Upcoming Potential Incompatibilities — Erlang/OTP v29.0.1

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# Upcoming Potential Incompatibilities

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## Introduction

This document lists planned upcoming potential incompatibilities in Erlang/OTP.

## OTP 30

### erlang:fun_info(Fun, pid)

As of OTP 27, the functions `erlang:fun_info/1,2` always say that the local `init` process created all funs, regardless of which process or node the fun was originally created on.

In OTP 30, the `{pid,_}` element will be removed altogether. That is, `erlang:fun_info(Fun, pid)` will raise a `badarg` exception, and `erlang:fun_info(Fun)` will no longer include a `{pid,Pid}` item in the returned list. (This was originally scheduled to occur in OTP 28.)

## OTP 27

### Fun creator pid will always be local init process

As of OTP 27, the functions `erlang:fun_info/1,2` will always say that the local `init` process created all funs, regardless of which process or node the fun was originally created on.

In OTP 28, the `{pid,_}`element will be removed altogether.

### Feature maybe_expr will be enabled by default

As of OTP 27, the `maybe_expr` feature will be approved and enabled by default. That means that code that uses the unquoted atom `maybe` will fail to compile. All uses of `maybe` as an atom will need to be quoted. Alternatively, as a short-term solution, the `maybe_expr` feature can be disabled.

It is recommend to quote all uses of the atom `maybe` as soon as possible. The compiler option `warn_keywords` can be used to emit warnings about all occurrences of `maybe` without quotes.

### 0.0 and -0.0 will no longer be exactly equal

Currently, the floating point numbers `0.0` and `-0.0` have distinct internal representations. That can be seen if they are converted to binaries:

    1> <<0.0/float>>.
    <<0,0,0,0,0,0,0,0>>
    2> <<-0.0/float>>.
    <<128,0,0,0,0,0,0,0>>

However, when they are matched against each other or compared using the `=:=` operator, they are considered to be equal. Thus, `0.0 =:= -0.0` currently returns `true`.

In Erlang/OTP 27, `0.0 =:= -0.0` will return `false`, and matching `0.0` against `-0.0` will fail. When used as map keys, `0.0` and `-0.0` will be considered to be distinct.

The `==` operator will continue to return `true` for `0.0 == -0.0`.

To help to find code that might need to be revised, in OTP 27 there will be a new compiler warning when matching against `0.0` or comparing to that value using the `=:=` operator. The warning can be suppressed by matching against `+0.0` instead of `0.0`.

We plan to introduce the same warning in OTP 26.1, but by default it will be disabled.

### Singleton type variables will become a compile-time error

Before Erlang/OTP 26, the compiler would silenty accept the following spec:

    -spec f(Opts) -> term() when
        Opts :: {ok, Unknown} | {error, Unknown}.
    f(_) -> error.

In OTP 26, the compiler emits a warning pointing out that the type variable `Unknown` is unbound:

    t.erl:6:18: Warning: type variable 'Unknown' is only used once (is unbound)
    %    6|     Opts :: {ok, Unknown} | {error, Unknown}.
    %     |                  ^

In OTP 27, that warning will become an error.

### Escripts will be compiled by default

Escripts will be compiled by default instead of interpreted. That means that the `compiler` application must be available.

The old behavior of interpreting escripts can be restored by adding the following line to the script file:

    -mode(interpret).

In OTP 28, support for interpreting an escript will be removed.

### -code_path_choice will default to strict

This command line option controls if paths given in the command line, boot scripts, and the code server should be interpreted as is strict or relaxed.

OTP 26 and earlier defaults to `relaxed`, which means `-pa myapp/ebin` would attempt to load `-pa myapp/ebin` and `-pa myapp/myapp/ebin`. The option will default to strict in OTP 27.

### Archive fallbacks will be removed

OTP 26 and earlier allows an application to have part of its directories as regular folders and others as archives. This functionality was previously used by reltool but it is no longer the case from OTP 26. Support for archive fallbacks will be removed from the code server in OTP 27.

### Triple-Quoted Strings

Before Erlang/OTP 27 a sequence of 3 or more double-quote characters was grouped in pairs each meaning the empty string and if there was an odd number the last character was the start of a string. The empty strings were then concatenated and effectively disappeared.

In Erlang/OTP 27; 3 or more double-quote characters are interpreted as the start of a "Triple-Quoted String". See EEP 64.

Here follows some examples of code that would change meaning. Note that all these examples before Erlang/OTP 27.0 was strange since there was no sensible reason to write like that.

    """String Content"""
    %% Was interpreted as
    "" "String Content" ""
    %% Which becomes
    "String Content"
    %%
    %% In OTP 27 it is instead a syntax error since no text is allowed
    %% on the line after an opening triple-quote

    """
    String Content
    """
    %% Was interpreted as
    "" "
    String Content
    " ""
    %% Which becomes
    "
    String Content
    "
    %%
    %% In OTP 27 it is instead interpreted as a
    %% Triple-Quoted String equivalent to
    "String Content"

    """"
    ++ foo() ++
    """"
    %% Became
    "" ++ foo() ++ ""
    %%
    %% In OTP 27 it is instead interpreted as a
    %% Triple-Quoted String (triple-or-more) equivalent to
    "++ foo() ++"

From Erlang/OTP 26.1 up to 27.0 the compiler issues a warning for a sequence of 3 or more double-quote characters since that is almost certainly a mistake or something like a result of bad automatic code generation. If a users gets that warning, the code should be corrected for example by inserting appropriate spaces between the empty strings, or removing the redundant ones alltogether, which will have the same meaning before and after Erlang/OTP 27.

## OTP 28

### The re module will use a different regular expression engine

The functionality of module `re` is currently provided by the PCRE library, which is no longer actively maintained. Therefore, in OTP 28, we will switch to a different regular expression library.

The source code for PCRE used by the `re` module has been modified by the OTP team to ensure that a regular expression match would yield when matching huge input binaries and/or when using demanding (back-tracking) regular expressions. Because of the those modifications, moving to a new version of PCRE has always been a time-consuming process because all of the modifications had to be applied by hand again to the updated PCRE source code.

Most likely, the new regular expression library will be RE2. RE2 guarantees that the match time is linear in the length of input string, and it also eschews recursion to avoid stack overflow. That should make it possible to use RE2 without modifying its source code. For more information about why RE2 is a good choice, see WhyRE2.

Some of implications of this change are:

- We expect that the functions in the `re` module will continue to be supported, although some of the options are likely to be dis-continued.
- It is likely that only pattern matching of UTF8-encoded binaries will be supported (not Latin1-encoded binaries).
- In order to guarantee the linear-time performance, RE2 does not support all the constructs in regular expression patterns that PCRE do. For example, backreferences and look-around assertions are not supported. See Syntax for a description of what RE2 supports.
- Compiling a regular expression is likely to be slower, and thus more can be gained by explicitly compiling the regular expression before matching with it.

### Fun creator pid will be removed

As of OTP 28, the function `erlang:fun_info/1` will not include the `{pid,_}` element and the function `erlang:fun_info/2` will no longer accept `pid` as the second argument.

### Support for interpreting escripts will be removed

Escripts will be compiled, and it will no longer be possible to force an escript to be interpreted by using the directive `-mode(interpret)`.

[]{}{: #max_connections_open }

### New Inets http client option

New option in Inets' http client `httpc:set_options([{max_connections_open, N}])` is introduced to prevent bandwidth exhaustion while trying to complete multiple requests at once, making the remote server close connection before the request is finished. The option is set for maintaining backwards compatibility allowing infinite amount of connections, but will be decreased in OTP 30 allowing as smooth request flow as possible.

## OTP 29

### It will no longer be possible to disable feature maybe_expr

As of OTP 29, the `maybe_expr` feature will become permanent and no longer possible to disable. All uses of `maybe` as an atom will need to be quoted.

It is recommend to quote all uses of the atom `maybe` as soon as possible. The compiler option `warn_keywords` can be used to emit warnings about all occurrences of `maybe` without quotes.

### cprof and eprof will be replaced by tprof

As of OTP 29, the `cprof` and `eprof` will be removed in favor of `tprof` added in OTP 27.

### CWD or '.' is now placed last in code path

As of OTP 29, the CWD is added last instead of first in code path. It can be added first with -pa '.' if needed.

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