NixOS/nix — pull request #90 by shlevy (2013-01-26 00:10:20+00:00)
Title: prim_functionArgs: More info about default values.

Rather than using a bool to indicate the presence or absence of a
default value for a given argument, provide a function that, when
called, evaluates to what the default value of that argument would be if
the arguments you called the function with were passed to the original
function. If an argument has no default, its corresponding attribute
will evaluate to null instead of a function as described above.