Learn Haskell in One Video

Posted by Derek Banas on Aug 1, 2015 in Web Design | 0 comments

This time we’ll learn Haskell in one video. This has been the most requested language and since I’ve been working on a project with it I thought I’d make the most all encompassing Haskell tutorial online.

I cover Installation, Data Types, Math Functions, :t, Lists, : Operator, Head / Tail, !! Operator, Take, Elem, Creating Ranges, Cycle, Filter, ZipWith, TakeWhile, Foldl, List Comprehensions, Tuples, Zip, Functions, Compiling, Type Declarations, Recursive Functions, Guards, Where, (x:y), As, Higher Order Functions, Map, (x:xs), Passing a Function into a Function, Returning a Function, Lambda, If, Case, Modules, Enumerations, Polymorphic Types, $ Operator, . Operator, Type Classes, Type Instance, Custom Typeclass, File I/O, Fibonacci Sequence and more.

If you like videos like this, it helps to tell Google with a click here [googleplusone]

Cheat Sheet From the Video

-- Haskell is a functional programming language
-- Everything is immutable so once a value is set it is set forever
-- Functions can be passed as a parameter to other functions 
-- Recursion is used often 
-- Haskell has no for, while, or technically variables, but it does have 
-- constants
-- Haskell is lazy in that it doesn't execute more then is needed and instead 
-- just checks for errors

-- Best Free Haskell Book
-- http://learnyouahaskell.com/chapters

-- Type ghci to open it up in your terminal
-- Load script with :l haskelltut
-- :quit exits the GHCi

-- Import a module
import Data.List
import System.IO

{- 
Beginning of multiline comment
-}

-- ---------- DATA TYPES ----------
-- Haskell uses type inference meaning it decides on the data type based on the -- value stored in it
-- Haskell is statically typed and can't switch type after compiling
-- Values can't be changed (Immutable)
-- You can use :t in the terminal to get the data type (:t value)

-- Int : Whole number -2^63 - 2^63
-- :: Int defines that maxInt is an Int
maxInt = maxBound :: Int
minInt = minBound :: Int

-- Integer : Unbounded whole number

-- Float : Single precision floating point number
-- Double : Double precision floating point number (11 pts precision)
bigFloat = 3.99999999999 + 0.00000000005

-- Bool : True or False
-- Char : Single unicode character denoted with single quotes
-- Tuple : Can store a list made up of many data types

-- You declare the permanent value of a variable like this
always5 :: Int
always5 = 5 

-- ---------- MATH ----------
-- Something crazy to start
sumOfVals = sum [1..1000]

addEx = 5 + 4
subEx = 5 - 4
multEx = 5 * 4
divEx = 5 / 4

-- mod is a prefix operator
modEx = mod 5 4

-- With back ticks we can use it as an infix operator
modEx2 = 5 `mod` 4

-- Negative numbers must be surrounded with parentheses
negNumEx = 5 + (-4)

-- If you define an Int you must use fromIntegral to use it with sqrt
-- :t sqrt shows that it returns a floating point number
num9 = 9 ::Int
sqrtOf9 = sqrt (fromIntegral num9)

-- Built in math functions
piVal = pi
ePow9 = exp 9
logOf9 = log 9
squared9 = 9 ** 2
truncateVal = truncate 9.999
roundVal = round 9.999
ceilingVal = ceiling 9.999
floorVal = floor 9.999

-- Also sin, cos, tan, asin, atan, acos, sinh, tanh, cosh, asinh, atanh, acosh

trueAndFalse = True && False
trueOrFalse = True || False
notTrue = not(True)

-- Remember you use :t in the terminal to get the data type (:t value)
-- You can also see how functions use data types with :t

-- :t (+) = Num a => a -> a -> a
-- Type a is in the type class num, we receive 2 of them and return 1

-- :t truncate = (RealFrac a, Integral b) => a -> b

-- ---------- LISTS ----------
-- Lists are singly linked and you can only add to the front of it
				
-- Lists store many elements of the same type
primeNumbers = [3,5,7,11]

-- Concatenate lists (Can be slow if your using a large list)
morePrimes = primeNumbers ++ [13,17,19,23,29]

-- You can use the cons operator to construct a list
favNums = 2 : 7 : 21 : 66 : []

-- You can make a list of lists
multList = [[3,5,7],[11,13,17]]

-- Quick way to add 1 value to the front of a list
morePrimes2 = 2 : morePrimes

-- Get number of elements in the list
lenPrime = length morePrimes2

-- Reverse the list
revPrime = reverse morePrimes2

-- return True if list is empty
isListEmpty = null morePrimes2

-- Get the number in index 1
secondPrime = morePrimes2 !! 1

-- Gets the 1st value in a list
firstPrime = head morePrimes2

-- Gets the last value
lastPrime = last morePrimes2

-- Gets everything but the first value
primeTail = tail morePrimes2

-- Gets everything but the last value
primeInit = init morePrimes2

-- Get specified number of elements from the front of a list
first3Primes = take 3 morePrimes2

-- Return values left after removing specified values
removedPrimes = drop 3 morePrimes2

-- Check if value is in list
is7InList = 7 `elem` morePrimes2

-- Get max value
maxPrime = maximum morePrimes2

-- Get minimum value
minPrime = minimum morePrimes2

-- Sum values in list
sumPrimes = sum morePrimes2

-- Get product of values in list (Value all can evenly divide by)
newList = [2,3,5]
prodPrimes = product newList

-- Create list from 0 to 10
zeroToTen = [0..10]

-- Create list of evens by defining the step between the first 2 values
evenList = [2,4..20]

-- You can use letters as well
letterList = ['A','C'..'Z']

-- You can generate an infinite list and Haskell will only generate what you 
-- need
infinPow10 = [10,20..]

-- repeat repeats a value a defined number of times
many2s = take 10 (repeat 2)

-- replicate generates a value a specified number of times
many3s = replicate 10 3

-- cycle replicates the values in a list indefinitely 
cycleList = take 10 (cycle [1,2,3,4,5])

-- You could perform operations on all values in a list
-- Cycle through the list storing each value in x which is multiplied by 2 and
-- then stored in a new list
listTimes2 = [x * 2 | x <- [1..10]]

-- We can filter the results with conditions
listTimes3 = [x * 3 | x <- [1..20], x*3 <= 50]

-- Return all values that are divisible by 13 and 9
divisBy9N13 = [x | x <- [1..500], x `mod` 13 == 0, x `mod` 9 == 0]

-- Sort a list
sortedList = sort [9,1,8,3,4,7,6]

-- zipwith can combine lists using a function
sumOfLists = zipWith (+) [1,2,3,4,5] [6,7,8,9,10]

-- Filter returns a list of items that match a condition
listBiggerThen5 = filter (>5) sumOfLists

-- takeWhile returns list items until the condition is false
evensUpTo20 = takeWhile (<=20) [2,4..]

-- foldl applies the operation on each item of a list
-- foldr applies these operations from the right
multOfList = foldl (*) 1 [2,3,4,5]

-- ---------- LIST COMPREHENSION ----------

-- We can generate a list from 1 to 10 to the power of 3
pow3List = [3^n | n <- [1..10]]

-- We can filter the results to only show values divisible by 9
pow3ListDiv9 = [3^n | n <- [1..10], 3^n `mod` 9 == 0]

-- Generate a multiplication table by multiplying x * y where y has the values
-- 1 through 10 and where x does as well
multTable = [[x * y | y <- [1..10]] | x <- [1..10]]

-- ---------- TUPLES ----------
-- Stores list of multiple data types, but has a fixed size

randTuple = (1,"Random tuple")

-- A tuple pair stores 2 values
bobSmith = ("Bob Smith",52)

-- Get the first value
bobsName = fst bobSmith

-- Get the second value
bobsAge = snd bobSmith

-- zip can combine values into tuple pairs 
names = ["Bob","Mary","Tom"]
addresses = ["123 Main","234 North","567 South"]

namesNAddress = zip names addresses 

-- ---------- FUNCTIONS ----------
-- ghc --make haskelltut compiles your program and executes the main function

-- Functions must start with lowercase letters

-- We can define functions and values in the GHCi with let
-- let num7 = 7
-- let getTriple x = x * 3

-- getTriple num7 = 21

-- main is a function that can be called in the terminal with main
main = do
	-- Prints the string with a new line
	putStrLn "What's your name: "
	
	-- Gets user input and stores it in name
	-- <- Pulls the name entered from an IO action
	name <- getLine
	
	putStrLn ("Hello " ++ name)
	
-- Create function addMe
-- x is a parameter and the operation follows the equals sign
-- The data type passed in will work if it makes sense
-- Every function must return something
-- A function name can't begin with a capital letter
-- A function that doesn't receive parameters is called a definition or name

-- You can define a type declaration for functions
-- funcName :: param1 -> param2 -> returnType
addMe :: Int -> Int -> Int

-- funcName param1 param2 = operations (Returned Value)
-- Execute with : addMe 4 5
addMe x y = x + y

-- Without type declaration you can add floats as well
sumMe x y = x + y

-- You can also add tuples : addTuples (1,2) (3,4) = (4,6)
addTuples :: (Int, Int) -> (Int, Int) -> (Int, Int)
addTuples (x, y) (x2, y2) = (x + x2, y + y2)

-- You can perform different actions based on values
whatAge :: Int -> String
whatAge 16 = "You can drive"
whatAge 18 = "You can vote"
whatAge 21 = "You're an adult"

-- The default
whatAge x = "Nothing Important"

-- Define that we expect an Int in and out
factorial :: Int -> Int

-- If 0 return a 1 (Recursive Function)
factorial 0 = 1
factorial n = n * factorial (n - 1)

-- 3 * factorial (2) : 6
-- 2 * factorial (1) : 2
-- 1 * factorial (0) : 1

-- You could also use product to calculate factorial
productFactorial n = product [1..n]

-- We can use guards that provide different actions based on conditions
isOdd :: Int -> Bool
isOdd n
	-- if the modulus using 2 equals 0 return False
	| n `mod` 2 == 0 = False
	
	-- Else return True
	| otherwise = True
	
-- This could be shortened to
isEven n = n `mod` 2 == 0

-- Use guards to define the school to output
whatGrade :: Int -> String
whatGrade age
	| (age >= 5) && (age <= 6) = "Kindergarten"
	| (age > 6) && (age <= 10) = "Elementary School"
	| (age > 10) && (age <= 14) = "Middle School"
	| (age > 14) && (age <= 18) = "High School"
	| otherwise = "Go to college"
	
-- The where clause keeps us from having to repeat a calculation
batAvgRating :: Double -> Double -> String
batAvgRating hits atBats
	| avg <= 0.200 = "Terrible Batting Average"
	| avg <= 0.250 = "Average Player"
	| avg <= 0.280 = "Your doing pretty good"
	| otherwise = "You're a Superstar"
	where avg = hits / atBats 
	
-- You can access list items by separating letters with : or get everything but
-- the first item with xs
getListItems :: [Int] -> String
getListItems [] = "Your list is empty"
getListItems (x:[]) = "Your list contains " ++ show x
getListItems (x:y:[]) = "Your list contains " ++ show x ++ " and " ++ show y
getListItems (x:xs) = "The first item is " ++ show x ++ " and the rest are " 
	++ show xs
	
-- We can also get values with an As pattern
getFirstItem :: String -> String
getFirstItem [] = "Empty String"
getFirstItem all@(x:xs) = "The first letter in " ++ all ++ " is "
	++ [x]
	
-- ---------- HIGHER ORDER FUNCTIONS ----------
-- Passing of functions as if they are variables

times4 :: Int -> Int
times4 x = x * 4

-- map applies a function to every item in the list
listTimes4 = map times4 [1,2,3,4,5]

-- Let's make map
multBy4 :: [Int] -> [Int]
multBy4 [] = []

-- Takes the 1st value off the list x, multiplies it by 4 and stores it in the 
-- new list
-- xs is then passed back into multBy4 until there is nothing left of the list -- to process (Recursion)
multBy4 (x:xs) = times4 x : multBy4 xs

-- Check if strings are equal with recursion
areStringsEq :: [Char] -> [Char] -> Bool
areStringsEq [] [] = True
areStringsEq (x:xs) (y:ys) = x == y && areStringsEq xs ys
areStringsEq _ _ = False

-- PASSING A FUNCTION INTO A FUNCTION
-- (Int -> Int) says we expect a function that receives an Int and returns an 
-- Int
doMult :: (Int -> Int) -> Int

-- We receive the function and pass 3 into it
doMult func = func 3

-- We pass in the function that multiplies by 4
num3Times4 = doMult times4

-- RETURNING A FUNCTION FROM A FUNCTION
getAddFunc :: Int -> (Int -> Int)

-- We can pass in the values to the function
getAddFunc x y = x + y

-- We could also get a function that adds 3 for example
adds3 = getAddFunc 3

fourPlus3 = adds3 4

-- We could use this function with map as well
threePlusList = map adds3 [1,2,3,4,5]

-- ---------- LAMBDA ----------
-- How we create functions without a name
-- \ represents lambda then you have the arguments -> and result

dbl1To10 = map (\x -> x * 2) [1..10]

-- ---------- CONDITIONALS ----------

-- Comparison Operators : < > <= >= == /=
-- Logical Operators : && || not

-- Every if statement must contain an else
doubleEvenNumber y = 
	if (y `mod` 2 /= 0)
		then y
		else y * 2
		
-- We can use case statements 
getClass :: Int -> String
getClass n = case n of
	5 -> "Go to Kindergarten"
	6 -> "Go to elementary school"
	_ -> "Go some place else"
	
-- ---------- MODULES ----------
-- You can group functions into modules. I showed previously how to load them
-- You can create your own module by creating a file that contains all your 
-- functions and then list the functions at the top like this
-- module SampFunctions (getClass, doubleEvenNumber) where
-- They can then be imported with import SampFunctions

-- ---------- ENUMERATION TYPES ----------
-- Used when you want a list of possible types
-- Provide name, a list and then Show converts into a String for printing

data BaseballPlayer = Pitcher
					| Catcher
					| Infield
					| Outfield
				deriving Show

barryBonds :: BaseballPlayer -> Bool
barryBonds Outfield = True

barryInOF = print(barryBonds Outfield)

-- ---------- CUSTOM TYPES ----------
-- You can store multiple values sort of like a struct to create custom types
data Customer = Customer String String Double
	deriving Show
	
-- Define Customer and its values
tomSmith :: Customer
tomSmith = Customer "Tom Smith" "123 Main St" 20.50

-- Define how we'll find the right customer (By Customer) and the return value
getBalance :: Customer -> Double
getBalance (Customer _ _ b) = b

tomSmithBal = print (getBalance tomSmith)

-- We can define a type with all possible values
data RPS = Rock | Paper | Scissors

shoot :: RPS -> RPS -> String
shoot Paper Rock = "Paper Beats Rock"
shoot Rock Scissors = "Rock Beats Scissors"
shoot Scissors Paper = "Scissors Beat Paper"
shoot Scissors Rock = "Scissors Loses to Rock"
shoot Paper Scissors = "Paper Loses to Scissors"
shoot Rock Paper = "Rock Loses to Paper"
shoot _ _ = "Error"

-- We could define 2 versions of a type
-- First 2 floats are center coordinates and then radius for Circle
-- First 2 floats are for upper left hand corner and bottom right hand corner 
-- for the Rectangle
data Shape = Circle Float Float Float | Rectangle Float Float Float Float
	deriving (Show)

-- :t Circle = Float -> Float -> Float -> Shape

-- Create a function to calculate area of shapes
area :: Shape -> Float
area (Circle _ _ r) = pi * r ^ 2
area (Rectangle x y x2 y2) = (abs (x2 - x)) * (abs (y2 -y))

-- Could also be area (Rectangle x y x2 y2) = (abs $ x2 - x) * (abs $ y2 -y)
-- $ means that anything that comes after it will take precedence over anything 
-- that comes before (Alternative to adding parentheses)

-- The . operator allows you to chain functions to pass output on the right to
-- the input on the left
-- sumValue = putStrLn (show (1 + 2)) becomes
sumValue = putStrLn . show $ 1 + 2

-- Get area of shapes
areaOfCircle = area (Circle 50 60 20)
areaOfRectangle = area $ Rectangle 10 10 100 100

-- ---------- TYPE CLASSES ----------
-- Num, Eq, Ord and Show are type classes
-- Type classes correspond to sets of types which have certain operations 
-- defined for them.
-- Polymorphic functions, which work with multiple parameter types, define
-- the types it works with through the use of type classes
-- For example (+) works with parameters of the type Num
-- :t (+) = Num a => a -> a -> a
-- This says that for any type a, as long as a is an instance of Num, + can take
-- 2 values and return an a of type Num

-- Create an Employee and add the ability to check if they are equal
data Employee = Employee { name :: String,	
						   position :: String,
						   idNum :: Int 
						   } deriving (Eq, Show)
						   
samSmith = Employee {name = "Sam Smith", position = "Manager", idNum = 1000}
pamMarx = Employee {name = "Pam Marx", position = "Sales", idNum = 1001}

isSamPam = samSmith == pamMarx

-- We can print out data because of show
samSmithData = show samSmith

-- Make a type instance of the typeclass Eq and Show
data ShirtSize = S | M | L

instance Eq ShirtSize where
	S == S = True
	M == M = True
	L == L = True
	_ == _ = False

instance Show ShirtSize where
	show S = "Small"
	show M = "Medium"
	show L = "Large"
	
-- Check if S is in the list
smallAvail = S `elem` [S, M, L]

-- Get string value for ShirtSize
theSize = show S

-- Define a custom typeclass that checks for equality
-- a represents any type that implements the function areEqual
class MyEq a where
	areEqual :: a -> a -> Bool
	
-- Allow Bools to check for equality using areEqual
instance MyEq ShirtSize where
	areEqual S S = True
	areEqual M M = True
	areEqual L L = True
	areEqual _ _ = False

newSize = areEqual M M

-- ---------- I/O ----------

sayHello = do
	-- Prints the string with a new line
	putStrLn "What's your name: "
	
	-- Gets user input and stores it in name
	name <- getLine
	
	-- $ is used instead of the parentheses
	putStrLn $ "Hello " ++ name
	
-- File IO
-- Write to a file 
writeToFile = do

	-- Open the file using WriteMode
	theFile <- openFile "test.txt" WriteMode
	
	-- Put the text in the file
	hPutStrLn theFile ("Random line of text")
	
	-- Close the file
	hClose theFile
	
readFromFile = do

	-- Open the file using ReadMode
	theFile2 <- openFile "test.txt" ReadMode
	
	-- Get the contents of the file
	contents <- hGetContents theFile2
	putStr contents
	
	-- Close the file
	hClose theFile2
	
-- ---------- EXAMPLE : FIBONACCI SEQUENCE ----------

-- Calculate the Fibonacci Sequence
-- 1, 1, 2, 3, 5, 8, ...

-- 1 : 1 : says to add 2 1s to the beginning of a list
-- | for every (a, b) add them
-- <- stores a 2 value tuple in a and b
-- tail : get all list items minus the first
-- zip creates pairs using the contents from 2 lists being the lists fib and the 
-- list (tail fib)

fib = 1 : 1 : [a + b | (a, b) <- zip fib (tail fib) ]

-- First time through fib = 1 and (tail fib) = 1
-- The list is now [1, 1, 2] because a: 1 + b: 1 = 2

-- The second time through fib = 1 and (tail fib) = 2
-- The list is now [1, 1, 2, 3] because a: 1 + b: 2 = 3

fib300 = fib !! 300 -- Gets the value stored in index 300 of the list

-- take 20 fib returns the first 20 Fibonacci numbers

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

290

291

292

293

294

295

296

297

298

299

300

301

302

303

304

305

306

307

308

309

310

311

312

313

314

315

316

317

318

319

320

321

322

323

324

325

326

327

328

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346

347

348

349

350

351

352

353

354

355

356

357

358

359

360

361

362

363

364

365

366

367

368

369

370

371

372

373

374

375

376

377

378

379

380

381

382

383

384

385

386

387

388

389

390

391

392

393

394

395

396

397

398

399

400

401

402

403

404

405

406

407

408

409

410

411

412

413

414

415

416

417

418

419

420

421

422

423

424

425

426

427

428

429

430

431

432

433

434

435

436

437

438

439

440

441

442

443

444

445

446

447

448

449

450

451

452

453

454

455

456

457

458

459

460

461

462

463

464

465

466

467

468

469

470

471

472

473

474

475

476

477

478

479

480

481

482

483

484

485

486

487

488

489

490

491

492

493

494

495

496

497

498

499

500

501

502

503

504

505

506

507

508

509

510

511

512

513

514

515

516

517

518

519

520

521

522

523

524

525

526

527

528

529

530

531

532

533

534

535

536

537

538

539

540

541

542

543

544

545

546

547

548

549

550

551

552

553

554

555

556

557

558

559

560

561

562

563

564

565

566

567

568

569

570

571

572

573

574

575

576

577

578

579

580

581

582

583

584

585

586

587

588

589

590

591

592

593

594

595

596

597

598

599

600

601

602

603

604

605

606

607

608

609

610

611

612

613

614

615

616

617

618

619

620

621

-- Haskell is a functional programming language

-- Everything is immutable so once a value is set it is set forever

-- Functions can be passed as a parameter to other functions

-- Recursion is used often

-- Haskell has no for, while, or technically variables, but it does have

-- constants

-- Haskell is lazy in that it doesn't execute more then is needed and instead

-- just checks for errors

-- Best Free Haskell Book

-- http://learnyouahaskell.com/chapters

-- Type ghci to open it up in your terminal

-- Load script with :l haskelltut

-- :quit exits the GHCi

-- Import a module

import Data.List

import System.IO

Beginning of multiline comment

-- ---------- DATA TYPES ----------

-- Haskell uses type inference meaning it decides on the data type based on the -- value stored in it

-- Haskell is statically typed and can't switch type after compiling

-- Values can't be changed (Immutable)

-- You can use :t in the terminal to get the data type (:t value)

-- Int : Whole number -2^63 - 2^63

-- :: Int defines that maxInt is an Int

maxInt = maxBound :: Int

minInt = minBound :: Int

-- Integer : Unbounded whole number

-- Float : Single precision floating point number

-- Double : Double precision floating point number (11 pts precision)

bigFloat = 3.99999999999 + 0.00000000005

-- Bool : True or False

-- Char : Single unicode character denoted with single quotes

-- Tuple : Can store a list made up of many data types

-- You declare the permanent value of a variable like this

always5 :: Int

always5 = 5

-- ---------- MATH ----------

-- Something crazy to start

sumOfVals = sum [1..1000]

addEx = 5 + 4

subEx = 5 - 4

multEx = 5 * 4

divEx = 5 / 4

-- mod is a prefix operator

modEx = mod 5 4

-- With back ticks we can use it as an infix operator

modEx2 = 5 `mod` 4

-- Negative numbers must be surrounded with parentheses

negNumEx = 5 + (-4)

-- If you define an Int you must use fromIntegral to use it with sqrt

-- :t sqrt shows that it returns a floating point number

num9 = 9 ::Int

sqrtOf9 = sqrt (fromIntegral num9)

-- Built in math functions

piVal = pi

ePow9 = exp 9

logOf9 = log 9

squared9 = 9 ** 2

truncateVal = truncate 9.999

roundVal = round 9.999

ceilingVal = ceiling 9.999

floorVal = floor 9.999

-- Also sin, cos, tan, asin, atan, acos, sinh, tanh, cosh, asinh, atanh, acosh

trueAndFalse = True && False

trueOrFalse = True || False

notTrue = not(True)

-- Remember you use :t in the terminal to get the data type (:t value)

-- You can also see how functions use data types with :t

-- :t (+) = Num a => a -> a -> a

-- Type a is in the type class num, we receive 2 of them and return 1

-- :t truncate = (RealFrac a, Integral b) => a -> b

-- ---------- LISTS ----------

-- Lists are singly linked and you can only add to the front of it

-- Lists store many elements of the same type

primeNumbers = [3,5,7,11]

-- Concatenate lists (Can be slow if your using a large list)

morePrimes = primeNumbers ++ [13,17,19,23,29]

-- You can use the cons operator to construct a list

favNums = 2 : 7 : 21 : 66 : []

-- You can make a list of lists

multList = [[3,5,7],[11,13,17]]

-- Quick way to add 1 value to the front of a list

morePrimes2 = 2 : morePrimes

-- Get number of elements in the list

lenPrime = length morePrimes2

-- Reverse the list

revPrime = reverse morePrimes2

-- return True if list is empty

isListEmpty = null morePrimes2

-- Get the number in index 1

secondPrime = morePrimes2 !! 1

-- Gets the 1st value in a list

firstPrime = head morePrimes2

-- Gets the last value

lastPrime = last morePrimes2

-- Gets everything but the first value

primeTail = tail morePrimes2

-- Gets everything but the last value

primeInit = init morePrimes2

-- Get specified number of elements from the front of a list

first3Primes = take 3 morePrimes2

-- Return values left after removing specified values

removedPrimes = drop 3 morePrimes2

-- Check if value is in list

is7InList = 7 `elem` morePrimes2

-- Get max value

maxPrime = maximum morePrimes2

-- Get minimum value

minPrime = minimum morePrimes2

-- Sum values in list

sumPrimes = sum morePrimes2

-- Get product of values in list (Value all can evenly divide by)

newList = [2,3,5]

prodPrimes = product newList

-- Create list from 0 to 10

zeroToTen = [0..10]

-- Create list of evens by defining the step between the first 2 values

evenList = [2,4..20]

-- You can use letters as well

letterList = ['A','C'..'Z']

-- You can generate an infinite list and Haskell will only generate what you

-- need

infinPow10 = [10,20..]

-- repeat repeats a value a defined number of times

many2s = take 10 (repeat 2)

-- replicate generates a value a specified number of times

many3s = replicate 10 3

-- cycle replicates the values in a list indefinitely

cycleList = take 10 (cycle [1,2,3,4,5])

-- You could perform operations on all values in a list

-- Cycle through the list storing each value in x which is multiplied by 2 and

-- then stored in a new list

listTimes2 = [x * 2 | x <- [1..10]]

-- We can filter the results with conditions

listTimes3 = [x * 3 | x <- [1..20], x*3 <= 50]

-- Return all values that are divisible by 13 and 9

divisBy9N13 = [x | x <- [1..500], x `mod` 13 == 0, x `mod` 9 == 0]

-- Sort a list

sortedList = sort [9,1,8,3,4,7,6]

-- zipwith can combine lists using a function

sumOfLists = zipWith (+) [1,2,3,4,5] [6,7,8,9,10]

-- Filter returns a list of items that match a condition

listBiggerThen5 = filter (>5) sumOfLists

-- takeWhile returns list items until the condition is false

evensUpTo20 = takeWhile (<=20) [2,4..]

-- foldl applies the operation on each item of a list

-- foldr applies these operations from the right

multOfList = foldl (*) 1 [2,3,4,5]

-- ---------- LIST COMPREHENSION ----------

-- We can generate a list from 1 to 10 to the power of 3

pow3List = [3^n | n <- [1..10]]

-- We can filter the results to only show values divisible by 9

pow3ListDiv9 = [3^n | n <- [1..10], 3^n `mod` 9 == 0]

-- Generate a multiplication table by multiplying x * y where y has the values

-- 1 through 10 and where x does as well

multTable = [[x * y | y <- [1..10]] | x <- [1..10]]

-- ---------- TUPLES ----------

-- Stores list of multiple data types, but has a fixed size

randTuple = (1,"Random tuple")

-- A tuple pair stores 2 values

bobSmith = ("Bob Smith",52)

-- Get the first value

bobsName = fst bobSmith

-- Get the second value

bobsAge = snd bobSmith

-- zip can combine values into tuple pairs

names = ["Bob","Mary","Tom"]

addresses = ["123 Main","234 North","567 South"]

namesNAddress = zip names addresses

-- ---------- FUNCTIONS ----------

-- ghc --make haskelltut compiles your program and executes the main function

-- Functions must start with lowercase letters

-- We can define functions and values in the GHCi with let

-- let num7 = 7

-- let getTriple x = x * 3

-- getTriple num7 = 21

-- main is a function that can be called in the terminal with main

main = do

-- Prints the string with a new line

putStrLn "What's your name: "

-- Gets user input and stores it in name

-- <- Pulls the name entered from an IO action

name <- getLine

putStrLn ("Hello " ++ name)

-- Create function addMe

-- x is a parameter and the operation follows the equals sign

-- The data type passed in will work if it makes sense

-- Every function must return something

-- A function name can't begin with a capital letter

-- A function that doesn't receive parameters is called a definition or name

-- You can define a type declaration for functions

-- funcName :: param1 -> param2 -> returnType

addMe :: Int -> Int -> Int

-- funcName param1 param2 = operations (Returned Value)

-- Execute with : addMe 4 5

addMe x y = x + y

-- Without type declaration you can add floats as well

sumMe x y = x + y

-- You can also add tuples : addTuples (1,2) (3,4) = (4,6)

addTuples :: (Int, Int) -> (Int, Int) -> (Int, Int)

addTuples (x, y) (x2, y2) = (x + x2, y + y2)

-- You can perform different actions based on values

whatAge :: Int -> String

whatAge 16 = "You can drive"

whatAge 18 = "You can vote"

whatAge 21 = "You're an adult"

-- The default

whatAge x = "Nothing Important"

-- Define that we expect an Int in and out

factorial :: Int -> Int

-- If 0 return a 1 (Recursive Function)

factorial 0 = 1

factorial n = n * factorial (n - 1)

-- 3 * factorial (2) : 6

-- 2 * factorial (1) : 2

-- 1 * factorial (0) : 1

-- You could also use product to calculate factorial

productFactorial n = product [1..n]

-- We can use guards that provide different actions based on conditions

isOdd :: Int -> Bool

isOdd n

-- if the modulus using 2 equals 0 return False

| n `mod` 2 == 0 = False

-- Else return True

| otherwise = True

-- This could be shortened to

isEven n = n `mod` 2 == 0

-- Use guards to define the school to output

whatGrade :: Int -> String

whatGrade age

| (age >= 5) && (age <= 6) = "Kindergarten"

| (age > 6) && (age <= 10) = "Elementary School"

| (age > 10) && (age <= 14) = "Middle School"

| (age > 14) && (age <= 18) = "High School"

| otherwise = "Go to college"

-- The where clause keeps us from having to repeat a calculation

batAvgRating :: Double -> Double -> String

batAvgRating hits atBats

| avg <= 0.200 = "Terrible Batting Average"

| avg <= 0.250 = "Average Player"

| avg <= 0.280 = "Your doing pretty good"

| otherwise = "You're a Superstar"

where avg = hits / atBats

-- You can access list items by separating letters with : or get everything but

-- the first item with xs

getListItems :: [Int] -> String

getListItems [] = "Your list is empty"

getListItems (x:[]) = "Your list contains " ++ show x

getListItems (x:y:[]) = "Your list contains " ++ show x ++ " and " ++ show y

getListItems (x:xs) = "The first item is " ++ show x ++ " and the rest are "

++ show xs

-- We can also get values with an As pattern

getFirstItem :: String -> String

getFirstItem [] = "Empty String"

getFirstItem all@(x:xs) = "The first letter in " ++ all ++ " is "

++ [x]

-- ---------- HIGHER ORDER FUNCTIONS ----------

-- Passing of functions as if they are variables

times4 :: Int -> Int

times4 x = x * 4

-- map applies a function to every item in the list

listTimes4 = map times4 [1,2,3,4,5]

-- Let's make map

multBy4 :: [Int] -> [Int]

multBy4 [] = []

-- Takes the 1st value off the list x, multiplies it by 4 and stores it in the

-- new list

-- xs is then passed back into multBy4 until there is nothing left of the list -- to process (Recursion)

multBy4 (x:xs) = times4 x : multBy4 xs

-- Check if strings are equal with recursion

areStringsEq :: [Char] -> [Char] -> Bool

areStringsEq [] [] = True

areStringsEq (x:xs) (y:ys) = x == y && areStringsEq xs ys

areStringsEq _ _ = False

-- PASSING A FUNCTION INTO A FUNCTION

-- (Int -> Int) says we expect a function that receives an Int and returns an

-- Int

doMult :: (Int -> Int) -> Int

-- We receive the function and pass 3 into it

doMult func = func 3

-- We pass in the function that multiplies by 4

num3Times4 = doMult times4

-- RETURNING A FUNCTION FROM A FUNCTION

getAddFunc :: Int -> (Int -> Int)

-- We can pass in the values to the function

getAddFunc x y = x + y

-- We could also get a function that adds 3 for example

adds3 = getAddFunc 3

fourPlus3 = adds3 4

-- We could use this function with map as well

threePlusList = map adds3 [1,2,3,4,5]

-- ---------- LAMBDA ----------

-- How we create functions without a name

-- \ represents lambda then you have the arguments -> and result

dbl1To10 = map (\x -> x * 2) [1..10]

-- ---------- CONDITIONALS ----------

-- Comparison Operators : < > <= >= == /=

-- Logical Operators : && || not

-- Every if statement must contain an else

doubleEvenNumber y =

if (y `mod` 2 /= 0)

then y

else y * 2

-- We can use case statements

getClass :: Int -> String

getClass n = case n of

5 -> "Go to Kindergarten"

6 -> "Go to elementary school"

_ -> "Go some place else"

-- ---------- MODULES ----------

-- You can group functions into modules. I showed previously how to load them

-- You can create your own module by creating a file that contains all your

-- functions and then list the functions at the top like this

-- module SampFunctions (getClass, doubleEvenNumber) where

-- They can then be imported with import SampFunctions

-- ---------- ENUMERATION TYPES ----------

-- Used when you want a list of possible types

-- Provide name, a list and then Show converts into a String for printing

data BaseballPlayer = Pitcher

| Catcher

| Infield

| Outfield

deriving Show

barryBonds :: BaseballPlayer -> Bool

barryBonds Outfield = True

barryInOF = print(barryBonds Outfield)

-- ---------- CUSTOM TYPES ----------

-- You can store multiple values sort of like a struct to create custom types

data Customer = Customer String String Double

deriving Show

-- Define Customer and its values

tomSmith :: Customer

tomSmith = Customer "Tom Smith" "123 Main St" 20.50

-- Define how we'll find the right customer (By Customer) and the return value

getBalance :: Customer -> Double

getBalance (Customer _ _ b) = b

tomSmithBal = print (getBalance tomSmith)

-- We can define a type with all possible values

data RPS = Rock | Paper | Scissors

shoot :: RPS -> RPS -> String

shoot Paper Rock = "Paper Beats Rock"

shoot Rock Scissors = "Rock Beats Scissors"

shoot Scissors Paper = "Scissors Beat Paper"

shoot Scissors Rock = "Scissors Loses to Rock"

shoot Paper Scissors = "Paper Loses to Scissors"

shoot Rock Paper = "Rock Loses to Paper"

shoot _ _ = "Error"

-- We could define 2 versions of a type

-- First 2 floats are center coordinates and then radius for Circle

-- First 2 floats are for upper left hand corner and bottom right hand corner

-- for the Rectangle

data Shape = Circle Float Float Float | Rectangle Float Float Float Float

deriving (Show)

-- :t Circle = Float -> Float -> Float -> Shape

-- Create a function to calculate area of shapes

area :: Shape -> Float

area (Circle _ _ r) = pi * r ^ 2

area (Rectangle x y x2 y2) = (abs (x2 - x)) * (abs (y2 -y))

-- Could also be area (Rectangle x y x2 y2) = (abs $ x2 - x) * (abs $ y2 -y)

-- $ means that anything that comes after it will take precedence over anything

-- that comes before (Alternative to adding parentheses)

-- The . operator allows you to chain functions to pass output on the right to

-- the input on the left

-- sumValue = putStrLn (show (1 + 2)) becomes

sumValue = putStrLn . show $ 1 + 2

-- Get area of shapes

areaOfCircle = area (Circle 50 60 20)

areaOfRectangle = area $ Rectangle 10 10 100 100

-- ---------- TYPE CLASSES ----------

-- Num, Eq, Ord and Show are type classes

-- Type classes correspond to sets of types which have certain operations

-- defined for them.

-- Polymorphic functions, which work with multiple parameter types, define

-- the types it works with through the use of type classes

-- For example (+) works with parameters of the type Num

-- :t (+) = Num a => a -> a -> a

-- This says that for any type a, as long as a is an instance of Num, + can take

-- 2 values and return an a of type Num

-- Create an Employee and add the ability to check if they are equal

data Employee = Employee { name :: String,

position :: String,

idNum :: Int

} deriving (Eq, Show)

samSmith = Employee {name = "Sam Smith", position = "Manager", idNum = 1000}

pamMarx = Employee {name = "Pam Marx", position = "Sales", idNum = 1001}

isSamPam = samSmith == pamMarx

-- We can print out data because of show

samSmithData = show samSmith

-- Make a type instance of the typeclass Eq and Show

data ShirtSize = S | M | L

instance Eq ShirtSize where

S == S = True

M == M = True

L == L = True

_ == _ = False

instance Show ShirtSize where

show S = "Small"

show M = "Medium"

show L = "Large"

-- Check if S is in the list

smallAvail = S `elem` [S, M, L]

-- Get string value for ShirtSize

theSize = show S

-- Define a custom typeclass that checks for equality

-- a represents any type that implements the function areEqual

class MyEq a where

areEqual :: a -> a -> Bool

-- Allow Bools to check for equality using areEqual

instance MyEq ShirtSize where

areEqual S S = True

areEqual M M = True

areEqual L L = True

areEqual _ _ = False

newSize = areEqual M M

-- ---------- I/O ----------

sayHello = do

-- Prints the string with a new line

putStrLn "What's your name: "

-- Gets user input and stores it in name

name <- getLine

-- $ is used instead of the parentheses

putStrLn $ "Hello " ++ name

-- File IO

-- Write to a file

writeToFile = do

-- Open the file using WriteMode

theFile <- openFile "test.txt" WriteMode

-- Put the text in the file

hPutStrLn theFile ("Random line of text")

-- Close the file

hClose theFile

readFromFile = do

-- Open the file using ReadMode

theFile2 <- openFile "test.txt" ReadMode

-- Get the contents of the file

contents <- hGetContents theFile2

putStr contents

-- Close the file

hClose theFile2

-- ---------- EXAMPLE : FIBONACCI SEQUENCE ----------

-- Calculate the Fibonacci Sequence

-- 1, 1, 2, 3, 5, 8, ...

-- 1 : 1 : says to add 2 1s to the beginning of a list

-- | for every (a, b) add them

-- <- stores a 2 value tuple in a and b

-- tail : get all list items minus the first

-- zip creates pairs using the contents from 2 lists being the lists fib and the

-- list (tail fib)

fib = 1 : 1 : [a + b | (a, b) <- zip fib (tail fib) ]

-- First time through fib = 1 and (tail fib) = 1

-- The list is now [1, 1, 2] because a: 1 + b: 1 = 2

-- The second time through fib = 1 and (tail fib) = 2

-- The list is now [1, 1, 2, 3] because a: 1 + b: 2 = 3

fib300 = fib !! 300 -- Gets the value stored in index 300 of the list

-- take 20 fib returns the first 20 Fibonacci numbers

	Facebook
	YouTube
	Twitter
	LinkedIn

Learn Haskell in One Video

Leave a Reply Cancel reply