Using an integer reference in a let expression

Question

I am attempting a code sample in Lesson 5, Step 6 of the Try Ocaml tutorial

We were supposed to fix this code sample:

let one =
  let accum = ref -54 in
  for i = 1 to ten do accum := !accum + i done ;
  !accum

and here is my attempt:

let one =
  let accum = ref -54 in (
      for i = 1 to 10 do
        accum := accum + i
      done
    ;
      !accum
    )

but unfortunately I am receiving the error message:

line 2, characters 14-17: Error: This expression has type 'a -> 'a ref but an expression was expected of type int

Richard-Degenne · Accepted Answer · 2017-04-24 08:00:34Z

2

You're missing parentheses around -54.

let one =
  let accum = ref (-54) in
  for i=1 to 10 do
    accum := !accum + i
  done;
  !accum
;;

ref is a function that has type 'a -> 'a ref, and the minus operator (-) has type int -> int -> int. Here, 54 is an int but ref is not, hence the type error message.

answered Apr 24, 2017 at 8:00

Richard-Degenne

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2 Comments

hcarty Over a year ago

You can also use ~- which is the prefix integer negation operator, avoiding the need to wrap extra parens.

Richard-Degenne Over a year ago

Nice, I didn't know that.

mookid · Accepted Answer · 2017-04-24 15:24:55Z

0

One weirdness of the lexer of ocaml is that -54 corresponds to two tokens.

Hence, your code corresponds to let accum = ref (-) 54 in

which yields the mentioned type error. The solution is to add parenthesis and write (-54).

answered Apr 24, 2017 at 15:24

mookid

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