I think many people know about Data.Some.Newtype⁠—⁠it uses quite a cute (and cursed) trick to encode a simple existential wrapper as a newtype, hence avoiding an unwanted indirection in the runtime representation.

But there's more to data than existentials⁠—⁠so how far do these tricks go? The answer is: surprisingly far! Let's see what other GADTs we can slim down.

Magic words:

psql -U postgres

Some interesting flags (to see all, use -h or --help depending on your psql version):

• -E: will describe the underlaying queries of the \ commands (cool for learning!)
• -l: psql will list all databases and then exit (useful if the user you connect with doesn't has a default database, like at AWS RDS)

$NAME is a language-agnostic tool for managing your Postgres schema, designed to make downtime-free schema changes as easy as possible to develop and run.

$NAME improves database migration ergonomics in five major ways:

1. It stores previous versions of your schema in source control and auto-generates scaffolding for new migrations by "diffing" them with the current schema. This ensures that your production schema never drifts out of sync from the schema defined by your application.

2. It automates many recipes for doing tricky migrations like column renames "online," without requiring downtime or a synchronized application deploy. Naive methods of renaming a column require multiple application deploys and costly table rebuilds. $NAME's recipe doesn't require either.

Peter Naur's classic 1985 essay "Programming as Theory Building" argues that a program is not its source code. A program is a shared mental construct (he uses the word theory) that lives in the minds of the people who work on it. If you lose the people, you lose the program. The code is merely a written representation of the program, and it's lossy, so you can't reconstruct

Defining records in Haskell causes accessor functions for the record's fields to be defined. There is no seperate namespace for these accessor functions.

Be able to

• use records in Haskell, which share field names.
• use lenses for accessing these fields

The following are appendices from Optics By Example, a comprehensive guide to optics from beginner to advanced! If you like the content below, there's plenty more where that came from; pick up the book!

• Operators
• Composition Guide
• Substitution Guide
• Optical Constarints and Types