Pinerl brings the Pin operator from the Elixir language to Erlang by the means of a parse transform. The pin operator must be applied to any variables in patterns that are matched by value, rather than bound there. In Elixir, the pin operator is a necessity to be able to match existing values, since the default behaviour is to shadow the old value. That is, of course, not the case for Erlang, but the use of the pin operator makes the programmer's intent explicit, making it easier to read. It also discoverers a real and common mistake in Erlang code – unintentional matching – that is not found by any other warnings.

When compiled with Pinerl, code like this will give you a warning

X0 = math:pi(),
Y0 = math:e(),
X0 = Y0 + X0,

sample:3: Warning: variable 'X0' matched without pin

Reminding you that you either should have either used a new name X1 for your new value, or the pin operator, depending on whether you intended to match. Here is the same code but with the pin operator applied, making the authors intent to match explicit:

X0 = math:pi(),
Y0 = math:e(),
?PIN(X0) = Y0 + X0,

Of course, using the pin operator on a new binding also gives you a warning:

X0 = math:pi(),
Y0 = math:e(),
?PIN(X1) = Y0 + X0,

sample:3: Warning: variable 'X1' is unbound in pin

As the implementation is a parse transform, the syntax for the Pin operator must be syntactically valid Erlang, but preferably semantically invalid. The current implementation uses a code-block for this purpose:

X = 1,
begin X end = 3.

Although it is strongly recommended to use the ?PIN(X) macro, which can be included from pinerl.hrl:

-define(PIN(X), begin X end).

It is recommended you use Pinerl in your entire project to minimise confusion. Therefore, you should include the parse transform in your compiler options ("+{parse_transform,pinerl_transform}"). In any files where the ?PIN(X) macro is needed, include the header:

-include_lib("pinerl/include/pinerl.hrl").

Add the following to your Makefile:

DEPS += pinerl
dep_pinerl = git git://github.com/margnus1/pinerl.git master
ERLC_OPTS += "+{parse_transform,pinerl_transform}"

Beware that if you do not otherwise define ERLC_OPTS, you will override the default options provided by Erlang.mk. You can add the following line (taken from erlang.mk) before the previous to set the default options explicitly:

ERLC_OPTS ?= -Werror +debug_info +warn_export_vars +warn_shadow_vars \
    +warn_obsolete_guard # +bin_opt_info +warn_export_all +warn_missing_spec

Add the following to your deps and erl_opts lists in rebar.config:

{deps, [{pinerl, ".*",
         {git, "git://github.com/margnus1/pinerl.git", {branch, "master"}}}
       ]}.
{erl_opts, [{parse_transform,pinerl_transform}]}.