13th_Sept_list_and_trees

// refining list definition

// A list of a certain data type is null or a pair
// whose head is of that data type or 
// whose tail is a list of that data type

// example: list of numbers

// should be null, or a pair whose head is a number and 
// whose tail is a list of numbers


// const lst = null; -> can be a list of any type

// we can have a list of mixed types - it would just be known as "list"
// now we are trying to define a list of a particular type






// Calculating the length of the list

// recursive

function length(xs){
    return is_null(xs)
        ? 0
        : 1 + length(tail(xs));
}

// iterative

function lenght_iter(xs){
    
    function len(ys, sofar){
        
        return is_null(ys)
            ? sofar
            : len(tail(ys), 1 + sofar);
        
    }
    
    return len(xs, 0);
    
}