haskell-basics

Basic Haskell syntax and examples learned through Learn You a Haskell for Great Good

How to complie and run Haskell code

I use GHC as the book does. Download the Haskell Platform through here.

• start the interactive mode of GHC: ghci at the prompt.
• load a haskell file: :l functions where the file name is functions.hs
• reload the current script: :r

Reserved Words

• let can be used to define a name in GHCI. Doing let a = 1 inside GHCI is the equivalent of writing a = 1 in a script and then loading it.

Boolean Algebra

• && means and
• || means or
• not negates a True or False

Testing for Equality

• == means equal
• /= means unequal

but they can compare only the same type

Infix Function

If a function takes two parameters, we can also call it as an infix function by surrounding it with backticks. div 92 10 is equal to 92 `div` 10

Built-in Functions

• succ A: takes anything that has a defined successor and returns that successor.
• min A B: takes two things that can be put in an order and returns the lesser one.
• max A B: takes two things that can be put in an order and returns the greater one.

Build-in Functions for List Operations

• [A] ++ [B]: puts two list together. Haskell goes through the whole elements of the left side list to append the right list.

ghci> [1,2,3,4] ++ [9,10,11,12]
[1,2,3,4,9,10,11,12]

• A : [B]: puts an element to the head of the list

ghci> 'A' : " SMALL CAT"
"A SMALL CAT"

• [A] !! B: takes an element out of the a list by index. The indices start at 0.

ghci> "Steve Buscemi" !! 6
'B'

• head [A]: takes a list and returns its head.

ghci> head [5,4,3,2,1]
5

• tail [A]: takes a list and returns its tail. In other words, it chops off a list's head.

ghci> tail [5,4,3,2,1]
[4,3,2,1]

• last [A]: takes a list and returns its last element.

ghci> last [5,4,3,2,1]
1

• init [A]: takes a list and returns everything except its last element.

ghci> init [5,4,3,2,1]
[5,4,3,2]

head, tail, last, and init will throw an exception when used on empty lists.

• length [A]: takes a list and returns its length.

ghci> length [5,4,3,2,1]
5

• null [A]: checks if a list is empty. If it is, it returns True, otherwise it returns False. Use this function instead of xs == [] (if you have a list called xs)

ghci> null [1,2,3]
False
ghci> null []
True

• reverse [A]: reverses a list.

ghci> reverse [5,4,3,2,1]
[1,2,3,4,5]

• take A [B]: takes a number and a list. It extracts that many elements from the beginning of the list.

ghci> take 3 [5,4,3,2,1]
[5,4,3]
ghci> take 1 [3,9,3]
[3]
ghci> take 5 [1,2]
[1,2]
ghci> take 0 [6,6,6]
[]

• drop A [B]: drops the number of elements from the beginning of a list.

ghci> drop 3 [8,4,2,1,5,6]
[1,5,6]
ghci> drop 0 [1,2,3,4]
[1,2,3,4]
ghci> drop 100 [1,2,3,4]
[]

• maximum [A]: takes a list of elements that can be put in some kind of order and returns the biggest one.

ghci> minimum [8,4,2,1,5,6]
1

• minimum [A]: takes a list of elements that can be put in some kind of order and returns the smallest one.

ghci> maximum [1,9,2,3,4]
9

• sum [A]: takes a list of numbers and returns their sum.

ghci> sum [5,2,1,6,3,2,5,7]
31

• product [A]: takes a list of numbers and returns their product.

ghci> product [6,2,1,2]
24
ghci> product [1,2,5,6,7,9,2,0]
0

• elem [A]: takes a thing and a list of things and returns if that thing is an element of the list. It's usually called as an infix function because it's easier to read that way.

ghci> 4 `elem` [3,4,5,6]
True
ghci> 10 `elem` [3,4,5,6]
False

List in Haskell

[1,2,3] is actually just syntactic sugar for 1:2:3:[]. [] is an empty list.

String in Haskell

string type in Haskell is just a list of character. That is, "hello" is just syntax suger for ['h','e','l','l','o'].