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This is needed to avoid this https://github.com/mesonbuild/meson/issues/13774 when we go back to making our subproject directory `src`.
245 lines
5 KiB
Markdown
245 lines
5 KiB
Markdown
# String interpolation
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String interpolation is a language feature where a [string], [path], or [attribute name][attribute set] can contain expressions enclosed in `${ }` (dollar-sign with curly brackets).
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Such a construct is called *interpolated string*, and the expression inside is an [interpolated expression](#interpolated-expression).
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[string]: ./types.md#type-string
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[path]: ./types.md#type-path
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[attribute set]: ./types.md#attribute-set
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> **Syntax**
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>
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> *interpolation_element* → `${` *expression* `}`
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## Examples
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### String
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Rather than writing
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```nix
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"--with-freetype2-library=" + freetype + "/lib"
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```
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(where `freetype` is a [derivation]), you can instead write
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[derivation]: @docroot@/glossary.md#gloss-derivation
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```nix
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"--with-freetype2-library=${freetype}/lib"
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```
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The latter is automatically translated to the former.
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A more complicated example (from the Nix expression for [Qt](http://www.trolltech.com/products/qt)):
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```nix
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configureFlags = "
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-system-zlib -system-libpng -system-libjpeg
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${if openglSupport then "-dlopen-opengl
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-L${mesa}/lib -I${mesa}/include
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-L${libXmu}/lib -I${libXmu}/include" else ""}
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${if threadSupport then "-thread" else "-no-thread"}
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";
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```
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Note that Nix expressions and strings can be arbitrarily nested;
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in this case the outer string contains various interpolated expressions that themselves contain strings (e.g., `"-thread"`), some of which in turn contain interpolated expressions (e.g., `${mesa}`).
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To write a literal `${` in an regular string, escape it with a backslash (`\`).
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> **Example**
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>
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> ```nix
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> "echo \${PATH}"
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> ```
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>
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> "echo ${PATH}"
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To write a literal `${` in an indented string, escape it with two single quotes (`''`).
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> **Example**
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>
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> ```nix
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> ''
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> echo ''${PATH}
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> ''
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> ```
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>
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> "echo ${PATH}\n"
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`$${` can be written literally in any string.
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> **Example**
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>
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> In Make, `$` in file names or recipes is represented as `$$`, see [GNU `make`: Basics of Variable Reference](https://www.gnu.org/software/make/manual/html_node/Reference.html#Basics-of-Variable-References).
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> This can be expressed directly in the Nix language strings:
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>
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> ```nix
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> ''
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> MAKEVAR = Hello
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> all:
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> @export BASHVAR=world; echo $(MAKEVAR) $${BASHVAR}
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> ''
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> ```
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>
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> "MAKEVAR = Hello\nall:\n\t@export BASHVAR=world; echo $(MAKEVAR) $\${BASHVAR}\n"
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See the [documentation on strings][string] for details.
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### Path
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Rather than writing
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```nix
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./. + "/" + foo + "-" + bar + ".nix"
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```
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or
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```nix
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./. + "/${foo}-${bar}.nix"
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```
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you can instead write
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```nix
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./${foo}-${bar}.nix
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```
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### Attribute name
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<!--
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FIXME: these examples are redundant with the main page on attribute sets.
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figure out what to do about that
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-->
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Attribute names can be interpolated strings.
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> **Example**
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>
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> ```nix
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> let name = "foo"; in
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> { ${name} = 123; }
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> ```
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>
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> { foo = 123; }
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Attributes can be selected with interpolated strings.
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> **Example**
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>
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> ```nix
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> let name = "foo"; in
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> { foo = 123; }.${name}
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> ```
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>
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> 123
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# Interpolated expression
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An expression that is interpolated must evaluate to one of the following:
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- a [string]
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- a [path]
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- an [attribute set] that has a `__toString` attribute or an `outPath` attribute
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- `__toString` must be a function that takes the attribute set itself and returns a string
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- `outPath` must be a string
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This includes [derivations](./derivations.md) or [flake inputs](@docroot@/command-ref/new-cli/nix3-flake.md#flake-inputs) (experimental).
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A string interpolates to itself.
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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](@docroot@/store/store-object.md).
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[store path]: @docroot@/store/store-path.md
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> **Example**
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>
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> ```console
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> $ mkdir foo
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> ```
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>
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> Reference the empty directory in an interpolated expression:
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>
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> ```nix
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> "${./foo}"
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> ```
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>
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> "/nix/store/2hhl2nz5v0khbn06ys82nrk99aa1xxdw-foo"
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A derivation interpolates to the [store path] of its first [output](./derivations.md#attr-outputs).
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> **Example**
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>
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> ```nix
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> let
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> pkgs = import <nixpkgs> {};
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> in
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> "${pkgs.hello}"
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> ```
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>
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> "/nix/store/4xpfqf29z4m8vbhrqcz064wfmb46w5r7-hello-2.12.1"
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An attribute set interpolates to the return value of the function in the `__toString` applied to the attribute set itself.
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> **Example**
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>
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> ```nix
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> let
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> a = {
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> value = 1;
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> __toString = self: toString (self.value + 1);
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> };
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> in
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> "${a}"
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> ```
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>
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> "2"
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An attribute set also interpolates to the value of its `outPath` attribute.
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> **Example**
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>
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> ```nix
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> let
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> a = { outPath = "foo"; };
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> in
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> "${a}"
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> ```
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>
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> "foo"
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If both `__toString` and `outPath` are present in an attribute set, `__toString` takes precedence.
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> **Example**
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>
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> ```nix
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> let
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> a = { __toString = _: "yes"; outPath = throw "no"; };
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> in
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> "${a}"
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> ```
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>
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> "yes"
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If neither is present, an error is thrown.
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> **Example**
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>
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> ```nix
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> let
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> a = {};
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> in
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> "${a}"
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> ```
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>
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> error: cannot coerce a set to a string: { }
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>
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> at «string»:4:2:
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>
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> 3| in
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> 4| "${a}"
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> | ^
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