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@parsonsmatt
parsonsmatt / dbcomp.markdown
Created May 28, 2020
haskell database library comparison
View dbcomp.markdown

there's kind of a ladder of complexity, where you have safety vs complexity vs how much to learn vs what the lib does for you. hasql and postgresql-simple have a 1 on safety and "what the lib does for you" but also have a 1 on complexity and how much to learn. if you know SQL, you can write raw string queries. But you also don't get any safety or migrations or composability. persistent is a 2 on the "how much to learn", and around a 3 on "how much it does for you." You get migrations, datatype definitions, very safe easy/common-case functions. The functions it gives you are perfectly safe, but you need to use esqueleto or raw SQL for fancy stuff so it's like 3 on safety. There's not a huge amount to learn, maybe it's a 2 here. esqueleto is a lot more powerful, but requires a lot more learning - so it's like a 4 on what it does for you and a 3 on how much to learn. It's not perfectly safe, but it does more safely than persistent, so it gets a 4. beam is complicated and difficult to learn. It has a 5 on how m

@parsonsmatt
parsonsmatt / prismatic.hs
Created Jun 5, 2018
I figured out a nice way to pluck exceptions out of a constraint!
View prismatic.hs
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE PartialTypeSignatures #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
@parsonsmatt
parsonsmatt / mbc.hs
Created Feb 21, 2020
Compatibility function to make MonadUnliftIO and MonadBaseControl IO work together
View mbc.hs
{-
Oh no! You're lost in a twisty maze of IO lifting and unlifting, and you've come to
an impossible fork: you're currently in some monad that is `MonadBaseControl IO m`,
but you need to call a function in `MonadUnliftIO`. AND you need to call functions
in your original monad transformer, too!
We can make this work, but it's a bit hairy.
MonadUnliftIO is a strictly less powerful type class than `MonadBaseControl IO`, so
@parsonsmatt
parsonsmatt / skolem.hs
Created Feb 7, 2020
What's goin on with my skolems
View skolem.hs
-- This reproduction can be played with using `ghcid --allow-eval`
{-# language RankNTypes #-}
import Control.Monad.ST
newtype IdT m a = IdT { unIdT :: m a }
-- $> :set -XRankNTypes
--
@parsonsmatt
parsonsmatt / no-thanks-acid-state.md
Created Dec 17, 2018
acid state antirecommendation
View no-thanks-acid-state.md

Someone asked whether or not acid-state was production ready. I shared my experiences:

parsonsmatt [11:32 AM] @nikolap it's used by cardano-wallet and Hackage. Based on my experience with acid-state, I'd say it is not a good choice for production data storage. For local desktop apps, SQLite is a much better choice, and for real production apps, Postgresql is king.

parsonsmatt [11:44 AM] acid-state did not have a test suite, at all, until I implemented the very first tests (for TemplateHaskell code generation) earlier this year. It has picked up some tests since then, but I'm still not confident in it's correctness.

It claims to be resilient to unplugging the power cord, but I doubt that, as it's not resilient to Ctrl-C: https://github.com/acid-state/acid-state/issues/79

@parsonsmatt
parsonsmatt / timestamp.hs
Created Aug 6, 2019
How long is your Template Haskell taking to run?
View timestamp.hs
timestamp :: String -> Q a -> Q a
timestamp name action = do
start <- runIO getCurrentTime
runIO $ putStrLn $ "Starting " <> name <> ": " <> show start
a <- action
end <- runIO getCurrentTime
runIO $ do
putStrLn $ "Ending " <> name <> ": " <> show end
putStrLn $ "Elapased " <> name <> ": " <> show (diffUTCTime end start)
pure a
View drf.hs
{-# language DuplicateRecordFields, TypeApplications #-}
module DuplicateRecordFields where
data User = User { name :: String }
data Dog = Dog { name :: String }
-- Works
nameAlias :: User -> String
nameAlias = name
View log.hs
{-# LANGUAGE TypeApplications, GADTs, FlexibleInstances, OverloadedLists #-}
module History where
import Lib
import Control.Monad (join)
import qualified Data.Map as Map
import Data.Map (Map)
import qualified Data.List.NonEmpty as NEL
import Data.List.NonEmpty (NonEmpty(..))
View animal2.sql
-- We're modeling the following Haskell datatype:
--
-- data Animal = Cat Name Age | Dog Name OwnerId
--
-- We're going to factor the common 'Name' field into the animal table.
--
-- The data that is specific for each field will go on a table with that field.
-- First we create the animal_type.
CREATE TYPE animal_type AS ('cat', 'dog');
View setisnotafunctor.hs
{-# LANGUAGE DeriveFunctor #-}
module SetIsNotAFunctor where
import Data.Set (Set)
import qualified Data.Set as Set
-- | A type for annotating a value where equality and ordering only care
-- about the value.
data Ann ann a = Ann { annotation :: ann, value :: a }
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