Control Flow

Nix is a functional language to behave as no side-effect, all variables are isolated in scope and all controls flow statement can have implicit return.

Let-in

let..in statement is used to create a isolated scope to perform operation on variable declared after let and requires a return after in.

let
    foo = 1;
in
    foo # foo is returned

Since it supports implicit return, you store the value to a variable.

let
  foo =
    let
      bar = 123; 
    in
    bar + 1; 
in
foo

NOTE

Child scope inside let..in statement can access variables from parent scope.

Value Fallback

foo or "foo"
# equvalent to
if foo != null then foo else "foo"

With Local Expanding

with works similarly to let..in, it creates a new scope and implicitly declares all attributes of a set as variables into the new scope.

let
  foo = {
    bar = 123;
    baz = 123;
  };
in
with foo;
{
  inherit bar; # foo.bar and foo.baz is available in scope as variables #
  inherit baz; 
}

If a attribute expanded by with conflicts with any variable in scope by its name, the variable is preferred.

let
  foo = {
    bar = 123;
  };
  bar = 456; 
in
with foo;
{
  inherit bar; # bar = 456 #
}

Assertion