CODEWITHSUNDEEP
ocaml
Terrence Brannon
Terrence Brannon

Reputation: 4978

Using an integer reference in a let expression

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

Upvotes: 2

Views: 153

Answers (2)

mookid
mookid

Reputation: 1172

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).

Upvotes: 0

Richard-Degenne
Richard-Degenne

Reputation: 2959

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.

Upvotes: 2

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