hmaurer

apply :: (t -> a) -> (t -> b) -> (a, b)
apply f g x = (f x, g x)

hmaurer

Useful Scalac Flags

So, there have been some discussions and angry tweets recently about irritating Scala "features" (like value discarding and auto-tupling) that can actually be turned off by selecting the right compiler flag in conjunction with -Xfatal-warnings. I highly recommend a set of options something like those below.

scalacOptions ++= Seq(
  "-deprecation",           
  "-encoding", "UTF-8",       // yes, this is 2 args
  "-feature",                
 "-language:existentials",

hmaurer

... = begin
        suc (a + suc b)
      ==< cong suc (lemma2 a b) >
        suc (suc (a + b))
      ==< cong (suc.suc) (comm a b) >
        suc (suc (b + a))
      ==< cong suc (sym (lemma2 b a)) >
        suc (b + suc a)
      []

hmaurer

suc (a + suc b)   == suc (b + suc a)
        ^
        |
        |
suc (suc (a + b)) == suc (b + suc a)
                             ^
                             |
                             |
suc (suc (a + b)) == suc (suc (b + a))
                   ^

hmaurer

module Printf

%default total

data Format = FInt Format -- %d
            | FString Format -- %s
            | FOther Char Format -- [a-zA-Z0-9]
            | FEnd --

format : List Char -> Format

hmaurer

This is my recommended path for learning Haskell.

Something to keep in mind: don't sweat the stuff you don't understand immediately. Just keep moving.

Primary course

Installing Haskell

Ubuntu PPA

hmaurer

require 'tire'
require 'nobrainer'

NoBrainer.connect 'rethinkdb://server/company'

class Employee
  include NoBrainer::Document

  field :name
  field :title

hmaurer

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'rack', github: 'rack/rack'
    gem 'i18n', github: 'svenfuchs/i18n'
    gem 'sqlite3'
  GEMFILE

hmaurer

role Group {
    method elements { ... }
    method id { ... }
    method op($l, $r) { ... }
}

macro assert($fact) {
    quasi {
        die "FAILED: ", $fact.Str
            unless {{{$fact}}};

hmaurer

module A3gis where

  import Data.Traversable
  import Data.Map
  import Data.Maybe

  foo :: forall k v. (Eq v, Ord k) => Map k v -> Map k v -> Maybe (Map k v)
  foo m1 m2 = sequence $ unionWith go (Just <$> m1) (Just <$> m2)
    where
      go x y = if x == y then x else Nothing