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>
NixOS · Dec 20, 2023 · 5f05df6 · 5f05df6
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diff --git a/doc/manual/src/SUMMARY.md.in b/doc/manual/src/SUMMARY.md.in
@@ -27,6 +27,7 @@
   - [Language Constructs](language/constructs.md)
     - [String interpolation](language/string-interpolation.md)
     - [Lookup path](language/constructs/lookup-path.md)
+    - [String context](language/string-context.md)
   - [Operators](language/operators.md)
   - [Derivations](language/derivations.md)
     - [Advanced Attributes](language/advanced-attributes.md)

diff --git a/doc/manual/src/glossary.md b/doc/manual/src/glossary.md
@@ -218,6 +218,19 @@
 
   [output path]: #gloss-output-path
 
+- [deriving path]{#gloss-deriving-path}
+
+  A [store]-level expression (not Nix language expression) that denotes a [store object].
+  There are two forms:
+
+  - *constant*: A constant deriving path is just a [store path]
+    It can be made valid by copying it into the store, from the evaluator, command line interface or another store.
+
+  - *output*: An output deriving path is a pair of a [store path] to a [derivation] and an [output] name.
+
+  Deriving paths are necessary because in general, we do not know what the [output path] path of an [output] will be until it is [realise]d.
+  Deriving paths provide a way to refer to unrealised outputs in order to be able to tell Nix to realise them.
+
 - [deriver]{#gloss-deriver}
 
   The [store derivation] that produced an [output path].

diff --git a/doc/manual/src/language/string-context.md b/doc/manual/src/language/string-context.md
@@ -0,0 +1,142 @@
+# 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 recipe without manually keeping track of where the "ingredients" come from, and then the Nix language does that bookkeeping implicitly to come up with the right derivation inputs.
+
+> In line with this goal, string contexts are *not* explicitly manipulated in idiomatic Nix code.
+> Strings with non-empty contexts are only concatenated and eventually passed to `builtins.derivation`.
+> Plain strings with empty contexts are the only ones to be inspected, e.g. using comparison with `==`.
+
+String context elements come in different forms:
+
+- [*constant*]{#string-context-element-constant}
+- [*output*]{#string-context-element-output}
+- [*derivation deep*]{#string-context-element-derivation-deep}
+
+*Constant* and *output* string contexts elements are just
+[deriving paths](@docroot@/glossary.md#gloss-deriving-path);
+those are just the names of the two kinds of deriving path.
+See the documentation on deriving paths for further details.
+
+*derivation deep* is an advanced feature intended to be used with the
+[`exportReferencesGraph`](./advanced-attributes.html#adv-attr-exportReferencesGraph)
+advanced derivation feature.
+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 their outputs too.
+
+## 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#) 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.
+
+The remainder of this section will focus on step 1: making strings with individual string context elements on which to apply `builtins.getContext`.
+
+[`builtins.appendContext`]: ./builtins.md#builtins-appendContext
+
+### Constant string context elements
+
+A constant string context element is just a constant [deriving path];
+a constant deriving path is just a [store path].
+We therefore want to use [`builtins.storePath`] to create a string with a single constant string context element:
+
+> **Example**
+>
+> ```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
+
+> **Example**
+>
+> This is best illustrated with a built-in function that is still experimental: [`builtins.ouputOf`].
+> This example will *not* work the 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 elements
+
+The best way to illustrate this is with [`builtins.addDrvOutputDependencies`].
+We take a regular constant string context element pointing to a derivation, and transform it into a "Derivation deep" string context element.
+
+> **Example**
+>
+> ```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
diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
@@ -75,7 +75,11 @@ static RegisterPrimOp primop_unsafeDiscardOutputDependency({
     .name = "__unsafeDiscardOutputDependency",
     .args = {"s"},
     .doc = R"(
-      Create a copy of the given string where every "derivation deep" string context element is turned into a constant string context element.
+      Create a copy of the given string where every
+      [derivation deep](@docroot@/language/string-context.md#string-context-element-derivation-deep)
+      string context element is turned into a
+      [constant](@docroot@/language/string-context.md#string-context-element-constant)
+      string context element.
 
       This is the opposite of [`builtins.addDrvOutputDependencies`](#builtins-addDrvOutputDependencies).
 
@@ -137,7 +141,11 @@ static RegisterPrimOp primop_addDrvOutputDependencies({
     .name = "__addDrvOutputDependencies",
     .args = {"s"},
     .doc = R"(
-      Create a copy of the given string where a single consant string context element is turned into a "derivation deep" string context element.
+      Create a copy of the given string where a single
+      [constant](@docroot@/language/string-context.md#string-context-element-constant)
+      string context element is turned into a
+      [derivation deep](@docroot@/language/string-context.md#string-context-element-derivation-deep)
+      string context element.
 
       The store path that is the constant string context element should point to a valid derivation, and end in `.drv`.