How to generate a random number in Scheme.

I couldn't find a function that generates a random number.
Instead, I make that function using "".

I wont to get current time for a seed.
But I couldn't a function for it.

(define (rand seed)
  (let ((x seed))
    (lambda ()
      (set! x (remainder (+ (* 13 x) 5) 24))
      x)))

(define a (rand 1))

(display (a))
(newline)
(display (a))
(newline)
(display (a))
(newline)

18
23
16

Closure of Scheme.

The simplest closure sample of Scheme.

(define (f x)
  (let ((y x))
    (lambda ()
      (set! y (+ y 1))
      y)))

> (define a (f 1))
> (display (a))
2
> (display (a))
3
> (display (a))
4

Conditional branches of Haskell.

I'm studying Haskell.

Haskell has some conditional statements.

1. pattern matching

fact 0 = 1
fact n = n * fact (n-1)

2. guard

fact n | n==0 = 1
       | otherwise = n * fact (n-1)

3. if

fact n = if n==0 then 1
                 else n * fact (n-1)

4. case

fact n = case n of
           0 -> 1
           _ -> n * fact (n-1)

I try Haskell.

I'll conduct study sessions at my office this week.
I learned the basics of Haskell for it.
It's interesting to be able to create an infinite list by delay evaluation.

Output Fibonacci number without recurrence.

It is possible to write a code for Fibonacci number without recurrence.
Alternatively the code include a loop.

def fib(n)
  a, b, c = 0, 1, 0

  n.times do
    c = a
    a = b
    b += c
  end

  a
end

p fib(10)

Use closure for event handler.

This is a sample program.
A number displayed on a label increases when a button is clicked.

public partial class Form1 : Form
{
   public Form1()
   {
       InitializeComponent();

       this.button1.Click += new EventHandler(button1_Click);
   }

   int num = 0;

   void button1_Click(object sender, EventArgs e)
   {
       this.label1.Text = num.ToString();
       num++;
   }
}

This code uses a member field "num".


I rewrite this code using closure.

public partial class Form1 : Form
{
   public Form1()
   {
       InitializeComponent();

       this.button1.Click += make_button1_Click(0);
   }

   EventHandler make_button1_Click(int num)
   {
       return delegate(object sender, EventArgs e)
       {
           this.label1.Text = num.ToString();
           num++;
       };
   }
}

This code uses no member field.
Closure memorize the variable "num".

I tried F# programming.

F# code for Fibonacci number.

let fib n =
  let rec fib_iter n a b =
    match n with
      | 0 -> a
      | _ -> fib_iter (n - 1) b (a + b) in
  fib_iter n 0 1;;
    
printfn "%d" (fib 10);;

F# code for a maximum value.

let lst = [3;1;4;1;5;9;2];;

let rec fmax l x =
  match l with
    | [] -> x
    | lh::lt ->
      fmax lt
        (if x > lh then x
         else lh);;

printfn "%d" (fmax lst 0);;

I feel that the expression of F# is more powerful than Scheme.