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Set-ExecutionPolicy -ExecutionPolicy Unrestricted -Scope Process

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# http://stackoverflow.com/a/9701130
GIT_COMMITTER_DATE="`date`" git commit --amend --date "`date`"

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#!/usr/bin/env stack
-- stack --resolver nightly-2017-09-13 --install-ghc exec ghci --package vector

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}

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#!/usr/bin/env stack
-- stack --resolver lts-10.8 --install-ghc exec ghci --package lens

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens (Getter, makeLenses, view)

data Foo = Foo
  { _fooInt    :: Int
  , _fooString :: String

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A coworker and I were discussing RankNTypes and why the extension is used in lens. RankNTypes has confused me for a while, but making the connection to lens gave me a good intuition (strangely?). Here’s a recap in case it’s useful to anyone else:

If you have a normal higher-order function:

foo :: Num a => (a -> a) -> a
foo f = f 1

We can pass foo any function that fits the (a -> a) with a Num constraint:

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#!/usr/bin/env stack
-- stack --resolver lts-10.8 --install-ghc exec ghci

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE LambdaCase #-}

-- A key type for pretend use in looking up a doggo in a database.
newtype Key a = Key { unKey :: String }

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-- subForestWithParent returns the subforest of the input 'Tree', where each
-- node value in each 'Tree' of the subforest is paired with the value of its
-- parent node.
--
-- ghci> let tree = Node 1 [Node 2 [Node 4 []], Node 3 [Node 5 [Node 7 []], Node 6 []]]
-- ghci> putStrLn . drawTree . fmap show $ tree
-- 1
-- |
-- +- 2
-- |  |

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printTree :: Show a => Tree a -> IO ()
printTree = putStrLn . Tree.drawTree . fmap show

tree :: Tree Int
tree = Tree.unfoldTree buildNode 1 where
  buildNode :: Int -> (Int, [Int])
  buildNode n
    | 2 * n + 1 <= 7 = (n, [2 * n, 2 * n + 1])
    | otherwise = (n, [])

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#!/usr/bin/env stack
-- stack --resolver lts-16.3 --install-ghc exec ghci

{-# OPTIONS_GHC -Wall -Werror #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}

import Data.Set (Set)
import qualified Data.Set as Set

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#!/usr/bin/env stack
-- stack --resolver lts-15.6 script --package text

{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE OverloadedStrings #-}

import Data.Text (Text)
import Prelude
import qualified Data.Text as Text
import qualified Data.Text.IO as Text.IO