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140 lines
7.2 KiB
Markdown
140 lines
7.2 KiB
Markdown
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# Import From Derivation
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The value of a Nix expression can depend on the contents of a [store object](@docroot@/glossary.md#gloss-store-object).
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Passing an expression `expr` that evaluates to a [store path](@docroot@/glossary.md#gloss-store-path) to any built-in function which reads from the filesystem constitutes Import From Derivation (IFD):
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- [`import`](./builtins.md#builtins-import)` expr`
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- [`builtins.readFile`](./builtins.md#builtins-readFile)` expr`
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- [`builtins.readFileType`](./builtins.md#builtins-readFileType)` expr`
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- [`builtins.readDir`](./builtins.md#builtins-readDir)` expr`
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- [`builtins.pathExists`](./builtins.md#builtins-pathExists)` expr`
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- [`builtins.filterSource`](./builtins.md#builtins-filterSource)` f expr`
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- [`builtins.path`](./builtins.md#builtins-path)` { path = expr; }`
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- [`builtins.hashFile`](./builtins.md#builtins-hashFile)` t expr`
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- `builtins.scopedImport x drv`
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When the store path needs to be accessed, evaluation will be paused, the corresponding store object [realised], and then evaluation resumed.
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[realised]: @docroot@/glossary.md#gloss-realise
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This has performance implications:
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Evaluation can only finish when all required store objects are realised.
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Since the Nix language evaluator is sequential, it only finds store paths to read from one at a time.
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While realisation is always parallel, in this case it cannot be done for all required store paths at once, and is therefore much slower than otherwise.
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Realising store objects during evaluation can be disabled by setting [`allow-import-from-derivation`](../command-ref/conf-file.md#conf-allow-import-from-derivation) to `false`.
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Without IFD it is ensured that evaluation is complete and Nix can produce a build plan before starting any realisation.
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## Example
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In the following Nix expression, the inner derivation `drv` produces a file with contents `hello`.
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```nix
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# IFD.nix
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let
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drv = derivation {
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name = "hello";
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builder = "/bin/sh";
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args = [ "-c" "echo -n hello > $out" ];
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system = builtins.currentSystem;
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};
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in "${builtins.readFile drv} world"
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```
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```shellSession
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nix-instantiate IFD.nix --eval --read-write-mode
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```
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```
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building '/nix/store/348q1cal6sdgfxs8zqi9v8llrsn4kqkq-hello.drv'...
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"hello world"
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```
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The contents of the derivation's output have to be [realised] before they can be read with [`readFile`](./builtins.md#builtins-readFile).
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Only then evaluation can continue to produce the final result.
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## Illustration
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As a first approximation, the following data flow graph shows how evaluation and building are interleaved, if the value of a Nix expression depends on realising a [store object].
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Boxes are data structures, arrow labels are transformations.
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```
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+----------------------+ +------------------------+
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| Nix evaluator | | Nix store |
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| .----------------. | | |
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| | Nix expression | | | |
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| '----------------' | | |
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| evaluate | | |
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| V | | |
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| .------------. | | .------------------. |
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| | derivation |----|-instantiate-|->| store derivation | |
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| '------------' | | '------------------' |
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| | | realise |
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| | | V |
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| .----------------. | | .--------------. |
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| | Nix expression |<-|----read-----|----| store object | |
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| '----------------' | | '--------------' |
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| evaluate | | |
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| V | | |
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| .------------. | | |
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| | value | | | |
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| '------------' | | |
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+----------------------+ +------------------------+
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```
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In more detail, the following sequence diagram shows how the expression is evaluated step by step, and where evaluation is blocked to wait for the build output to appear.
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```
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.-------. .-------------. .---------.
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|Nix CLI| |Nix evaluator| |Nix store|
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'-------' '-------------' '---------'
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|evaluate IFD.nix| |
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|--------------->| |
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| evaluate `"${readFile drv} world"` |
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| evaluate `readFile drv` |
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| evaluate `drv` as string |
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| |instantiate /nix/store/...-hello.drv|
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| |----------------------------------->|
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| : |
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| : realise /nix/store/...-hello.drv |
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| :----------------------------------->|
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| : |
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| |--------.
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| : | |
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| (evaluation blocked) | echo hello > $out
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| : | |
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| |<-------'
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| : /nix/store/...-hello |
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| |<-----------------------------------|
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| resume `readFile /nix/store/...-hello` |
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| | readFile /nix/store/...-hello |
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| |----------------------------------->|
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| | hello |
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| |<-----------------------------------|
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| resume `"${"hello"} world"` |
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| resume `"hello world"` |
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| "hello world" | |
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|<---------------| |
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.-------. .-------------. .---------.
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|Nix CLI| |Nix evaluator| |Nix store|
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'-------' '-------------' '---------'
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```
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