---
linguagem: haskell
tradutor/contribuidor:
    - ["Lucas Tonussi", "http://www.inf.ufsc.br/~tonussi/"]
---

As linguagens funcionais são linguagens de programação com base em avaliação
de funções matemáticas (expressões), evitando-se o conceito de mudança de
estado com alteração de dados. Neste aspecto, este paradigma é oposto ao
paradigma imperativo que se baseia em alterações de estados.

A programação funcional começou no cálculo lambda, que foi base teórica para
o desenvolvimento deste paradigma de programação.


```haskell
-- Para comentar a linha basta dois traços seguidos.

{- Abre chaves traço e traço fecha chaves cria um campo
   para comentário em múltiplas linhas.
-}

----------------------------------------------------
-- 1. Tipos Primitivos de Dados e Operadores
----------------------------------------------------

-- Numerais

0 -- 3
1 -- 1
2 -- 2 ...

-- Alguns Operadores Fundamentais

7 + 7 -- 7 mais 7
7 - 7 -- 7 menos 7
7 * 7 -- 7 vezes 7
7 / 7 -- 7 dividido por 7

-- Divisões não são inteiras, são fracionádas por padrão da linguagem
28736 / 82374 -- 0.3488479374559934


-- Divisão inteira
82374 `div` 28736 -- 2

-- Divisão modular
82374 `mod` 28736 -- 24902

-- Booleanos como tipo primitivo de dado
True -- Verdadeiro
False -- Falso

-- Operadores unitário
not True -- Nega uma verdade
not False -- Nega uma falácia


-- Operadores binários
7 == 7 -- 7 é igual a 7 ?
7 /= 7 -- 7 é diferente de 7 ?
7 < 7 -- 7 é menor que 7 ?
7 > 7 -- 7 é maior que 7 ?


{- Haskell é uma linguagem que tem uma sintáxe bastante familiar na
   matemática, por exemplo em chamadas de funções você tem:

   NomeFunção ArgumentoA ArgumentoB ArgumentoC ...
-}

-- Strings e Caractéres
"Texto entre abre áspas e fecha áspas define uma string"
'a' -- Caractere
'A' -- Caractere

'Strings entre aspas simples sobe um erro' -- Erro léxico!

-- Concatenação de Strings
"StringA" ++ "StringB" -- "StringAStringB"

-- Você pode listar uma string pelos seus caractéres
"AbBbbcAbbcbBbcbcb" !! 0 -- 'A'
"AbBbbcAbbcbBbcbcb" !! 1 -- 'b'
"AbBbbcAbbcbBbcbcb" !! 2 -- 'B'

----------------------------------------------------
-- Listas e Túplas
----------------------------------------------------

-- A construção de uma lista precisa ser de elementos homogêneos
[1, 2, 3, 4, 5] -- Homogênea
[1, a, 2, b, 3] -- Heterogênea (Erro)

-- Haskell permite que você crie sequências
[1..5]

{- Haskell usa avaliação preguiçosa o que
   Permite você ter listas "infinitas"
-}

-- Uma lista "infinita" cuja razão é 1
[1..]

-- O 777º elemento de uma lista de razão 1
[1..] !! 777 -- 778

-- União de listas [lista_0] ++ [lista_1] ++ [lista_i]
[1..5] ++ [6..10] ++ [1..4] -- [1,2,3,4,5,6,7,8,9,10,1,2,3,4]

-- Adiciona um cabeçalho a sua lista e desloca a cauda
0:[1..10] -- [0, 1, 2, 3, 4, 5]
'a':['a'..'e'] -- "aabcde"

-- Indexação em uma lista
[0..] !! 5 -- 5

-- Operadores de Listas usuais
head ['a'..'e'] -- Qual o cabeçalho da lista ?
tail ['a'..'e'] -- Qual a cauda da lista ?
init ['a'..'e'] -- Qual a lista menos o último elemento ?
last ['a'..'e'] -- Qual o último elemento ?

-- list comprehensions
[x*2 | x <- [1..5]] -- [2, 4, 6, 8, 10]

-- with a conditional
[x*2 | x <- [1..5], x*2 > 4] -- [6, 8, 10]

-- Every element in a tuple can be a different type, but a tuple has a
-- fixed length.
-- A tuple:
("haskell", 1)

-- accessing elements of a tuple
fst ("haskell", 1) -- "haskell"
snd ("haskell", 1) -- 1

----------------------------------------------------
-- 3. Functions
----------------------------------------------------
-- A simple function that takes two variables
add a b = a + b

-- Note that if you are using ghci (the Haskell interpreter)
-- You'll need to use `let`, i.e.
-- let add a b = a + b

-- Using the function
add 1 2 -- 3

-- You can also put the function name between the two arguments
-- with backticks:
1 `add` 2 -- 3

-- You can also define functions that have no letters! This lets
-- you define your own operators! Here's an operator that does
-- integer division
(//) a b = a `div` b
35 // 4 -- 8

-- Guards: an easy way to do branching in functions
fib x
  | x < 2 = x
  | otherwise = fib (x - 1) + fib (x - 2)

-- Pattern matching is similar. Here we have given three different
-- definitions for fib. Haskell will automatically call the first
-- function that matches the pattern of the value. 
fib 1 = 1
fib 2 = 2
fib x = fib (x - 1) + fib (x - 2)

-- Pattern matching on tuples:
foo (x, y) = (x + 1, y + 2)

-- Pattern matching on lists. Here `x` is the first element
-- in the list, and `xs` is the rest of the list. We can write
-- our own map function:
myMap func [] = []
myMap func (x:xs) = func x:(myMap func xs)

-- Anonymous functions are created with a backslash followed by
-- all the arguments.
myMap (\x -> x + 2) [1..5] -- [3, 4, 5, 6, 7]

-- using fold (called `inject` in some languages) with an anonymous
-- function. foldl1 means fold left, and use the first value in the
-- list as the initial value for the accumulator.
foldl1 (\acc x -> acc + x) [1..5] -- 15

----------------------------------------------------
-- 4. More functions
----------------------------------------------------

-- currying: if you don't pass in all the arguments to a function,
-- it gets "curried". That means it returns a function that takes the 
-- rest of the arguments.

add a b = a + b
foo = add 10 -- foo is now a function that takes a number and adds 10 to it
foo 5 -- 15

-- Another way to write the same thing
foo = (+10)
foo 5 -- 15

-- function composition
-- the (.) function chains functions together.
-- For example, here foo is a function that takes a value. It adds 10 to it,
-- multiplies the result of that by 5, and then returns the final value.
foo = (*5) . (+10)

-- (5 + 10) * 5 = 75
foo 5 -- 75

-- fixing precedence
-- Haskell has another function called `$`. This changes the precedence
-- so that everything to the left of it gets computed first and then applied
-- to everything on the right. You can use `.` and `$` to get rid of a lot
-- of parentheses:

-- before
(even (fib 7)) -- true

-- after
even . fib $ 7 -- true

----------------------------------------------------
-- 5. Type signatures
----------------------------------------------------

-- Haskell has a very strong type system, and everything has a type signature.

-- Some basic types:
5 :: Integer
"hello" :: String
True :: Bool

-- Functions have types too.
-- `not` takes a boolean and returns a boolean:
-- not :: Bool -> Bool

-- Here's a function that takes two arguments:
-- add :: Integer -> Integer -> Integer

-- When you define a value, it's good practice to write its type above it:
double :: Integer -> Integer
double x = x * 2

----------------------------------------------------
-- 6. Control Flow and If Statements
----------------------------------------------------

-- if statements
haskell = if 1 == 1 then "awesome" else "awful" -- haskell = "awesome"

-- if statements can be on multiple lines too, indentation is important
haskell = if 1 == 1
            then "awesome"
            else "awful"

-- case statements: Here's how you could parse command line arguments
case args of
  "help" -> printHelp
  "start" -> startProgram
  _ -> putStrLn "bad args"

-- Haskell doesn't have loops because it uses recursion instead.
-- map applies a function over every element in an array

map (*2) [1..5] -- [2, 4, 6, 8, 10]

-- you can make a for function using map
for array func = map func array

-- and then use it
for [0..5] $ \i -> show i

-- we could've written that like this too:
for [0..5] show

-- You can use foldl or foldr to reduce a list
-- foldl <fn> <initial value> <list>
foldl (\x y -> 2*x + y) 4 [1,2,3] -- 43

-- This is the same as
(2 * (2 * (2 * 4 + 1) + 2) + 3)

-- foldl is left-handed, foldr is right-
foldr (\x y -> 2*x + y) 4 [1,2,3] -- 16

-- This is now the same as
(2 * 3 + (2 * 2 + (2 * 1 + 4)))

----------------------------------------------------
-- 7. Data Types
----------------------------------------------------

-- Here's how you make your own data type in Haskell

data Color = Red | Blue | Green

-- Now you can use it in a function:


say :: Color -> String
say Red = "You are Red!"
say Blue = "You are Blue!"
say Green =  "You are Green!"

-- Your data types can have parameters too:

data Maybe a = Nothing | Just a

-- These are all of type Maybe
Just "hello"    -- of type `Maybe String`
Just 1          -- of type `Maybe Int`
Nothing         -- of type `Maybe a` for any `a`

----------------------------------------------------
-- 8. Haskell IO
----------------------------------------------------

-- While IO can't be explained fully without explaining monads,
-- it is not hard to explain enough to get going.

-- When a Haskell program is executed, the function `main` is
-- called. It must return a value of type `IO ()`. For example:

main :: IO ()
main = putStrLn $ "Hello, sky! " ++ (say Blue) 
-- putStrLn has type String -> IO ()

-- It is easiest to do IO if you can implement your program as 
-- a function from String to String. The function 
--    interact :: (String -> String) -> IO ()
-- inputs some text, runs a function on it, and prints out the 
-- output.

countLines :: String -> String
countLines = show . length . lines

main' = interact countLines

-- You can think of a value of type `IO ()` as representing a
-- sequence of actions for the computer to do, much like a
-- computer program written in an imperative language. We can use
-- the `do` notation to chain actions together. For example:

sayHello :: IO ()
sayHello = do 
   putStrLn "What is your name?"
   name <- getLine -- this gets a line and gives it the name "name"
   putStrLn $ "Hello, " ++ name
   
-- Exercise: write your own version of `interact` that only reads
--           one line of input.
   
-- The code in `sayHello` will never be executed, however. The only
-- action that ever gets executed is the value of `main`. 
-- To run `sayHello` comment out the above definition of `main` 
-- and replace it with:
--   main = sayHello

-- Let's understand better how the function `getLine` we just 
-- used works. Its type is:
--    getLine :: IO String
-- You can think of a value of type `IO a` as representing a
-- computer program that will generate a value of type `a` 
-- when executed (in addition to anything else it does). We can
-- store and reuse this value using `<-`. We can also 
-- make our own action of type `IO String`:

action :: IO String
action = do
   putStrLn "This is a line. Duh"
   input1 <- getLine 
   input2 <- getLine
   -- The type of the `do` statement is that of its last line.
   -- `return` is not a keyword, but merely a function 
   return (input1 ++ "\n" ++ input2) -- return :: String -> IO String

-- We can use this just like we used `getLine`:

main'' = do
    putStrLn "I will echo two lines!"
    result <- action 
    putStrLn result
    putStrLn "This was all, folks!"

-- The type `IO` is an example of a "monad". The way Haskell uses a monad to
-- do IO allows it to be a purely functional language. Any function that
-- interacts with the outside world (i.e. does IO) gets marked as `IO` in its
-- type signature. This lets us reason about what functions are "pure" (don't
-- interact with the outside world or modify state) and what functions aren't.

-- This is a powerful feature, because it's easy to run pure functions
-- concurrently; so, concurrency in Haskell is very easy.


----------------------------------------------------
-- 9. The Haskell REPL
----------------------------------------------------

-- Start the repl by typing `ghci`.
-- Now you can type in Haskell code. Any new values
-- need to be created with `let`:

let foo = 5

-- You can see the type of any value with `:t`:

>:t foo
foo :: Integer

-- You can also run any action of type `IO ()`

> sayHello
What is your name?
Friend!
Hello, Friend!

```

There's a lot more to Haskell, including typeclasses and monads. These are the big ideas that make Haskell such fun to code in. I'll leave you with one final Haskell example: an implementation of quicksort in Haskell:

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

Haskell is easy to install. Get it [here](http://www.haskell.org/platform/).

You can find a much gentler introduction from the excellent
[Learn you a Haskell](http://learnyouahaskell.com/) or
[Real World Haskell](http://book.realworldhaskell.org/).