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Week2 Notes of "Haskell Building Blocks" -- "Functional Programming in Haskell" MOOC

Contents

Boolean Values and Expressions

Equality and Comparison Operators

  • Boolean Values are either True and False.
  • Double-equals operator == is used for testing value equality.
  • Not-equals operator is /= (looks like an equals sign with a line through it).
  • Haskell cannot compare two values that have different types.
  • Haskell supports the standard comparison / relational operators, <, <=, >, >=.

Testing List membership with the elem function

  • elem function returns true if a value is part of a list, and false otherwise.
elem 1 [1,2,3]  -- > True

Using infix and prefix operations

  • Haskell permits any two-argument function to be written as an infix operator using backquote (`) characters.
1 `elem` [1,2,3]    -- > True
42 `max` 13         -- > 42
(+) 1 1             -- > 2

More Boolean Operations

  • The not function returns the opposite boolean value, the logical complement.
not True            -- > False
not (not False)     -- > False
  • && infix operator is a boolean conjunction (AND function).
True && True        -- > True
False && True       -- > False
  • || infix operator is a boolean disjunction (logical OR); dual of the AND operation.
True || False       -- > True
False || False      -- > False
  • xor function returns true when its two boolean arguments are different.
True `xor` False        -- > True
False `xor` False       -- > False
  • Haskell supports multi-input boolean operations with and and or functions that take a list of boolean values as a single input.
and [False, True, False, True]      -- > False
or [True, True, False]              -- > True
  • if expressions evaluate to either the then value or the else value, based on the if value.
if 2*2==4 then "happy" else "sad"   -- > "happy"
if True then 42 else pi             -- > 42.0

Zipping Lists

  • zip function combines a pair of lists into a list of pairs.
    • If zip function is used on lists which are of different lengths, length of the output list will be the length of the shortest list.
  • zip3 function combines three lists into a triplet.
  • By adding import Data.List, we get pre-baked definitions of zip3 ... zip7 and zipWith3 ... zipWith7.
  • zipWith function is a generalization of zip.
    • While zip can only combine elements from two input lists by creating pairs of those elements, zipWith allows us to provide a function that tells us how to combine the elements from the input lists.
    • zipWith function applies a function to the result of zip.
    • zipWith function can take a lambda function for the operation.
zip [1,2,3] [4,5]                           -- > [(1,4),(2,5)]
zipWith max [1,2,3] [0,2,4]                 -- > [1,2,4]
zipWith (+) [1,2,3] [0,2,4]                 -- > [1,4,7]
zipWith (\x->(\y->(x,y))) [1,2,3] "abc"     -- > [(1,'a'),(2,'b'),(3,'c')]    -- Note: Strings in Haskell are list of characters

Input / Output

  • Input is with getLine and output is with putStrLn.
  • putStrLn (like println in Java or print in Python) prints a character string to the terminal.
  • getLine function reads in a character string from user input.
    • After getLine function is invoked, text should be typed at the > prompt followed by pressing the enter key.
  • read reads values as strings, and converts them into other types.
    • A type annotation (for example: ::Int) is mandatory; otherwise it is not clear what type of number the input String is meant to represent.
  • show takes a value and returns a String representation of that value.
    • show function is the dual of the read function.
  • read and show functions synthesize values from Strings and vice versa.
  • print does the composition of putStrLn and show.
putStrLn ("hello " ++ "world" ++ "!!")  -- > "hello world!!" -- :: String
do {
    putStrLn "what is your name?"
    x <- getLine
    putStrLn ("hello " ++ x)
}
read "42" :: Int                        -- > 42     -- :: Int
show 42                                 -- > "42"             -- :: String
print 42                                -- > 42              -- :: IO ()

I/O Monad

  • Input and output (I/O) operations are impure.
  • Use do to specify a sequence of actions.
  • Use <- inside a do to associate input values with names.
  • Any value or function that involves I/O has IO in its type.
  • Note: <- is for associating names with values in do blocks;
    • Whereas -> is used for defining functions.
let greet() = do
    planet <- getLine
    home <- getLine
    putStrLn ("greetings " ++ planet ++ "ling.")
    putStrLn ("I am from " ++ home ++ ".")
    putStrLn "Take me to your leader."

Installing Haskell

Two bundled distributions of GHC are: