How to do multiple things in a Haskell function?-CodePudding

I was wondering, how can we do multiple things in a Haskell function?

For example, I have this code:

number = 0

increment :: Int -> Int
increment i = i   1

function :: String -> String
function s = s    " world" AND increment number

I was wondering, how can we do something like this? I'm searching everywhere but I don't find a solution for this simple example :O

number
0

:function "hello"
"hello world" (AND the number = 1 now)

number
1

note: I know that AND is not a syntax in Haskell but I wanted to make you understand what I wanted to say :)

CodePudding user response：

You can't modify variables in Haskell (they are not, in fact, variable). What you can do is return new values:

f (string, number) = (string    " world", number   1)

which would be used like this:

Prelude> f ("hello", 0)
("hello world",1)

If you called this function with a variable instead of a literal, it would not change the value of that variable, because Haskell is designed to prevent that from happening:

Prelude> let n = 0
Prelude> f ("hello", n)
("hello world",1)
Prelude> print n
0

CodePudding user response：

the problem is: in haskell you don't have "mutable states" like in other programming languages: so "number = 1" wouldn't work in terms of "setting a State"

you could combine them as a result:

increment :: Int -> Int
increment i = i   1 

stringCombine :: String -> String
stringcombine str = str    "world"

combine i str = (increment i, stringCombine str)

or if you have a function with a side effect (like print, putStrLn) you have to use the IO Monad:

doSideEffects :: Int -> String -> IO Int
doSideEffects i str = do
   putStrLn $ str    "world"
   return $ increment i