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

Examples

String

Rather than writing

"--with-freetype2-library=" + freetype + "/lib"

(where freetype is a derivation), you can instead write

"--with-freetype2-library=${freetype}/lib"

The latter is automatically translated to the former.

A more complicated example (from the Nix expression for Qt):

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

./. + "/" + foo + "-" + bar + ".nix"

or

./. + "/${foo}-${bar}.nix"

you can instead write

./${foo}-${bar}.nix

Attribute name

Attribute names can be created dynamically with string interpolation:

let name = "foo"; in
{
  ${name} = "bar";
}

{ foo = "bar"; }