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# String interpolation
String interpolation is a language feature where a [string], [path], or [attribute name] can contain expressions enclosed in `${ }` (dollar-sign with curly brackets).
Such a string is an *interpolated string*, and an expression inside is an *interpolated expression*.
Interpolated expressions must evaluate to one of the following:
- a [string]
- a [path]
- a [derivation]
[string]: ./values.md#type-string
[path]: ./values.md#type-path
[attribute name]: ./values.md#attribute-set
[derivation]: ../glossary.md#gloss-derivation
## Examples
### String
Rather than writing
```nix
"--with-freetype2-library=" + freetype + "/lib"
```
(where `freetype` is a [derivation]), you can instead write
```nix
"--with-freetype2-library=${freetype}/lib"
```
The latter is automatically translated to the former.
A more complicated example (from the Nix expression for [Qt](http://www.trolltech.com/products/qt)):
```nix
configureFlags = "
-system-zlib -system-libpng -system-libjpeg
${if openglSupport then "-dlopen-opengl
-L${mesa}/lib -I${mesa}/include
-L${libXmu}/lib -I${libXmu}/include" else ""}
${if threadSupport then "-thread" else "-no-thread"}
";
```
Note that Nix expressions and strings can be arbitrarily nested;
in this case the outer string contains various interpolated expressions that themselves contain strings (e.g., `"-thread"`), some of which in turn contain interpolated expressions (e.g., `${mesa}`).
### Path
Rather than writing
```nix
./. + "/" + foo + "-" + bar + ".nix"
```
or
```nix
./. + "/${foo}-${bar}.nix"
```
you can instead write
```nix
./${foo}-${bar}.nix
```
### Attribute name
Attribute names can be created dynamically with string interpolation:
```nix
let name = "foo"; in
{
${name} = "bar";
}
```
{ foo = "bar"; }
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