Reason: Omitting `;` from the last expression in a function

[ccorcos]

I was playing around with imperative control structures and had the following code:

type result 'a = {mutable list: list 'a};

let firstN (n: int) (l: list 'a) :list 'a => {
  let result: result 'a = {list: []};
  let start = 0;
  let stop = min (n - 1) (List.length l);
  for i in start to stop {
    result.list = List.append [List.nth l i] result.list
  }
};

The formatter would work, but I'd get a compile error that the return type was unit. That made sense, so I tried to return the result.list:

type result 'a = {mutable list: list 'a};

let firstN (n: int) (l: list 'a) :list 'a => {
  let result: result 'a = {list: []};
  let start = 0;
  let stop = min (n - 1) (List.length l);
  for i in start to stop {
    result.list = List.append [List.nth l i] result.list
  }
  result.list;
};

Now the formatter wouldn't work and the compiler said it was looking for a closing block for the for-loop. I was confused and couldn't figure out what was wrong.

The solution ended up being this:

type result 'a = {mutable list: list 'a};

let firstN (n: int) (l: list 'a) :list 'a => {
  let result: result 'a = {list: []};
  let start = 0;
  let stop = min (n - 1) (List.length l);
  for i in start to stop {
    result.list = List.append [List.nth l i] result.list
  };
  result.list
};

I noticed that when I put semi-colons at the end of the for loop block and at the end of the List.append expression in the first example, the formatter took them away. By complete coincidence, I put at the end of the for-loop block after putting result.list in there, and ah-ha! it worked.

---

long story short, semi-colons being omitted from the last expression in a block can cause a lot of confusion! when the formatter gets rid of a semi-colon, I don't expect to have to add it back when I add another line to try to make the compiler happy.

[jordwalke]

Interestingly, @yunxing and I realized that enforcing final semicolons eliminates some confusion around punning of single field records. Doing so might kill two birds with one stone.

[TheSpyder]

ah, so this _had_ been logged already. I'd also like to see semicolons retained at the end of functions that have multiple expressions to avoid confusion. They are already implicitly mandatory for single-expression functions, due to the lack of braces:

let f = (x) => x + 1;

[jordwalke]

I don't think you'd get any objection to adding the final semicolons back. Feel free to send a PR (I think this one is pretty easy).

[mrkaspa]

I like more rust's approach on semicolon, it's more similar to ocaml's one, the expression to be returned does not have semicolons

[jordwalke]

Reason's current parser also does not require the final semicolon. It does, however, print it - and this is so that you can more easily copy/paste lines between easily. Also, printing the final semicolon prepares us to finally accept single field punned records like {this}.

[jordwalke]

I think I'll close this out unless there's any specific request in light of recent changes (we automatically add the final ; back)