Koji Miyazato viercc

GHC itself

(I didn't think of kind polymorphism and don't know how that relates to FlexibleInstances)

-- Fixed is owned

Let's define a type class Searchable.

class Searchable a where
    epsilon :: (a -> Bool) -> a

    -- [Law]
    --  (∃x :: a. p a == True) => p (epsilon p) = True

We want to show that the following instance is valid.

	{-
	https://www.reddit.com/r/haskell/comments/g6kc33/intersection_of_infinite_lists/
	-}
	import qualified Data.Set as Set

	-------- Original and Step 1 --------

	intersection :: (Eq a) => [[a]] -> [[a]] -> Int -> [([a], [a])]
	intersection l1 l2 i =
	[(x, y) \| x <- (take i l1), y <- (take i l2), not (common x y == [])]

	-- Just wrote down in the web editor.
	-- Not tested at all but I hope you get the idea

	import qualified Data.HashTable.ST.Basic as HT
	import Data.List.NonEmpty(NonEmpty(..))
	import qualified Data.List.NonEmpty as NE

	type MyHashTable s k v = HT.HashTable s k (NonEmpty v)

	insert :: MyHashTable s k v -> k -> v -> ST s ()

	-- https://www.reddit.com/r/haskell/comments/fbfhum/monthly_hask_anything_march_2020/fl4fgek/

	{-# LANGUAGE PatternSynonyms #-}
	{-# LANGUAGE StandaloneDeriving #-}
	{-# LANGUAGE UndecidableInstances #-}
	{-# LANGUAGE QuantifiedConstraints #-}
	{-# LANGUAGE RankNTypes #-}

	import Data.Word
	import Data.Functor.Identity

	-- Breaking first-class-instances
	{-#
	LANGUAGE
	TemplateHaskell,
	RankNTypes,
	TypeFamilies,
	KindSignatures,
	FlexibleInstances,
	ConstraintKinds
	#-}

	$ ghcjs -O2 test.hs
	[1 of 1] Compiling Main ( test.hs, test.js_o )
	Linking test.jsexe (Main)
	$ (cd test.jsexe/ && node all.js)
	(0,1)
	(1,2)
	(2,4)
	(3,8)
	(4,16)
	(5,32)

	{-

	@lexi_lambda (at twitter)
	https://mobile.twitter.com/lexi_lambda/status/1192930938537332736?s=19



	-}

	{-# LANGUAGE RankNTypes #-}

	class Foo()
	// フィールドが一つもないクラスを定義し、そのクラスのオブジェクトを作ると
	// コンパイラ自体がSegmentation Faultを起こす

	//@one_field = 0
	@function g()
	ans = "0"
	return ans

	x = Foo()