[Caml-list] type inference of record field that is an array

[Caml-list] type inference of record field that is an array

[Tim Leonard]

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Is type inference working as expected here, or is this a bug?

# type record_type = { array_field : int array };;
type record_type = { array_field : int array; }
# let test_function_1 x = Array.set x.array_field 1 3;;
val test_function_1 : record_type -> unit = <fun>
# let test_function_2 x = x.array_field.[1] <- 3;;
Characters 24-46:
  let test_function_2 x = x.array_field.[1] <- 3;;
                          ^^^^^^^^^^^^^^^^^^^^^^
Warning 3: deprecated: String.set
Use Bytes.set instead.
Characters 24-37:
  let test_function_2 x = x.array_field.[1] <- 3;;
                          ^^^^^^^^^^^^^
Error: This expression has type int array
       but an expression was expected of type bytes = string
# 

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Re: [Caml-list] type inference of record field that is an array

[Ivan Gotovchits]

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Hi Tim,

The `x.[i] <- v` syntax maps to `String.set` function, that has type
`string -> char -> unit`, the array subscription syntax uses parentheses
instead of the brackets. So in your case, it should be

    x.array_field.(1) <- 3


If you are concerned with the warning, that is emitted before the error.
Then this is expected, as `x.[i] <- v` is a syntactic sugar to the
`String.set` and it is translated on the lexing level. Thus the type
checker never sees the array/string/bigarray subscript operators (as they
are not present in the AST)

Regards,
Ivan

On Tue, May 16, 2017 at 12:58 PM, Tim Leonard <Tim@timleonard.us> wrote:

> Is type inference working as expected here, or is this a bug?
>
> # type record_type = { array_field : int array };;
> type record_type = { array_field : int array; }
> # let test_function_1 x = Array.set x.array_field 1 3;;
> val test_function_1 : record_type -> unit = <fun>
> # let test_function_2 x = x.array_field.[1] <- 3;;
> Characters 24-46:
>   let test_function_2 x = x.array_field.[1] <- 3;;
>                           ^^^^^^^^^^^^^^^^^^^^^^
> Warning 3: deprecated: String.set
> Use Bytes.set instead.
> Characters 24-37:
>   let test_function_2 x = x.array_field.[1] <- 3;;
>                           ^^^^^^^^^^^^^
> Error: This expression has type int array
>        but an expression was expected of type bytes = string
> #
>

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