Save function status in Haskell

Question

I want to implement a button that every time it is clicked changes state in Haskell. If it is on when press it should turn off and vice versa.

In c++ I would save the state in a global variable, but in Haskell I have no idea!

Answer 1

Haskell is a purely functional language. It is impossible to save the state of something. The only way to achieve what you want is to pass to a function two things at once: the previous state of the button and the user action. The function then must decide the new state of the button depending on the action.

A simple example:

-- Novo tipo para representar o botão
data Botao = Ligado | Desligado deriving (Eq, Show)

-- Novo tipo para representar uma acao
data Acao = Pressiona | Nada deriving (Eq, Show)

-- Recebe o estado de um botao, mais uma ação e retorna um novo estado
runBotao :: Botao -> Acao -> Botao 
runBotao Ligado    Pressiona = Desligado -- Se ligado e pressionado, muda para desligado
runBotao Desligado Pressiona = Ligado    -- Se desligado e pressionado, muda para ligado
runBotao s         Nada      = s         -- Se não houver ação, permanece no estado original

You can test in Ghci, for example:

ghci> Ligado `runBotao` Nada `runBotao` Pressiona `runBotao` Pressiona
Ligado

In this example, a button starts Ligado, then there is no action, then it is pressed and finally it is pressed again.

To work with states more thoroughly and more widely, I recommend you read about the monad State (unfortunately it seems that this chapter has not yet been translated into Portuguese).