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Document string context (NixOS#8595)
* Document string context Now what we have enough primops, we can document how string contexts work. Co-authored-by: Robert Hensing <roberth@users.noreply.github.com> Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com> Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io> Co-authored-by: Felix Uhl <iFreilicht@users.noreply.github.com>
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| # String context | ||
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| > **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*. | ||
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| 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. | ||
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| > **Note** | ||
| > | ||
| > String contexts are *not* explicitly manipulated in idiomatic Nix language code. | ||
| String context elements come in different forms: | ||
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| - [deriving path]{#string-context-element-derived-path} | ||
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| 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. | ||
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| - [Constant string context elements]{#string-context-constant} | ||
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| > **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]: ./values.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 |
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