My work at Lob involves a program called Dora that performs address verification. It validates addresses, repairs them to something valid or rejects them and tells you the issue. It’s basically a very long pipeline processing an Address struct with lots of rules and steps that gradually enrich the data structure as it passes along the pipeline.

The original authors have since moved on so we had to come up with techniques to understand why Dora makes the decisions and modifications it does.

To that end, I came up with Audit (hex,
github)

It operates by adding an extra field to the data structure of interest: __audit_trail__.
You then wrap your updates with the audit macro.
This will record the current file and line number (hence the use of a macro) and store the current version of the data structure.
The __audit_trail__ field is effectively a linked list history of every version of the
data structure you wrapped in audit.

The to_string method will take a struct with the __audit_trail__
and pretty print the list deltas and with the line of code that caused it
(as a github link, a filename/line number and a code snippet from the actual line in the file)
e.g. some example output:

github url: https://github.com/lob/dora/tree/leverage_audit_hex_package/lib/dora/.../standard.ex#L234
local path: lib/dora/.../standard.ex:234
code: address = %Address{audit(address) | primary_number: primary}
diff: [{[:primary_number], {:add, "18"}}]
=====
github url: https://github.com/lob/dora/tree/leverage_audit_hex_package/lib/dora/.../standard.ex#L228
local path: lib/dora/.../standard.ex:228
code: address = %Address{audit(address) | street_name: street}
diff: [{[:street_name], {:add, "BERNICE"}}]
=====
github url: https://github.com/lob/dora/tree/leverage_audit_hex_package/lib/dora/.../standard.ex#L217
local path: lib/dora/.../standard.ex:217
code: address = %Address{audit(address) | street_suffix: suffix}
diff: [{[:street_suffix], {:add, "ST"}}]

To kick the tires, add the following to your deps:

      {:audit, "~> 0.1.6"},

Add the __audit_trail__ field to your struct.
And modify your instances of:

%struct{ v | ... }

to be:

%struct{ audit(v) | ... }

then do the following to your resulting data structure:

r 
|> Audit.to_string()
|> IO.puts