Haskell - Being lazy with class

Haskellbeing lazy with class

Tiago BaboPPRO@FEUP 2012

Lisp/Scheme1959

ML/OCaml1979

Miranda1985

Haskell 982003

functional language

statictyping

lazyevaluation

History

There are two main Haskell implementations:

▹ GHC. Probably the most popular, compiles to native code on a number of different architectures.

▹ Hugs. It’s a bytecode interpreter and the most portable and lightweight of the Haskell implementations.

History

Related languages

There are some languages inspired by Haskell:

▹ Different type system: Epigram, Agda.

▹ JVM-based: Frege, Jaskell.

▹ Other related languages: Curry.

▹ Testbed for many new ideas: Parallel Haskell, Eager Haskell, Generic Haskell, O’Haskell, Hume, Scotch, Disciple...

haskell

Haskell is a pure functional language. It means that:

▹ Variables never change after definition.

▹ Functions don’t have side effects.

▹ Functions always return the same output given the same input.

haskell

What haskell offer to the programmer?

▹ Purity. It doesn’t allow any side-effects.

▹ Laziness (non-strict). Nothing is evaluated until it has to be evaluated.

▹ Strong static typing. The compiler automatically infers a precise type for all values and it permits no implicit type conversions.

▹ Elegance. Stuff just work like you’d expect it to.

haskell and bugs

Haskell programs have fewer bugs because Haskell is:

▹ Pure. There are no side effects.

▹ Strongly typed. There can be no dubious use of types.

▹ Concise. Programs are shorter which make it easier to “take it all” at once.

haskell and bugs

Haskell programs have fewer bugs because Haskell is:

▹ High Level. Haskell programs most often reads out almost exactly like the algorithm description.

▹ Memory Managed. There’s no worrying about dangling pointers, the Garbage Collector takes care of all that.

▹ Modular. Haskell offers stronger and more “glue” to compose your program from already developed modules.

Haskell has some disadvantages:

▹ Hard to learn and master. It’s even harder without a proper computer science background.

▹ You can’t write haskell-like code in other programming languages.

▹ Lacks libraries and support from who mantain and improve them.

haskell cons

code examples

fibonacci1 1 2 3 5 8 13 21 34 ..

List <int> fib(int i) {List <int> seq = new ArrayList(n);seq[0] = 1;seq[1] = 1;for(int i = 2; i < n; i++) {seq[i] = seq[i-2] + seq[i-1];}return seq;}

Java

fib = 1:1:zipWith (+) fib (tail fib)

Haskell

List <int> seq = new ArrayList(n);seq[0] = 1;seq[1] = 1;for(int i = 2; i < n; i++) {seq[i] = seq[i-2] + seq[i-1];

}

Java VS haskell

fib = 1:1:zipWith (+) fib (tail fib)

List <int> seq = new ArrayList(n);seq[0] = 1;seq[1] = 1;for(int i = 2; i < n; i++) {seq[i] = seq[i-2] + seq[i-1];

}

Java VS haskell

fib = 1:1:zipWith (+) fib (tail fib)

List <int> seq = new ArrayList(n);seq[0] = 1;seq[1] = 1;for(int i = 2; i < n; i++) {seq[i] = seq[i-2] + seq[i-1];

}

Java VS haskell

fib = 1:1:zipWith (+) fib (tail fib)

List <int> seq = new ArrayList(n);seq[0] = 1;seq[1] = 1;for(int i = 2; i < n; i++) {seq[i] = seq[i-2] + seq[i-1];

}

Java VS haskell

fib = 1:1:zipWith (+) fib (tail fib)

How it works?

fib = 1:1:zipWith (+) fib (tail fib)

take 2 fib -> ?

fib = 1 : 1 : ..tail fib = 1 : ..zipWith (+) fib (tail fib) = ..

take 2 fib -> [1,1]

lazy evaluation

fib = 1:1:zipWith (+) fib (tail fib)

take 3 fib -> ?

fib = 1 : 1 : 2 : ..tail fib = 1 : 2 : ..zipWith (+) fib (tail fib) = 2 : ..

take 3 fib -> [1,1,2]

How it works?

fib = 1:1:zipWith (+) fib (tail fib)

take 10 fib -> ?

fib = 1 : 1 : 2 : 3 : ..tail fib = 1 : 2 : 3 : ..zipWith (+) fib (tail fib) = 2 : 3 : ..

...

fib = 1 : 1 : 2 : 3 : 5 ..tail fib = 1 : 2 : 3 : 5 : ..zipWith (+) fib (tail fib) = 2 : 3 : 5 : ..

take 10 fib -> [1,1,2,3,5,8,13,21,34,55]

How it works?

and how about the types?

fib = 1:1:zipWith (+) fib (tail fib)

and how about the types?

int

int

fib = 1:1:zipWith (+) fib (tail fib)

and how about the types?

int

intList<int>

fib = 1:1:zipWith (+) fib (tail fib)

and how about the types?

int

intList<int>

List<int>

List<int>

fib = 1:1:zipWith (+) fib (tail fib)

and how about the types?

int

intList<int>

List<int>

List<int>List<int>

fib = 1:1:zipWith (+) fib (tail fib)

Quicksort[3,2,1,4] -> [1,2,3,4]

// To sort array a[] of size n: qsort(a,0,n-1)void qsort(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; qsort( a, lo, l-1 ); qsort( a, l+1, hi ); }}

C

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

Haskell

qsort(p:xs) = qsort [x | x<-xs, x<p] ++ [p] ++ qsort [x | x<-xs, x>=p]

Haskell

Factorial5! = 1x2x3x4x5

fac 0 = 1fac n | n > 0 = n * fac (n-1)

Haskellpattern

matching

fac n = product [1..n]

fac n = foldr1 (*) [1..n]

fac n = if n == 1 then 1 else n * fac (n-1)

fac n = case n of0 -> 1n -> n * fac (n-1)

Haskell

facs = scanl (*) 1 [1..]

fac n = facs !! n

haskell in industry

▹ Aerospace, defense, finance, web startups, and hardware design firms.

Haskell is used in many areas:

haskell in industry

automate processing of internet abuse complaints

programmatically manipulating a PHP code base

procedural city generation and simulation market

analysis of cryptographic protocols

haskell in industry

measure the counterparty risk on portfolios of financial derivates

job scheduling and brand matching

implement mathematical models and other complex

handwriting recognition system

Haskellbeing lazy with class

Tiago BaboPPRO@FEUP 2012

Links:http://haskell.org/ - Haskell official homepagehttp://youtu.be/cXY4fSA7DnM - Stanford Tutorialhttp://haifux.org/lectures/173/ - An overview of Haskell (Haggai Eran)http://en.wikipedia.org/wiki/Haskell_(programming_language) - Haskell History