mikehaertl

Learn you a Haskell - In a nutshell

This is a summary of the "Learn You A Haskell" online book under http://learnyouahaskell.com/chapters.

---

1. Introduction

• Haskell is a functional programming language.

pascalpoitras

Mouse

---

enable

---

Kartones

PSQL

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)

Grokzen

# Based on post from: https://snipt.net/chrisdpratt/symmetrical-manytomany-filter-horizontal-in-django-admin/#L-26
# Only reposting to avoid loosing it.

"""
When adding a many-to-many (m2m) relationship in Django, you can use a nice filter-style multiple select widget to manage entries. However, Django only lets you edit the m2m relationship this way on the forward model. The only built-in method in Django to edit the reverse relationship in the admin is through an InlineModelAdmin.

Below is an example of how to create a filtered multiple select for the reverse relationship, so that editing entries is as easy as in the forward direction.

IMPORTANT: I have no idea for what exact versions of Django this will work for, is compatible with or was intended for.

Spindel

Watchdogged

Place the service file (or a link to it) in /etc/systemd/system/ Place the watchdogged.py file somewhere ( and change the ExecStart portion in the .service to point at the file )

then do systemctl daemon-reload followed by systemctl start watchdogged.service

After this you can watch the progress using journalctl --follow -u watchdogged.service change the PROBABILITY variable to something else to watch it faster/later or succeed.

parsonsmatt

The question of "How do I design my application in Haskell?" comes up a lot. There's a bunch of perspectives and choices, so it makes sense that it's difficult to choose just one. Do I use plain monad transformers, mtl, just pass the parameters manually and use IO for everything, the ReaderT design pattern, free monads, freer monads, some other kind of algebraic effect system?!

The answer is: why not both/all?

Lately, I've been centering on a n application design architecture with roughly three layers:

Layer 1:

newtype AppT m a = AppT { unAppT :: ReaderT YourStuff m a } deriving ............ The ReaderT Design Pattern, essentially. This is what everything gets boiled down to, and what everything eventually gets interpreted in. This type is the backbone of your app. For some components, you carry around some info/state (consider [MonadMetrics](https://hackage