drv")
    >     "out")
    > ```
    > evaluates to
    > ```nix
    > {
    >   "/nix/store/fvchh9cvcr7kdla6n860hshchsba305w-hello-2.12.drv" = {
    >     outputs = [ "out" ];
    >   };
    > }