Introduction To Haskell

Lecture 1

An Unexpected Journey™

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Instructor

Nishant Shukla

ns4av@virginia.edu

Watchful Guardian

Professor Jack Davidson

jwd@virginia.edu

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Syllabus

Find it on the class website

(Slides, homework, and announcements)

More Than a Language

I know why you're here. ...why you hardly sleep, why night after night, you sit by your computer.

History

Alonzo Church invented λ-calculus

Then John McCarthy invented Lisp

The Origin

Haskell was made by a committee of really smart people to define an open standard*

More than 20 years old

• MIT
• Chalmers University
• Mitre Corp
• Victoria University of Wellington
• Simon Fraser University
• University of Cambridge
• Yale University
• University of Glasgow
• Microsoft Research Ltd

From Simon Peyton Jones' talk at OSCON

From Simon Peyton Jones' talk at OSCON

From Simon Peyton Jones' talk at OSCON

From Simon Peyton Jones' talk at OSCON

Features

1. Purely functional

Let's be a little formal

f is function from a set A to a set B.

f :: A → B

domain(f)   = 
codomain(f) = 
range(f)    ⊆ 
	    

domain(f)   = A
codomain(f) = B
range(f)    ⊆ B
	    

Functional:

Haskell, Lisp, ML, Scheme, Erlang

Focuses on the high-level "what"

Imperative:

C++, Java, Python, Pascal

Focuses on the low-level "how"

What does this code do?

void f(int a[], int lo, int hi) 
{
  int h, l, p, t;

  if (lo < hi) {
    l = lo;
    h = hi;
    p = a[hi];

    do {
      while ((l < h) && (a[l] <= p)) 
          l = l+1;
      while ((h > l) && (a[h] >= p))
          h = h-1;
      if (l < h) {
          t = a[l];
          a[l] = a[h];
          a[h] = t;
      }
    } while (l < h);

    a[hi] = a[l];
    a[l] = p;

    f( a, lo, l-1 );
    f( a, l+1, hi );
  }
}

	    

Sort in Haskell

qsort :: Ord a => [a] -> [a]
	      

qsort []     = []

qsort (p:xs) = (qsort lesser) ++ [p] ++ (qsort greater)
    where
        lesser  = filter (< p) xs
        greater = filter (>= p) xs
	    

No Side Effects

Haskell code:

count :: List -> Int
	    

C++ code:

int count( List l ) { ... }
	    

2. Statically Typed

• f x = x² + 1
• f :: Int → Int
• There is never confusion about types

  (Bool, Int, Char, etc)

• Strong formalism. The proof is the code.
• If your code compiles, you're 99% done

Types

Every function in haskell has a Type signature.

foo :: Int -> String
	    

I don't know what foo means,

but I know what it does!

3. Lazy

Lazy?

• Nothing is evaluated unless necessary

head (sort ls)
	      

The list will only be sorted enough to find the minimum

• Allows infinite data structures

[1..]
	      

Who uses Haskell? *

Language shapes the way we think, and determines what we can think about.

The limits of my language mean the limits of my world.

A language that doesn’t affect the way you think about programming, is not worth knowing.

Slide from ed.ac.uk

Homework

The Journey Begins

1. Install Haskell

2. Run the interactive interpreter

   
    $ ghci
   	    
    Prelude>
   		

3. Fill out this week's form