Notes on Programming in Haskell

Date: November 3 2021

Summary: My notes on an overview and summary on how to use Haskell

Keywords: #syntax ##summary #haskell #programming #languages #archive

Bibliography

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Table of Contents

Packages

Prelude - Haskell's Standard Library

Functions

All functions are pure within Haskell. This means that the same result is always produced with no side effects given the same input to a Haskell function.

Ar

Haskell Type System

Except only in very seldom cases, type annotations are optional in Haskell.

Bool

A two element set of True and False.

Char

A set of all Unicode characters.

String

A synonym for an infinite list of Char's.

()

A dummy value where there is only one instance of it ever. It is pronounced unit.

-- Unit typing example

-- Function declaration
f44 :: () -> Integer

-- Function definition
f44 () = 44

-- Function invocation
f44 () -- Returns the value 44

Type Coercion

Haskell provides unsafeCoerce to bypass the type system.

Syntax

Types

All concrete types start with a capital letter. Names of type variables start with a lowercase letter.

Functions

A function type is created by putting an arrow between two types.

f :: foo -> bar

A function definition uses the name of the function and formal parameters. The body of a function follows an equals sign. Furthermore, the body of a function is always an expression.

One of the strangest quirks about Haskell functions is that arguments are neither surrounded by parentheses nor separated by commas.

Defining Function in ghci

-- Defining a function within ghci
-- Requires the use of multiple lines as denoted
-- by :{ ... :} 
« Prelude » λ: :{
Prelude| add :: Integer -> Integer -> Integer -- Function declaration
Prelude| add x y = x + y -- Function definition
Prelude| :}

« Prelude » λ: (add 5 3) -- Compute sum of two numbers
8

Defining Function in a Script

-- Creating function declaration
add :: Integer -> Integer -> Integer

-- Creating function definition
add x y = x + y

-- Compute sum of two numbers
add 5 3

Variable Wildcards

Arguments can be discarded with a wildcard by the following notation:

fInt :: Integer -> ()
fInt _ = ()

Function Composition

Functions can be composed by putting a period between them (or a Unicode circle, "◦"):

-- Define two functions:
f1 :: A -> B
f2 :: B -> C

-- Compose them together:
f1 . f2

-- Or use alternative composition syntax:
f1 ◦ f2

Double colons

In Haskell, a double colon means, "has type of..."

f :: foo -> bar

Infix Operators

Any infix operator can be turned into a two-argument function by surrounding them with parentheses:

"Hello " ++ "world!"

Can be rewritten as:

(++) "Hello " "world!"

Equality

Functional Equality

Haskell enables you to express equality of functions:

mappend = (++)

This is also known as point-free equality as the arguments to these functions are not defined.

Extensional Equality

Haskell defines extensional equality loosely where

mappend s1 s2 = (++) s1 s2

is saying that the output of the function of the left is equivalent to the output of the function on the right. This is also known as point-wise equality as the arguments (points) are defined.

How To Cite

Zelko, Jacob. Notes on Programming in Haskell. https://jacobzelko.com/11032021155827-haskell-programming. November 3 2021.

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