Lisperati Haskell tutorial.pdf

by Conrad Barski, M.D. Mail

Note To Advanced Haskellers

Hi everyone,

welcome to my Haskell tutorial!

There's other tutorials out there, but you'll like this one the

best for sure : You can just cut and paste the code from this

tutorial bit by bit, and in the process, your new program will

create magically create more and more cool graphics along

the way... The final program will have less than 100 lines of

Haskell [1] and will organize a mass picnic in an arbitrarily-

shaped public park map and will print pretty pictures

showing where everyone should sit! ( Here's what the final

product will look like, if you're curious...)

The code in this tutorial is a simplified version of the code

I'm using to organize flash mob picnics for my art project,

picnicmob.org . .. Be sure to check out the site and sign up if

you live in one of the cities we're starting off with :-)

[1] - Lines of active code only, not counting optional function

signatures. Intermediate debug output lines excluded. May

cause eye irritation if viewed for prolonged periods of time.

Speak to your physician to find out if this tutorial is the best

treatment option for you.

How To Organize a Picnic on a Computer

Ok, so here's what the program is going to do... On the

picnicmob.org website, people answer a bunch of random

questions. What we want our program to do is take a picture of

a city park, a list of answers people gave on the questions, and

have it figure out where people should sit in the park so that

their neighbors are as similar as possible to them, based on

similar answers for the questions. We're going to do this using

a standard simulated annealing algorithm. (Read this for more

info... we'll be describing the basics below, too...)

Installing GHC Haskell On Your Computer

The only preparation you need to do for this tutorial is install

the Glasgow Haskell Compiler- You can get the latest version

from here . It's a very well supported and very fast compiler.

We're just going to use the basic GHC libraries in this tutorial-

No other installs are needed.

Hello World! Let's Have a Picnic!

Now you have all you need to run the "Hello World" program

below- Just copy the code into a file called tutorial.hs :

import Data.List

import Text.Regex

import System.Random

import Data.Ord

type Point = (Float,Float)

type Color = (Int,Int,Int)

type Polygon = [Point]

type Person = [Int]

type Link = [Point]

type Placement = [(Point,Person)]

type EnergyFunction a = a -> Int

type TemperatureFunction = Int -> Int -> Float

type TransitionProbabilityFunction = Int -> Int -> Float -> Float

type MotionFunction a = StdGen -> a -> (StdGen,a)

main = do

putStr "Hello World! Let's have a picnic! \n"

After you've copied the code to a file, just run your new

program from the command line as shown below:

> runHaskell tutorial.hs

Hello World! Let's have a picnic!

*phew* That was easy!

How Does This Code Work?

As you can see, there's lots of stuff in this "Hello World" that

doesn't seem necessary... Actually, only the last two lines

(starting at main = do ) are really needed to print our

message... The rest is just header stuff that we'll use later in

the tutorial...

At the top, you can see some things we're importing out of

the standard GHC libraries:

1. The Data.List module has extra functions for

slicing and dicing lists- Lists are fundamental in

Haskell programming and we'll be using them a lot in

this program.

2. The Text.Regex module let's us do funky things

with text strings using regular expressions - If you

don't know what this means, read this tutorial first .

Every programmer should know about regular

expressions.

3. The System.Random module, as you've probably

guessed, lets us use random numbers, essential for

simulated annealing.

4. The Data.Ord just contains a single function I'm

really fond of, called comparing . I'm not happy if I

can't use comparing , and I think we all want this to

be a happy tutorial...

The next few lines define different types that we're going to

use in our picnic program... Haskell programmers are really

fond of lots of types, because Haskell compilers are basically

just these crazy type-crunching machines that will take any

types you give them and if they find anything questionable

about how you're using types they will let you know early,

preventing your program from becoming buggy. The way

Haskell compilers handle types puts them head and shoulders

above most other compilers (Read more about Hindley-

Milner type inference here. )

The first few type lines should be pretty self-explanatory,

for a program that's going to work with pictures: A Point is

just a pair of floating numbers, a Color is an rgb triple, a

Polygon is just a list of Point s, a Person is just a list of

the answers they gave in the questionaire- Very simple stuff...

The lower type lines are a bit more difficult to understand-

They describe the types of the four functions that are used in

all simulated annealing algorithms... we'll be describing those

later on. For now, just know that in Haskell you want to look

at the last arrow, which looks like this > ... Anything to the

left of that arrow is a parameter into our function- The thing

to the right is the return value of the function. For instance,

the EnergyFunction is a function that takes an arbitrary

type a (which will be a seating arrangement of picnickers)

and returns an integer, which is the "energy" of our picnic. A

low energy will mean that all the picnickers are happy with

where they're sitting. We'll explain these funcitons more later.

What's Good about this Code?

What's good is that in Haskell we can create all kinds of

types to describe what we want to do and the Haskell

compiler is very good at catching potential errors for us

ahead of time. Defining types in Haskell is usually a choice

and not a necessity - This program would run just fine, even if

all the type declarations were missing... The Haskell compiler

would just figure them out on its own!

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: