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expand on interpolated expressions

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Valentin Gagarin 2023-10-07 02:27:06 +02:00
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120
doc/manual/src/language/string-interpolation.md
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@ -1,19 +1,12 @@
# String interpolation
String interpolation is a language feature where a [string], [path], or [attribute name] can contain expressions enclosed in `${ }` (dollar-sign with curly brackets).
String interpolation is a language feature where a [string], [path], or [attribute name][attribute set] can contain expressions enclosed in `${ }` (dollar-sign with curly brackets).
Such a string is an *interpolated string*, and an expression inside is an *interpolated expression*.
Interpolated expressions must evaluate to one of the following:
- a [string]
- a [path]
- a [derivation]
Such a construct is called *interpolated string*, and the expression inside is an [interpolated expression](#interpolated-expression).
[string]: ./values.md#type-string
[path]: ./values.md#type-path
[attribute name]: ./values.md#attribute-set
[derivation]: ../glossary.md#gloss-derivation
[attribute set]: ./values.md#attribute-set
## Examples
@ -80,3 +73,110 @@ let name = "foo"; in
```
{ foo = "bar"; }
# Interpolated expression
An interpolated expression must evaluate to one of the following:
- a [string]
- a [path]
- an [attribute set] that has a `__toString` attribute or an `outPath` attribute
- `__toString` must be a function that takes the attribute set itself and returns a string
- `outPath` must be a string
This includes [derivations](./derivations.md) or [flake inputs](@docroot@/command-ref/new-cli/nix3-flake.md#flake-inputs) (experimental).
A string interpolates to itself.
A path in an interpolated expression is first copied into the Nix store, and the resulting string is the [store path] of the newly created [store object](../glossary.md#gloss-store-object).
[store path]: ../glossary.md#gloss-store-path
> **Example**
>
> ```console
> $ mkdir foo
> ```
>
> Reference the empty directory in an interpolated expression:
>
> ```nix
> "${./foo}"
> ```
>
> "/nix/store/2hhl2nz5v0khbn06ys82nrk99aa1xxdw-foo"
A derivation interpolates to the [store path] of its first [output](./derivations.md#attr-outputs).
> **Example**
>
> ```nix
> let
> pkgs = import <nixpkgs> {};
> in
> "${pkgs.hello}"
> ```
>
> "/nix/store/4xpfqf29z4m8vbhrqcz064wfmb46w5r7-hello-2.12.1"
An attribute set interpolates to the return value of the function in the `__toString` applied to the attribute set itself.
> **Example**
>
> ```nix
> let
> a = {
> value = 1;
> __toString = self: toString (self.value + 1);
> };
> in
> "${a}"
> ```
>
> "2"
An attribute set also interpolates to the value of its `outPath` attribute.
> **Example**
>
> ```nix
> let
> a = { outPath = "foo"; };
> in
> "${a}"
> ```
>
> "foo"
If both `__toString` and `outPath` are present in an attribute set, `__toString` takes precedence.
> **Example**
>
> ```nix
> let
> a = { __toString = _: "yes"; outPath = throw "no"; };
> in
> "${a}"
> ```
>
> "yes"
If neither is present, an error is thrown.
> **Example**
>
> ```nix
> let
> a = {};
> in
> "${a}"
> ```
>
> error: cannot coerce a set to a string
>
> at «string»:4:2:
>
> 3| in
> 4| "${a}"
> | ^

12
doc/manual/src/language/values.md
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@ -112,18 +112,16 @@
environment variable `NIX_PATH` will be searched for the given file
or directory name.
When an [interpolated string][string interpolation] evaluates to a path, the path is first copied into the Nix store and the resulting string is the [store path] of the newly created [store object].
[store path]: ../glossary.md#gloss-store-path
[store object]: ../glossary.md#gloss-store-object
For instance, evaluating `"${./foo.txt}"` will cause `foo.txt` in the current directory to be copied into the Nix store and result in the string `"/nix/store/<hash>-foo.txt"`.
Note that the Nix language assumes that all input files will remain _unchanged_ while evaluating a Nix expression.
For example, assume you used a file path in an interpolated string during a `nix repl` session.
Later in the same session, after having changed the file contents, evaluating the interpolated string with the file path again might not return a new store path, since Nix might not re-read the file contents.
Later in the same session, after having changed the file contents, evaluating the interpolated string with the file path again might not return a new [store path], since Nix might not re-read the file contents.
Paths themselves, except those in angle brackets (`< >`), support [string interpolation].
[store path]: ../glossary.md#gloss-store-path
Paths, except those in angle brackets (`< >`), support [string interpolation] and can be used in [interpolated expressions].
[interpolated expressions]: ./string-interpolation.md#interpolated-expressions
At least one slash (`/`) must appear *before* any interpolated expression for the result to be recognized as a path.

4
src/libexpr/primops.cc
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@ -260,9 +260,7 @@ static RegisterPrimOp primop_import({
.doc = R"(
Load, parse and return the Nix expression in the file *path*.
The value *path* can be a path, a string, or an attribute set with an
`__toString` attribute or a `outPath` attribute (as derivations or flake
inputs typically have).
The *path* argument must meet the same criteria as an [interpolated expression](@docroot@/language/string-interpolation.md#interpolated-expression).
If *path* is a directory, the file `default.nix` in that directory
is loaded.