01-Haskell

Module 01: Introduction to Haskell

In this module you will focus on learning some of the basics of the Haskell programming language. If you already know some Haskell, you should focus on helping your partner(s) understand all the material in this module. However, keep in mind that people learn best by doing, not by being told. The driver should be whoever has the least experience with Haskell.

This file is a “literate Haskell document”: only lines preceded by > and a space (see below) are code; everything else (like this paragraph) is a comment, formatted using Markdown syntax. Literate Haskell documents have an extension of .lhs, whereas non-literate Haskell source files use .hs.

GHCi

  • Run ghci from a command prompt (or from the “Shell” tab in repl.it). At the resulting prompt, type :help.

  • Find the command to exit ghci. What is it? Exit ghci now.

  • Using the cd command at the command prompt, change to the directory containing this file, 01-Haskell.lhs (if using repl.it, you should already be in the correct directory).

  • Now start ghci again. Find the command to load a module. What is it? Load this file, 01-Haskell.lhs, into ghci. Note that by default, ghci will look for files in the same directory in which it was started.

Hint: to kill a runaway ghci evaluation, use Ctrl+C.

Basic Haskell declarations

Now consider the following Haskell code.

i :: Int
i = -35

n :: Integer
n = 25

c :: Char
c = 'Z'

b :: Bool
b = True

s :: String
s = "Hello, world!"

f :: Integer -> Integer
f n = 2*n + 1

g :: Integer -> Integer -> Integer
g m n = (m - n)*(m + n)

-- This is a comment
{- So
   is
   this -}

-- Uncomment me:
-- i = 12

  • Enter :type n at the ghci prompt. What does the :type command do?

  • What do you think :: means?

  • What do you think = means?

  • What do you think -> means?

  • Find the ghci command to reload the current module. Uncomment the line i = 12 above, save this file, and reload. What happens?

  • Does this change your answer to the question about what = means?

Arithmetic

  • At the ghci prompt, type each of the following expressions, and record the result. Feel free to experiment with other expressions as well.

    3 + 2
19 - 27
div 19 3
19 `div` 3
mod 19 3
19 `mod` 3
19 `divMod` 3
7 ^ 222
(-3) * (-7)
2*i + 3
i + n

  • Explain what happens when you evaluate i + n.

  • What are the smallest and largest possible Int values?

  • What are the smallest and largest possible Integer values?

(Haskell has floating-point values too, but we won’t use them much in this course.)

Booleans

  • Find out the syntax for each of the following operations in Haskell:

    • Boolean operations: and, or, not

    • Comparison: equal, not equal, less than, greater than, less or equal, greater or equal

    • if-expressions

    Of course, be sure to cite any resources you use!

  • Play around with the operators you discovered and try them on some examples. Record three of your most interesting experiments, the result, and what you learned from each.

Pairs

  • Type (n,c) at the ghci prompt. What is the result?

  • What is the type of (n,c)?

  • What is the result of fst (n,c)?

  • What is the result of snd (n,c)?

  • What is the type of fst? What does it do?

Values like (n,c) are called pairs, or more generally, tuples. (Haskell also has 3-tuples, 4-tuples, … but we will not use them.)

  • Define e such that fst (fst (snd (fst e))) == 6.

Functions

Evaluate the following expressions:

f 6
f 8
g 5 4
g 2 3

A function takes one or more input values and produces a single output value.

  • What is the Haskell syntax for applying a function to a single argument?

  • What is the Haskell syntax for applying a function to multiple arguments?

  • Write a function which takes two Integer values as input and returns True if and only if the first is greater than twice the second. What is the type of your function?

Pattern matching

wub :: Integer -> Integer
wub 0 = 1
wub n = n * wub (n-1)

dub :: Integer -> Integer
dub 0 = 0
dub 1 = 1
dub n = dub (n-1) + dub (n-2)

flub :: (Integer, Integer) -> Integer
flub p = fst p + 2 * snd p

gub :: (Integer, Integer) -> Integer
gub (x,y) = x + 2*y

  • Evaluate wub 0, wub 1, and wub 5.

  • Explain in words what wub does.

  • What does the line wub 0 = 1 mean?

  • What do you think would happen if the lines wub 0 = 1 and wub n = n * wub (n-1) were switched? Make a guess before trying it, and record your guess here.

  • Now try it. What happens? Why?

  • What happens when you evaluate wub (-3)? Why?

  • Evaluate wub (3+1) and wub 3+1. Can you explain the difference?

  • What does dub do?

  • What happens if the lines dub 0 = 0 and dub 1 = 1 are switched?

  • Call flub and gub on some example inputs. Record your results here. Do you notice a difference between the behavior of flub and gub?

  • Explain the difference between flub and gub.

  • Which do you prefer? Why?

Guards

hailstone :: Integer -> Integer
hailstone n
  | even n    = n `div` 2
  | otherwise = 3*n + 1

  • Try evaluating hailstone on some example inputs; record them here.

  • Try evaluating even on some example inputs. What does the even function do?

  • How is otherwise defined? (You’ll have to Google this one.)

  • Explain the behavior of hailstone.

Practice

  • Write a function inRange which takes two inputs, a pair of Integers and an Integer, and checks whether the Integer is in between the elements of the pair (inclusive). For example, inRange (2,4) 2, inRange (2,4) 3, and inRange (2,4) 4 should all be True, whereas inRange (2,4) 6 should be False. Note that inRange (4,2) 3 should also be True.