mirror of
https://github.com/privatevoid-net/nix-super.git
synced 2024-11-15 18:56:16 +02:00
eb7d7780b1
This is needed to avoid this https://github.com/mesonbuild/meson/issues/13774 when we go back to making our subproject directory `src`.
134 lines
5.6 KiB
Markdown
134 lines
5.6 KiB
Markdown
# 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 combine together values to create a build instructions for derivations without manually keeping track of where they come from.
|
|
Then the Nix language implicitly does that bookkeeping to efficiently obtain the closure of derivation inputs.
|
|
|
|
> **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;
|
|
> };
|
|
> }
|
|
> ```
|
|
|
|
[deriving path]: @docroot@/glossary.md#gloss-deriving-path
|
|
[store path]: @docroot@/glossary.md#gloss-store-path
|
|
[`builtins.storePath`]: ./builtins.md#builtins-storePath
|
|
|
|
- [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" ];
|
|
> };
|
|
> }
|
|
> ```
|
|
|
|
[`builtins.outputOf`]: ./builtins.md#builtins-outputOf
|
|
|
|
- [*derivation deep*]{#string-context-element-derivation-deep}
|
|
|
|
*derivation deep* is an advanced feature intended to be used with the
|
|
[`exportReferencesGraph` derivation attribute](./advanced-attributes.html#adv-attr-exportReferencesGraph).
|
|
A *derivation deep* string context element is a derivation path, and refers to both its outputs and the entire build closure of that derivation:
|
|
all its outputs, all the other derivations the given derivation depends on, and all the outputs of those.
|
|
|
|
> **Example**
|
|
>
|
|
> The best way to illustrate *derivation deep* string contexts is with [`builtins.addDrvOutputDependencies`].
|
|
> Take a regular constant string context element pointing to a derivation, and transform it into a "Derivation deep" string context element.
|
|
>
|
|
> ```nix
|
|
> builtins.getContext
|
|
> (builtins.addDrvOutputDependencies
|
|
> (builtins.storePath "/nix/store/fvchh9cvcr7kdla6n860hshchsba305w-hello-2.12.drv"))
|
|
> ```
|
|
> evaluates to
|
|
> ```nix
|
|
> {
|
|
> "/nix/store/fvchh9cvcr7kdla6n860hshchsba305w-hello-2.12.drv" = {
|
|
> allOutputs = true;
|
|
> };
|
|
> }
|
|
> ```
|
|
|
|
[`builtins.addDrvOutputDependencies`]: ./builtins.md#builtins-addDrvOutputDependencies
|
|
[`builtins.unsafeDiscardOutputDependency`]: ./builtins.md#builtins-unsafeDiscardOutputDependency
|
|
|
|
## Inspecting string contexts
|
|
|
|
Most basically, [`builtins.hasContext`] will tell whether a string has a non-empty context.
|
|
|
|
When more granular information is needed, [`builtins.getContext`] can be used.
|
|
It creates an [attribute set] representing the string context, which can be inspected as usual.
|
|
|
|
[`builtins.hasContext`]: ./builtins.md#builtins-hasContext
|
|
[`builtins.getContext`]: ./builtins.md#builtins-getContext
|
|
[attribute set]: ./types.md#attribute-set
|
|
|
|
## Clearing string contexts
|
|
|
|
[`buitins.unsafeDiscardStringContext`](./builtins.md#builtins-unsafeDiscardStringContext) will make a copy of a string, but with an empty string context.
|
|
The returned string can be used in more ways, e.g. by operators that require the string context to be empty.
|
|
The requirement to explicitly discard the string context in such use cases helps ensure that string context elements are not lost by mistake.
|
|
The "unsafe" marker is only there to remind that Nix normally guarantees that dependencies are tracked, whereas the returned string has lost them.
|
|
|
|
## Constructing string contexts
|
|
|
|
[`builtins.appendContext`] will create a copy of a string, but with additional string context elements.
|
|
The context is specified explicitly by an [attribute set] in the format that [`builtins.hasContext`] produces.
|
|
A string with arbitrary contexts can be made like this:
|
|
|
|
1. Create a string with the desired string context elements.
|
|
(The contents of the string do not matter.)
|
|
2. Dump its context with [`builtins.getContext`].
|
|
3. Combine it with a base string and repeated [`builtins.appendContext`] calls.
|
|
|
|
[`builtins.appendContext`]: ./builtins.md#builtins-appendContext
|