BFPG Meetup - September 2026 - Data Type Diff, Optics and Event Sourcing

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​Agenda

• ​18:00: Welcome and setup

• ​Presentation #1: Data Type Diff, Optics and Event Sourcing - Tony Morris

• ​20:00ish: Pack down, head to Criterion pub

​# Data Type Diff, Optics & Event-sourcing

Take two functions that ought to be inverses. Write down the two laws
that say so. Do not do anything else. You are now in possession of an
optic.

Two very small data types — `Diff` and `Patch` — describe how two values
differ, and how to apply a change to a value. Between them, they carry
two round-trip laws. From those laws, and no further effort, an `Iso'`
falls out in the total case, and a `Prism'` falls out in the fallible
case. The isomorphism laws are exactly the round-trip laws, wearing
different clothes.

The same story turns out to be the story of parsers and printers that
round-trip, and of event-sourced systems whose log and projections are
honest with each other. Along the way we distinguish real event-sourcing
from its "event-flavoured" imposter, via a property test that falls
directly out of the round-trip laws.

Familiarity with Haskell's `lens` is helpful, not required.