Let expressions

Sometimes you only want to give a name to something in a limited scope. You can do that with a let expression.

circumference radius =
    let
        pi = 3.14159
    in
    2 * pi * radius

Here, pi only exists in the circumference function.

A let expression can be helpful when you want to express logic as a series of steps.

daysToSeconds days =
    let
        hours = 24 * days
        minutes = 60 * hours
        seconds = 60 * minutes
    in
    seconds

You can define functions in let expressions too.

quadruple num =
    let
        double n =
            n * 2
    in
    double (double num)