Installing GHC

• First install GHCup https://www.haskell.org/ghcup/
• While installing make sure you install stack, cabal and HLS (Haskell Language Server) if you want IDE like support in your editor
• That’s it!

Using GHC

• Now start up ghci (interactive interpreter) by typing “ghci” in your terminal
• Once within ghci you can evaluate Haskell expressions!
• Let’s try it!

Standard Prelude

• Haskell comes with many builtin functions which you are always welcome to use
• The prelude can be found here: prelude
• Provides many useful list functions (some of which we will redefine)

Exploring The Prelude: Numeric Functions

• Addition:

  ghci> 1 + 2
  3

• Exponentiation:

  ghci> 3 ^ 2
  9

• Integer Division (note the backticks, shift ~):

  ghci> 5 ‘div‘ 2
  2

Exploring The Prelude: List Functions

• Get the first element of a list:

  ghci> head [1, 2, 3]
  1

• Get everything but the first element of a list:

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

• Get the nth element of a list:

  ghci> [1, 2, 3] !! 2
  3

Exploring The Prelude: List Functions Cont.

• Check if a list contains an element:

  ghci> elem 3 [1, 2, 4] 
  False

• Calculate the length of a list:

  ghci> length [1, 2, 3]
  3

• Reverse a list:

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

Exploring The Prelude: List Functions Cont.

• Drop the first n elements from a list:

  ghci> drop 2 [1, 2, 3]
  [3]

• Appending two lists:

  ghci> [1, 2] ++ [3, 4]
  [1, 2, 3, 4]

• Sum a list:

  ghci> sum [1, 2, 3]
  6

Function Application Java

Given the following Java Function:

public static int foo(int x, int y) {
    return x + (y * 2);
}

You apply it to a set of arguments as follows:

foo(2, 3)

Thus, function application in Java is done with parentheses.

Function Application Haskell

Instead of parentheses Haskell uses a space to denote function application.

For example here is the same function written in Haskell:

foo x y = x + (y * 2)

And here is that function being applied:

foo 2 3

Function Application Haskell Precedent

Function application has a higher priority than any other operator.

This means it will always happen first, for example what does the following expression evaluate to?

foo 2 3 * 3

Creating A Haskell File

• Haskell files use the .hs suffix
• General workflow:
  • Make edits in the Haskell file
  • Save
  • Open terminal with ghci and reload Haskell file
  • Fix errors if necessary
  • Test code
  • Repeat

Loading A Haskell File

• The easiest way to load in a Haskell file to ghci is to run the following in a terminal: “ghci foo.hs”
• Then after you make changes you can reload by typing “:reload” or “:r” inside ghci
• While inside ghci you have access to the prelude as well as functions defined within your file
• See this link for more details about ghci: ghci user guide

Naming Requirements and Layout

• Functions and arguments must begin with a lower-case letter
• By convention, names that represent a list end with an ’s’
• Sequences of definitions must all be in the same column

Exercise: Feet to Meters

Define a Haskell function, feet2Meters, which converts a given number of feet to meters. The conversion rate is 1 foot to 0.3048 meters.

For example a function to convert feet to inches could be defined as follows:

feet2Inches ft = ft * 12