commit ad8d2471a750841d192757675b9c14cb70fc67ea
Author: Michael Zhang <mail@mzhang.io>
Date:   Mon Jan 25 09:52:24 2021 -0600

    initiial

diff --git a/haskell-book.hs b/haskell-book.hs
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--- /dev/null
+++ b/haskell-book.hs
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+{-# LANGUAGE NoImplicitPrelude #-}
+
+import Prelude hiding (map, filter, foldl, foldr, takeWhile)
+import Debug.Trace
+
+-- Base
+
+map :: (a -> b) -> [a] -> [b]
+map f [] = []
+map f (x : xs) = f x : map f xs
+
+filter :: (a -> Bool) -> [a] -> [a]
+filter f [] = []
+filter f (x : xs) = if f x then x : filter f xs else filter f xs
+
+foldl :: (b -> a -> b) -> b -> [a] -> b
+foldl f e [] = e
+foldl f e (x : xs) = foldl f (f e x) xs
+
+foldr :: (a -> b -> b) -> b -> [a] -> b
+foldr f e [] = e
+foldr f e (x : xs) = f x (foldr f e xs)
+
+inserts :: a -> [a] -> [[a]]
+inserts x [] = [[x]]
+inserts x (y : ys) = (x : y : ys) : map (y :) (inserts x ys)
+
+perms1 :: [a] -> [[a]]
+perms1 [] = [[]]
+perms1 (x : xs) = [zs | ys <- perms1 xs, zs <- inserts x ys]
+
+-- Exercise 1.2
+
+uncons :: [a] -> Maybe (a, [a])
+uncons [] = Nothing
+uncons (hd : tl) = Just (hd, tl)
+
+-- Exercise 1.3
+
+wrap :: a -> [a]
+wrap x = [x]
+
+unwrap :: [a] -> a
+unwrap [x] = x
+
+single :: [a] -> Bool
+single [x] = True
+single _ = False
+
+-- Exercise 1.4
+
+reverse :: [a] -> [a]
+reverse = foldl (\a x -> x : a) []
+
+-- Exercise 1.5
+
+map2 :: (a -> b) -> [a] -> [b]
+map2 f = foldr (\x a -> f x : a) []
+
+filter2 :: (a -> Bool) -> [a] -> [a]
+filter2 f = foldr (\x a -> if f x then x : a else a) []
+
+-- Exercise 1.6
+
+composed :: (a -> b -> b) -> (a -> Bool) -> b -> [a] -> b
+composed f p e = foldr f e . filter p
+
+composed2 :: (a -> b -> b) -> (a -> Bool) -> b -> [a] -> b
+composed2 f p = foldr (\x a -> if p x then f x a else a)
+
+-- Exercise 1.7
+
+takeWhile :: (a -> Bool) -> [a] -> [a]
+takeWhile f = foldr (\x a -> if f x then x : a else []) []
+
+-- Exercise 1.8
+
+dropWhileEnd :: Eq a => (a -> Bool) -> [a] -> [a]
+dropWhileEnd f = foldr (\x a -> if f x && a == [] then [] else x : a) []
+
+-- Exercise 1.9
+-- This implementation is insanely slow because both `init` and `last` are O(n)
+
+-- Exercise 1.10
+-- e is the identity, op is associative
+
+-- Exercise 1.11
+
+integer :: [Int] -> Int
+integer = foldl (\a x -> 10 * a + x) 0
+
+fraction :: [Int] -> Double
+fraction = (/ 10) . foldr (\x a -> fromIntegral x + a / 10) 0
+
+-- Exercise 1.12
+-- Fuck this
+
+-- Exercise 1.13
+
+apply :: Int -> (a -> a) -> a -> a
+apply 0 f x = x
+apply n f x = apply (n - 1) f (f x)
+
+-- Exercise 1.14
+
+-- inserts2 :: Show a => a -> [a] -> [[a]]
+inserts2 n = foldr (\x (a, b) -> (trace ("{"++show a++","++show b++"}") (x : a), b)) ([], [])
+
+-- 1 [2,3,4]
+-- 
+-- 1:2:3:4
+-- map (2:) (inserts 1 [3,4])
+--   2: 1:3:4
+--   2: map (3:) (inserts 1 [4])
+--     2:3: 1:4
+--     2:3: map (4:) (inserts 1 [])
+--       2:3:4: 1
+
+-- Exercise 1.15
+
+remove :: Eq a => a -> [a] -> [a]
+remove x ys = filter (/= x) ys
+
+perms3 :: Eq a => [a] -> [[a]]
+perms3 [] = [[]]
+perms3 xs = [x : ys | x <- xs, ys <- perms3 (remove x xs)]
+
+-- Exercise 1.16
+
+
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