问题描述:

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?**

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.

Another possible error: `{n;_;_}`

should be `{n;_}`

Only 1 underscore is necessary. Think of it like the `_`

wildcard in a match statement.

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