OCaml Class 1: Mutability

• While you can't change which value is bound to a variable, you can change the value itself!

• Sometimes. Certain values are mutable and certain values are not.

• Mutable:

  Strings, arrays, records, refs.

  Immutable:

  Everything else (including lists)

• "Mutable values but not mutable variables? Isn't this a distinction without a difference?"

• No. This scheme allows the OCaml compiler both to:

  • generate more efficient code
  • catch more of your errors

• Each type of mutable value has it's own distinct syntax for mutation (updating); this helps catch errors:

• Strings: individual characters are mutable; result: efficient string manipulation (compare: Haskell, Erlang)

  str.[2] <- 'x'

  Arrays: individual array elements are mutable

  arr.(0) <- 56

• Let's make every other character in a string 'x':

    let str = "fuzzy" in
      for i = 0 to String.length str do
        if i mod 2 = 0 then
          str.[i] <- 'x'
        else
          ()
      done