Why ocamlc says I mismatched {} while I don't? - functional-programming

Question

I have written myPercolation.ml.
open MyUnionFind
module type MyPercolationSig = sig
type percolation
val create_percolation : int -> percolation
val open_site : percolation -> int -> int -> unit
val is_open : percolation -> int -> int -> bool
val is_full : percolation -> int -> int -> bool
val can_percolates : percolation -> bool
end
module MyPercolation : MyPercolationSig = struct
exception IndexOutOfBounds;;
type percolation =
{n : int;
sites: bool array;
union : MyUnionFind.union_find};;
let create_percolation n =
{n = n; sites = Array.make (n*n) false; union = MyUnionFind.create_union (n*n)};;
let open_site p i j =
let {n;_;union} = p
in
if not (is_open p i j) then
begin
sites.(index_of n i j) <- true;
if i - 1 >= 1 && i - 1 <= n && is_open n (i-1) j then
MyUnionFind.union union (index_of n i j) (index_of n (i-1) j)
else if i + 1 >= 1 && i + 1 <= n && is_open n (i+1) j then
MyUnionFind.union union (index_of n i j) (index_of n (i+1) j)
else if j - 1 >= 1 && j - 1 <= n && is_open n i (j-1) then
MyUnionFind.union union (index_of n i j) (index_of n i (j-1))
else if j + 1 >= 1 && j + 1 <= n && is_open n i (j+1) then
MyUnionFind.union union (index_of n i j) (index_of n i (j+1))
end;;
let index_of n i j = n * (i - 1) + j;;
let is_open {n;sites;_} i j =
if i < 1 || i > n || j < 1 || j > n then
raise IndexOutOfBounds
else
sites.(index_of n i j);;
let is_full {n;_;union} i j =
let rec is_connected_top j' =
if j = 0 then false
else
if MyUnionFind.is_connected union (index_of n i j) (index_of n 0 j') then true
else is_connected_top (j'-1)
in is_connected_top n;;
let can_percolates p =
let {n;_;_} = p
in
let rec is_full_bottom j =
if j = 0 then false
else
if is_full p n j then true
else is_full_bottom (j-1)
end
Please ignore the package MyUnionFind package. It is just a homemade implementation for union-find algorithm.
when I try to compile the myPercolation.ml, I got such an error:
$ ocamlc -c myPercolation.ml
File "myPercolation.ml", line 25, characters 11-12:
Error: Syntax error: '}' expected
File "myPercolation.ml", line 25, characters 8-9:
Error: This '{' might be unmatched
I think the error is talking about let {n;_;union} = p in function of let open_site p i j.
I have read through that line and all code many times, but I still don't see any mismatched {} in that line.
can anyone help please?

Answer 1

Another possible error: {n;_;_} should be {n;_} Only 1 underscore is necessary. Think of it like the _ wildcard in a match statement.

Answer 2

The expression let {n; _; union} = p is not well formed OCaml. I think what you want is let {n; union} = p. The way to handle fields you don't care about in a record pattern is not to mention them.
Update:
As rgrinberg points out, a much better way to describe the problem is that the _ has to appear as the last field. That's why the compiler is expecting to see } afterward. It might be good style to include the _ as an indicator that you're purposely matching only a subset of the fields of the record. You can, in fact, turn on a compiler option that checks for this.
Update 2:
The warning for incomplete record patterns is warning number 9, and also is associated with the letter R. Here's how to use R:
$ ocaml -w +R
OCaml version 4.00.0
# type r = { a: int; b: char };;
type r = { a : int; b : char; }
# let {a} = {a=3; b='x'} in a;;
Warning 9: the following labels are not bound in this record pattern:
b
Either bind these labels explicitly or add '; _' to the pattern.
- : int = 3
The command-line syntax is the same for the compiler.