Allow user-defined sigils via parse transforms

[williamthome]

This is a POC of custom sigils implementation for Erlang/OTP.

It is based on this comment on the Erlang Forums.

This is a minimal change and does not break any existing code.

How it works

The developer can use the built-in sigils, like s, S, b, and B, but it can also use any sigil it wants. However, it is obligated to handle the custom sigil with parse transform, otherwise, an invalid sigil exception will be raised at the compile time.

Why parse transform?

It was the first idea and seems to be the simplest one to implement this feature.

Example

Important

Please refer to the example project for the complete code.

-module(example).
-compile({parse_transform, json_sigil}).
-compile({parse_transform, sql_sigil}).
%% Sigils can be defined globally in rebar3.config (applies to all .erl files), e.g.:
%{erl_opts, [
%    {parse_transform, json_sigil},
%    {parse_transform, sql_sigil}
%]}.
-export([json/0]).
-export([sql/2]).

json() ->
    ~json"""
    {"key": "value"}
    """.

sql(Name, MinAge) when is_binary(Name); is_integer(MinAge) ->
    ~sql"""
    SELECT *
      FROM users
     WHERE name = {Name}
       AND age >= {MinAge}
    """.

%% Uncomment this function to raise an invalid sigil exception.
%invalid() ->
%    ~invalid"".

Running it

1> c(json_sigil), c(sql_sigil), c(example).
{ok,example}
2> example:json().
#{<<"key">> => <<"value">>}
3> example:sql(~"Bob", 18).
{sql,<<"SELECT *\n  FROM users\n WHERE name = $1\n   AND age >= $2">>,
     [<<"Bob">>,18]}
4> ~invalid"".
* 1:1: invalid sigil

Notes

I have no idea what the core team has planned for sigils.
This PR is just to initiate a discussion about the future of sigils :)

I'll continue the discussion on the same thread of the Erlang Forums of the f sigil proposal (#9512).

[github-actions]

CT Test Results

    3 files     99 suites   1h 9m 19s ⏱️
2 221 tests 2 170 ✅ 50 💤 1 ❌
2 593 runs  2 536 ✅ 56 💤 1 ❌

For more details on these failures, see this check.

Results for commit bc95953.

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// Erlang/OTP Github Action Bot