First steps

Relevant content from Chapter 2 of Programming in Haskell 2nd Edition

Glasgow Haskell Compiler

GHC is the leading implementation of Haskell, and compromises both a compiler and interpreter.

Starting GHCi

The interpreter can be started simply from the terminal command prompt $ by simply typing ghci

$ ghci
GHCi, version 9.2.5: https://www.haskell.org/ghc/  :? for help
Prelude>

GHCi prompt > means that the interpreter is now ready to evaluate an expression

It can now be used as a desktop calculator to evaluate simple numeric expressions

> 2 + 3 * 4
14

> (2 + 3) * 4
20

> sqrt (3 ^ 2 + 4 ^ 2)
5.0

The Standard Prelude

Haskell comes with numerous standard library functions.

Including but not limited to numeric functions such as +, *, ^ as well as useful functions on lists.

-- Returns the head (first element) of a list
> head [1, 2, 3, 4, 5]
1

-- Returns the tail of a list (everything but the first element)
> tail [1, 2, 3, 4, 5]
[2, 3, 4, 5]

-- Selects the nth element of a list
> [1, 2, 3, 4, 5] !! 2
3

-- Appending two lists
> [1, 2, 3] ++ [4, 5]
[1, 2, 3, 4, 5]

-- Calculate the length of a list
> length [1, 2, 3]
3

-- Reserve a list
> reverse [1, 2, 3]
[3, 2, 1]

-- Return the first n elements of a list
> take 3 [1, 2, 3, 4, 5]
[1, 2, 3]

-- Return the list without its first n elements
> drop 3 [1, 2, 3, 4, 5]

-- Calculate the sum of a list of numbers
> sum [1, 2, 3, 4, 5]
15

-- Calculate the product of a list of numbers
> product [1, 2, 3, 4, 5]
120

Function Application Syntax

TYPE	MATHEMATICS	HASKELL
FUNCTION APPLICATION	Parenthesis f(a,b)	Space f a b
MULTIPLICATION	Juxtaposition cd / Space c d	Multiplication symbol c * d

Moreover, in Haskell function application is assumed to have higher priority than all other operators.

f a + b denotes (f a) + b not f (a + b)

MATHEMATICS	HASKELL
f(x)	f x
f(x,y)	f x y
f(g(x))	f (g x)
f(x,g(y))	f x (g y)
f(x) g(y)	f x * g y

Haskell Scripts

As well as functions in the standard library, it is also possible to define your own functions.

These are defined with a script (.hs), which is a text file compromising a sequence of definitions.

To start the created script GHCi can be used

$ ghci test.hs

Now the standard file is loaded, and its methods can be used. Once the file has been changed it has to be reloaded, because GHCi does not automatically detect that the script has been changed.

To do that the reload command must be executed before the newly added or changed definitions can be used.

COMMAND	MEANING
:load name	Loading script name
:reload	Reloading current script
:set editor name	Set editor to name
:edit name	Set edit script name
:edit	Edit current script
:type expr	Show type of expr
:?	Show all commands
:quit	Quits GHCi

Naming Requirements

Function and argument names must begin with a lowercase letter.

myFun, fun1, arg_2, x'

By convention, list arguments usually have an s suffix on their name.

xs, ns, nss

Layout Rule

In a sequence of definitions, each definition must begin in precisely the same column.

a = 10
b = 20
c = 30

Whereas this would be invalid syntax.

a = 10
    b = 10
c = 10

This avoids the need for explicit braces and semicolons.

Implicit grouping (Indentation and white spaces relevant):

a = b + c
    where
        b = 1
        c = 2
d = a * 2

Explicit grouping (With layout expanded, indentation and white spaces not relevant)

a = b + c
    where
    { b = 1;
      c = 2 }
d = a * 2