Week 6: Notes

This week we began our study of the Haskell language. In addition to following our lectures, I recommend that you read chapters from our textbook Learn You a Haskell For Great Good! by Miran Lipovača, which is freely available online.

In this class we will use GHC (the Glasgow Haskell Compiler). You will want to download and install it now.

This week we discussed

• integers (types Int and Integer), including operators +, -, *, div, mod

• booleans (type Bool), including operators &&, ||, not

• comparison operators ==, /=, <, <=, >, >=

• if / then / else expressions

• function definitions

• type declarations

• pattern matching in function definitions

• lists, including the list constructor : and the concatenation operator ++

• characters (type Char) and strings (type String = [Char])

• basic built-in list functions: head, tail, length, take

• type variables in type declarations

• the Eq type class

• ranges

You can review these features in chapters 1-4 of the Learn You a Haskell book.

tutorial exercises

Here are some of the functions we wrote in the tutorial.

kth element

Write a function kth that returns the kth element of a list, where elements are numbered starting from 0. (Do not use the built-in operator !!, which does the same thing.)

kth :: Int -> [a] -> a
kth 0 (x : _) = x
kth n (_ : xs) = kth (n - 1) xs

infinite ints

Construct a list ints of integers from 1 to ∞.

-- intsFrom n returns an infinite list of all integers starting with n
--  i.e. [n, n + 1, n + 2, ...]

intsFrom :: Int -> [Int]
intsFrom n = n : intsFrom (n + 1)

-- a list of all integers starting with 1
allInts :: [Int]
allInts = intsFrom 1

cyclic list

Implement the built-in function cycle that takes a list L and returns an infinite list consisting of L repeated over and over.

my_cycle :: [a] -> [a]
my_cycle xs = xs ++ my_cycle xs