friedbrice

This now lives on GitHub: https://github.com/friedbrice/runhs

friedbrice

-- ghcid -c'stack repl --resolver nightly --package time' haskell-time.hs --allow-eval

----
-- # Haskell Time Crib Sheet
----

-- Excellent sources for this small guide include:
--
--   * https://two-wrongs.com/haskell-time-library-tutorial.html
--   * https://williamyaoh.com/posts/2019-09-16-time-cheatsheet.html

domenkozar

1. Put ci.yaml into .github/workflows/
2. Follow https://github.com/cachix/cachix-action#usage to setup Cachix and benefit from quick CI build times.

OliverJAsh

Option vs non-Option

	Option<T>	non-Option (T | undefined)
accessing property	userOption.map(user => user.age)	userNullish?.age
calling a method	userOption.map(user => user.fn())	userNullish?.fn()
providing fallback	ageOption.getOrElse(0)	ageNullish ?? 0
filter	ageOption.filter(checkIsOddNumber)	`ageNull

chrisdone

Statically checked overloaded strings

This gist demonstrates a trick I came up with which is defining IsString for Q (TExp a), where a is lift-able. This allows you to write $$("...") and have the string parsed at compile-time.

On GHC 9, you are able to write $$"..." instead.

This offers a light-weight way to enforce compile-time constraints. It's basically OverloadedStrings with static checks. The inferred return type

edmundsmith

Method for Emulating Higher-Kinded Types in Rust

Intro

I've been fiddling about with an idea lately, looking at how higher-kinded types can be represented in such a way that we can reason with them in Rust here and now, without having to wait a couple years for what would be a significant change to the language and compiler.

There have been multiple discussions on introducing higher-ranked polymorphism into Rust, using Haskell-style Higher-Kinded Types (HKTs) or Scala-looking Generalised Associated Types (GATs). The benefit of higher-ranked polymorphism is to allow higher-level, richer abstractions and pattern expression than just the rank-1 polymorphism we have today.

As an example, currently we can express this type:

ChrisPenner

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE DeriveGeneric #-}

module GenericMonoid where

import Data.Monoid
import Data.Maybe

jkachmar

{-# LANGUAGE ConstraintKinds            #-}
{-# LANGUAGE DataKinds                  #-}
{-# LANGUAGE FlexibleInstances          #-}
{-# LANGUAGE FunctionalDependencies     #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE InstanceSigs               #-}
{-# LANGUAGE LambdaCase                 #-}
{-# LANGUAGE NoImplicitPrelude          #-}
{-# LANGUAGE NoMonomorphismRestriction  #-}

dcastro

import org.scalacheck.cats.implicits._
import org.scalacheck.Gen
import io.circe.{Decoder, Encoder}

object Test extends App {
  case class X(i: Int)
  
  implicit val x: Encoder[X] = Wrapper[X].deriveInstance[Encoder]
  implicit val y: Decoder[X] = Wrapper[X].deriveInstance[Decoder].withErrorMessage("some custom error msg")
  val z: Gen[X]              = Wrapper[X].lift(Gen.choose(0, 10)) 

carymrobbins

{- TODO: The Num and Fractional instances don't implement all methods.
 - In practice this is fine but for completeness they should be done.
 -}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
module NumericLits where

data I = I
i :: I