AGENDA

Functional Programming
Haskell Features
Practical examples
"Hello World" in Haskell
Resources
Functional Programming

In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data.

Functional Programming (Cont.)

Treats computation as a series of mathematical functions (functional style)

Subset of compositional programming (pipe in Unix?)

Avoids states and mutable data.

Immutable

Purity

Functions a first class

functions can be a argument (HOF)

functions can be the return value

Abstraction

Mathematics model (when solving problems)

Currying

Recursive / Tail Call Optimisation
Expressions v.s Statements

Summing the integers 1 to 10 in Java

Java

int total = 0;
for (i = 1; i <= 10; ++i) {
   total = total+i;
}

Haskell

sum [1..10]

Haskell Features

Haskell in one slide

by SPJ

Purely functional

f(x) = x^2 + 1

No side effects
No more global variable
Pure function is easy to test

High order functions

map :: (a->b) -> [a] -> [b]
foldl :: (a -> b -> a) -> a -> [b] -> a


multiTwo x = x * 2
cal xs = map multiTwo xs

Currying

-- foldl :: (a -> b -> a) -> a -> [b] -> a

add x y = x + y

f = foldl add     -- f :: b -> [b] -> b
g = foldl add 0   -- g :: [b] -> b

sum = foldl add 0

bind function in JavaScript

Function composition


(.) :: (b -> c) -> (a -> b) -> a -> c
f x = x + 2
g x = x * x
h = f . g

Statically typed

f :: Int -> Int
Bool, Int, Char, etc
null value? then what is the type?

Pattern matching

fac :: Int -> Int
fac 0 = 1
fac n = n * fac (n-1)

Practical example

Trivial Abstraction example

function reverse (aList) {
  var result = [];
  for (item in aList) {
    result = result.unshift(item);
  }
  return result;
}

What patterns found?

function reduceL (fn, aList, init) {
  if (aList.length > 0) {
     return reduceL(fn, aList.slice(1), fn(init, aList[0]));
  } else {
     return init;
  }
}

function _pushR (xs, y) {
   xs.unshift(y);
   return xs;
}

function reverse (xs) { 
   return reduceL(_pushR, xs, []);
}

function multi(xs) {
    return reduceL(function (x,y) { return x*y; }, xs, 1);
}

notes: code written in JavaScript

The ancient RC process

I would like to make change to search-ecomm_13_12.js and I need to

Figure out all TML files that refer to search-ecomm_13_12.js
P4 edit those TML files
Update the reference path
- from .../resources/mojito/.../search-ecomm_13_12.js
- to .../RC/mojito/.../search-RC.js
Copy
- from .../resources/mojito/.../search-ecomm_13_12.js
- to .../RC/mojito/.../search-RC.js
p4 add it
Make change to the search-RC.js

Can a tool help me out like:

a-simple-tool --resource search-ecomm_13_12.js --branch pb_intl_eu

The ancient RC process - Types

Translate the scenario straightforwardly

findTMLs   :: Directory -> IO [TML]
grepTMLs   :: ResourceName -> [TML] -> IO [TML]
updateTMLs :: ResourceName -> [TML] -> IO [TML]
createRC   :: Directory -> ResourceName -> IO ()

runP4 :: CMD -> IO ()

The ancient RC process - Show me the code

https://github.scm.corp.ebay.com/Haskell/make-RC
Quick start a Hello world
1. Install Haskell platform
2. You will get following executable application
  - ghc
  - ghci
  - runghc
3. Create file hello-world.hs with content

module Main where
main :: IO ()
main = print "Hello, Haskell"

run it

runghc hello-world.hs

Resources

Wikipedia - Functional Programming
Haskell Wiki
- Installation / Libraries / IDE / Tools / etc..
Learn You a Haskell for Great Good
Lectures: Functional Programming Fundamentals - Dr. Erik Meijer
More resources links
Thank you

Haskellers say "If your code compiles, you're 99% done."

No Silver bullet!