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Cats

import cats._, cats.data._, cats.implicits._

Kernel

• Semigroup
• Band
• Commutative Semigroup

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Cats-collections

Exhaustive list of data structures here.

import cats.collection._

List of data structures: BitSet, Dequeue, Diet, Discrete, DisjointSets, Heap, ISet, AvlMap, PairingHeap, PartiallyOrderedSet, Range, AvlSet, TreeList

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Variance:

• Covariant ( return type )
• Contravariant ( argument type )

Covariant type on contravariant position:

case class Box[+A](value: A) {
  def set(a: A): Box[A] = Box(a)
}

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-- Source: https://hackage.haskell.org/package/profunctors-5.4/docs/Data-Profunctor.html

class Profunctor p where
  -- dimap | lmap, rmap
  dimap :: (a -> b) -> (c -> d) -> p b c -> p a d
  lmap :: (a -> b) -> p b c -> p a c
  rmap :: (b -> c) -> p a b -> p a c
  
class Profunctor p => Strong p where
  -- first' | second'

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Cats-effect Async.async has the following pitfall:

During the execution of async, the effect is guaranteed to run on the backer ContextShift required by the creation of the Async type class. The main issue is that, if the callback is running on its on blocking execution context, the IO will not switch back to the main execution context (or the one running before calling async). And the rest of your code will be executed on the blocking execution context. This could end up making your cpu bounded code run on a specialized thread pool that may not be ready to handle more threads running on it.

You can test it yourself by running the following snippet:

import cats._, cats.data._, cats.implicits._
import cats.effect._, cats.effect.implicits._
import fs2._, fs2.io._

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FoldLeftSC

A Foldable.foldLeftimplementation that short-circuits on a given predicate

---

The first attempt was a failure. Using foldM and Option.

def foldLeft[F[_]: Foldable, A, B](fa: F[A])(b: B)(f: (B, A) => B)(p: B => Boolean): B = 

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PostRest

PostgREST is a standalone web server which turns a PostgreSQL database into a RESTful API. It serves an API that is customized based on the structure of the underlying database.

Official documentation at http://postgrest.org/en/v5.2/index.html

There's a docker image in case of testing :-)

To configure your PostREST instance: http://postgrest.org/en/v5.2/install.html#configuration

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All public gists https://gist.github.com/monadplus
Copyright 2019, Arnau Abella Gassol
MIT License, http://www.opensource.org/licenses/mit-license.php

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Key formats in bitcoin

There are (at least) two ways to import private keys:

• WIF format: https://en.bitcoin.it/wiki/Wallet_import_format
• BIP32: https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki

WIF it's just the secKey with the checksum, compression flag and nothing else.

BIP32 also contains the derivation info: pubkey, depth, derivation-index, chain-code, parent fingerprint.

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IORef vs STRef vs MVar

• IORef: Mutable references in the IO monad.
• STRef: Mutable references in the (strict) ST monad.
• MVar: synchronized mutable variables between threads.

STRef provides mutable references, IORef provides mutable references, exception catching, threads, and of course IO.

Rule of thumb: use the weakest abstraction.