OCaml Class 1: Immutability

• Because OCaml is a functional language, variables are immutable

• You can't change the value of a function parameter: there's no assignment statement for function parameters!

  • Invalid!

    let f x = x := x + 1

  Function parameters are initialized at the call, but can't be changed thereafter (i.e. within the body of the function).

• Same for let-bindings! (Because let-bindings and function parameter bindings are, in fact, identical.)

• You might think you can change the binding with an inner let:

  • One x, another x...

    let f x = let x = x + 1 in x

  This is valid, useful, and idiomatic, but the inner x is a completely different binding, due to the nesting: you haven't changed f's x.

• But how do we write a while loop?

  	let running = true in
  	  while running do
  	    ???
  	  done

  The following loop will never terminate, because the inner variable running is not the same as the loop-control variable!

  	let running = true in
  	  while running do
  	    let running = false in
  	      (* anything *)
  	  done

• A purely functional programming language only has immutable variables. This is (proved) sufficient to write any possible program.

• But OCaml is not a purely functional language. It has immutable variable bindings but has mutable values.