Remove (.[]<-)

[nojb]

This operator should have been removed when the function String.set was removed in #10867.

Fixes #11342

[xavierleroy]

The only potential downside I see to this change is that it rules out the following valid OCaml 4 code:

module BytesNotation = struct
    module String =
      let get = Bytes.get
      let set = Bytes.set
    end
  end;;
# let b = Bytes.make 25 'x';;
val b : bytes = Bytes.of_string "xxxxxxxxxxxxxxxxxxxxxxxxx"
# BytesNotation.(b.[2] <- 'y');;
- : unit = ()
# BytesNotation.(b.[2]);;
- : char = 'y'

I don't really want support this kind of abuse of notations, however.

[Octachron]

Do we want a specialized error message? It is quite exceptional that the removal of a deprecated function might result in a syntax error in user code. It might be help confused users to have a specific explanation rather than just a Error: Syntax error located on the assignment arrow <- .

[xavierleroy]

I agree that a specialized error message would be nice, at least in 5.0. It could be removed in later releases.

[nojb]

I agree that a specialized error message would be nice, at least in 5.0. It could be removed in later releases.

I gave this a try, it looks like this:

$ TERM=dumb ocaml
OCaml version 5.1.0+dev1-2022-06-09
Enter #help;; for help.

# s.[i] <- x;;
Line 1, characters 0-10:
1 | s.[i] <- x;;
    ^^^^^^^^^^
Error: the infix operator (.[]<-) used to be bound to String.set, which has been removed in OCaml 5.0

[dbuenzli]

Error: the infix operator (.[]<-) used to be bound to String.set, which has been removed in OCaml 5.0

The message doesn't say what the problem is. You should say it is illegal syntax.

[nojb]

Error: the infix operator (.[]<-) used to be bound to String.set, which has been removed in OCaml 5.0

The message doesn't say what the problem is. You should say it is illegal syntax.

OK, I tweaked the message:

$ TERM=dumb ocaml
OCaml version 5.1.0+dev1-2022-06-09
Enter #help;; for help.

# s.[i] <- c;;
Line 1, characters 0-10:
1 | s.[i] <- c;;
    ^^^^^^^^^^
Error: Syntax error: the infix operator (.[]<-) used to be bound to String.set, which was removed in OCaml 5.0

[kit-ty-kate]

I’m a bit surprised by this change. Where is the (.![]<-) operator defined? I wasn’t able to locate it

[nojb]

This test only checks that the source file can be parsed.

[kit-ty-kate]

ah I see. Thanks!

[Octachron]

I am not that fond of having history-focused user messages. When reading

 Syntax error: the infix operator (.[]<-) used to be bound to String.set, which was removed in OCaml 5.0

a new user will have to search what was String.set in older versions of OCaml, understand why it existed, then why it was removed before finally making the link with their current issue.

Even regular users might scratch their head about the information that .[]<- was bound to String.set (or String.unsafe_set) which was always an implementation detail.

I would propose to focus on the current situation:

• strings are immutable
• an assignment operator makes no sense for immutable data types
• there is a mutable sequence of bytes data types: Bytes.t

Something like

Syntax error, strings are immutable, there are no assignment syntax for them.
Hint: Mutable sequence of bytes are available in the Bytes module.

but I am missing a good way to bring up Bytes.set. And there is some potential confusion for people that knowingly ignored the deprecation warning for .[]<- and got used to the idea that .[]<- worked on bytes rather than strings.

[nojb]

Syntax error, strings are immutable, there are no assignment syntax for them.
Hint: Mutable sequence of bytes are available in the Bytes module.

Thanks, this feels much better. I amended the patch:

OCaml version 5.1.0+dev1-2022-06-09
Enter #help;; for help.

# s.[i] <- c;;
Line 1, characters 0-10:
1 | s.[i] <- c;;
    ^^^^^^^^^^
Error: Syntax error: strings are immutable, there is no assignment syntax for them.
Hint: Mutable sequences of bytes are available in the Bytes module.
#

[xavierleroy]

Looks good to me.

[nojb]

Looks good to me.

Thanks. I rebased on trunk (with the Changes entry on 5.0 section). After merging I will cherry-pick to the 5.0 branch.

[gasche]

Hint: Mutable sequences of bytes are available in the Bytes module.
Instead of `s.[i] <- c`, one may write `Bytes.set buf i c`.

(I'm not suggesting to pretty-print the actual subterms of the user, that sounds like a dubious idea, but I think showing some concrete syntax can help.)

[nojb]

Hint: Mutable sequences of bytes are available in the Bytes module.
Instead of `s.[i] <- c`, one may write `Bytes.set buf i c`.

(I'm not suggesting to pretty-print the actual subterms of the user, that sounds like a dubious idea, but I think showing some concrete syntax can help.)

Personally feels a bit overkill to me, but am not opposed. If no-one voices an objection, I'll amend accordingly.

[gasche]

I think that for beginners there is a certain distance between "use the module Bytes" and (1) finding the Bytes documentation then (2) figuring out that Bytes.set is the right function and (3) how to use it. Seeing the concrete code can save a bit of time.

[Octachron]

I agree that we want to mention Bytes.set. Do we need to refer to the previous syntax, however

Hint: Mutable sequences of bytes are available in the Bytes module.
Hint: Did you mean to use `Bytes.set buffer index char`?

?

[nojb]

I think that for beginners there is a certain distance between "use the module Bytes" and (1) finding the Bytes documentation then (2) figuring out that Bytes.set is the right function and (3) how to use it. Seeing the concrete code can save a bit of time.

OK, I pushed a commit as suggested.

[nojb]

Hint: Did you mean to use Bytes.set buffer index char?

What about

Hint: Did you mean to use 'Bytes.set'?

?
(otherwise I'm afraid that buffer, index, char may end up confusing rather than clarifying)

[Octachron]

Yes, your version seems clearer to me.

[dbuenzli]

(otherwise I'm afraid that buffer, index, char may end up confusing rather than clarifying)

Agreed. If you don't reply with the names that are in the context it is going to be confusing.

[nojb]

OK, pushed a new version:

nojebar@PERVERSESHEAF:~/ocaml$ TERM=dumb runtime/ocamlrun ./ocaml -I stdlib
OCaml version 5.1.0+dev1-2022-06-09
Enter #help;; for help.

# s.[i] <- c;;
Line 1, characters 0-10:
1 | s.[i] <- c;;
    ^^^^^^^^^^
Error: Syntax error: strings are immutable, there is no assignment syntax for them.
Hint: Mutable sequences of bytes are available in the Bytes module.
Hint: Did you mean to use 'Bytes.set'?

[gasche]

I'm okay with the proposed final version.

[nojb]

Cherry-picked to 5.0: 67863ba