Common Mistakes

Functions

The space after : is required. Without a space, nix will parse the value as an url, and represent it as a string

nix-repl> :t x: x
a function

nix-repl> :t x:x
a string

Lists

Functions and lists use whitespace to do function application, however, list element delimitation takes precedence over function application.

For example, if someone were to try and use optional python integration on a package, they may write something like:

  extraPackages = [
    somePackage.override { withPython = true; }
  ];

In this example, it’s an array of two elements, somePackage.override is a function, and the other element is an attr set. This is more accurately represented as:

  extraPackages = [
    (somePackage.override) # type: Attr -> drv
    ({ withPython = true; }) # type: Attr
  ];

The correct usage of this would be:

  extraPackages = [
    (somePackage.override { withPython = true; }) # type: drv
  ];