ocharles

Magic

It’s folklore that if you’re summing a list of numbers, then you should always use strict foldl. Is that really true though? foldr is useful for lists when the function we use is lazy in its second argument. For (+) :: Int -> Int -> Int this is tyically not the case, but in some sense that’s because Int is “too strict”. An alternative representation of numbers is to represent them inductively. If we do this, sumation can be lazy, and foldr can do things that foldl simply can’t!

First, let’s define natural numbers inductively, and say how to add them:

data Nat = Zero | OnePlus Nat deriving Show

one :: Nat

clementd-fretlink

#!/usr/bin/env stack
-- stack --resolver lts-14.20 --install-ghc runghc --package either --package free
{-# LANGUAGE DeriveFunctor #-}
module Main
    where

import           Control.Applicative.Free
import           Data.Bifunctor           (first)
import           Data.Either.Combinators  (maybeToRight)
import           Data.Either.Validation

atacratic

This tutorial explains how Unison handles 'effectful' computations, like storing state or performing I/O, using abilities. It assumes you haven't come across abilities before, and covers everything from the ground up.

This is an unofficial tutorial, written before the one on unisonweb.org/docs. The approach taken here is slow and methodical. Your first stop should be the official tutorial, if you haven't seen it already.

This doc is a Unison transcript - the source is here.

Terminology note: other languages with ability systems typically call them 'effect handlers' or 'algebraic effects', but many of the ideas are the same.

Introducing abilities

kazu-yamamoto

{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE NumericUnderscores #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}

-- |
-- To reproduce with curl, run mainTls (from ghci) and use
--
-- @

ChrisPenner

Optics By Example Cheat Sheets

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

Operator Cheat Sheet

chrisdone

{-# LANGUAGE DataKinds #-}

-- | My database API.

module DBAPI where

import Data.Defaults

data ConnSpec p = ConnSpec
  { username :: !(Required p String)

pchiusano

-- This gist shows how we can use abilities to provide nicer syntax for any monad.
-- We can view abilities as "just" providing nicer syntax for working with the
-- free monad.

ability Monadic f where
  eval : f a -> a

-- Here's a monad, encoded as a first-class value with
-- two polymorphic functions, `pure` and `bind`
type Monad f = Monad (forall a . a -> f a) (forall a b . f a -> (a -> f b) -> f b)

docteurklein

<?php

$num = floatval($argv[1] ?? '1e6');
echo "iterations: $num\n";

function noop() {
}

$start = microtime(true);
printf("starting raw at:\t\t%.3F s\n", $start);

cscalfani

Monoids in Haskell, an Introduction

Why should programmers care about Monoids? Because Monoids are a common pattern that shows up over and over in programming. And when patterns show up, we can abstract them and leverage work we've done in the past. This allows us to quickly develop solutions on top of proven, stable code.

Add Commutative Property to a Monoid (Commutative Monoid) and you have something that can be executed in parallel. With the end of Moore's Law, parallelism is our only hope to increasing processing speeds.

What follows is what I've learned after studying Monoids. It is hardly complete, but hopefully will prove to be helpful as an introduction for others.

Monoid Lineage

dmitrika

const puppeteer = require('puppeteer');
const { defineSupportCode } = require('cucumber')

defineSupportCode(({ Before, Given, When, Then }) => {
  Before({ timeout: 60 * 1000 }, async function testCase() {
    this.browser = await puppeteer.launch()
  })

  Given('I am on google with puppeteer', { timeout: 60 * 1000 }, async function testCase() {
    this.page = await this.browser.newPage()