notes/os/linux/nixos/features/el/expr/let.txt

Let expressions

    This kind of expression is used to define local variables for inner expressions.

        nix-repl> let a = "foo"; in a
            "foo"

    The syntax is:
        first assign variables,
        then in,
        then an expression which can use the defined variables.

    The value of the whole let expression will be the value of the expression after the in.

        nix-repl> let a = 3; b = 4; in a + b
            7

    Let's write two let expressions, one inside the other:

        nix-repl> let a = 3; in let b = 4; in a + b
            7

    With let you cannot assign twice to the same variable. However, you can shadow outer variables:

        nix-repl> let a = 3; a = 8; in a
            error: attribute `a' at (string):1:12 already defined at (string):1:5
        nix-repl> let a = 3; in let a = 8; in a
            8

    You cannot refer to variables in a let expression outside of it:

        nix-repl> let a = (let b = 3; in b); in b
            error: undefined variable `b' at (string):1:31

    You can refer to variables in the let expression when assigning variables, like with recursive attribute sets:

        nix-repl> let a = 4; b = a + 5; in b
            9

    So beware when you want to refer to a variable from the outer scope, but it's also defined in the current let expression.
    The same applies to recursive attribute sets