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geas Build Status

Guess Erlang Application Scattering

Geas : (In Irish folklore) an obligation or prohibition magically imposed on a person.

By extension, obligation or prohibition imposed by an Erlang application or module, which may modify its scattering.


Geas is a tool detecting the runnable official Erlang release window for your project.

Geas will tell you also :

  • what are the offending functions in the beam/source files that reduce the available window.
  • if some beam files are compiled native.
  • the installed patches and recommend patches that should be installed depending your code.

For example , if a dependency is using application:ensure_all_started/1 your project can only be used starting R16B02 or if maps is used somewhere, starting 17.0. On contrary, pg module cannot be used after 17.5 but is back again in 23.0.

For more details, see Wiki.

Geas is also available on

When using it ?

  • Each time you prepare a project release or update a dependancy.
  • When you plan to add a dependancy to your project
  • When writing your README, to inform your project's users on possible release window
  • To limit test of CI tools on only possible release window


geas is available as a module, but you will probably prefere to use geas plugins with your usual build tool.

Plugin is available for and rebar .

Output example

Example on a test project using cowboy :

   R15                   24.0       Geas database                       2.7.5
   R16B                             cowboy                              1.0.4
                         19.3       cowlib                              1.0.2
                                    ranch                               1.2.1
   R16B03     x86_64     18.1       test                                0.0.1
   R16B03     x86_64     18.1       Global project                      0.0.1

C : 23.0
R : 23.0.1 23.0.2 23.0.3
T : >=R16B03 <=18.1

R16B      erlang:integer_to_binary/1

R16B      erlang:integer_to_binary/1

R16B      erlang:integer_to_binary/1

R15B02    crypto:hash/2

19.3      crypto:rand_bytes/1

R16B03    ssh_message:encode_host_key/1

18.1      ssh_message:encode_host_key/1
===> Current version is incompatible with release window

Current (C) Erlang release found is 23.0 and no patches (P) was detected. The global project can run starting R16B03 up to 18.1 in such case, due to use of a function introduced in R16B03 and another one removed in 18.2. Native x86 compilation was detected on test module. Recommended patches (R) are proposed when an application used in code is referenced in a not installed patche, even if in this exemple, current Erlang release cannot run the project.

Tuning output

Output can be tuned depending some environment variables.

Note : starting version 2.6, variables can be set in rebar.config.

Environment variables cheatsheet :

Key Value type Value Comment Details Since
GEAS_USE_SRC boolean [0 / 1] Use source code instead beam files ...
GEAS_MY_RELS string Erlang release list List possible releases ...
GEAS_EXC_RELS string Erlang release list Exclude some releases ...
GEAS_DISC_RELS boolean [0 / 1] Show discarded buggy Erlang releases ...
GEAS_LOG string Log level list Log informations ...
GEAS_TIPS boolean [0 / 1] Give tips on patches to apply ...
GEAS_RANGE string SEMVER range Set an OTP range for project ...
GEAS_FRAME string SEMVER range Set an OTP frame for project ... 2.6
GEAS_UPDATE boolean [0 / 1] Force update of OTP version table 2.6
GEAS_HTTP_OPTS string httpc options Erlang term string with a final dot 2.6

Under compat table output, some informations can be shown depending those environment variables and analyze result. A single capital letter is used as tag for each information.

  • C = Current : Current Erlang major.minor release used (for example 20.2)
  • P = Patches : List of patches detected to be installed on current release, if any (for example 20.2.2)
  • T = Total : All releases inside computed release window GEAS_MY_RELS=""
  • L = Local : All local releases given by user inside computed window GEAS_MY_RELS="18.1 19.0"
  • D = Discarded : Show releases that was automatically discarded GEAS_DISC_RELS=1
  • E = Excluded : User excluded release list GEAS_EXC_RELS="R16B03"
  • R = Recommended : Patches that should be applied, because code is using module impacted by a patch for current version GEAS_TIPS=1

Quick Start

git clone git://
cd geas
make shell


Contributions are welcome. Please use pull-requests.


Guess Erlang Application Scattering





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