CODEWITHSUNDEEP
haskell
user13425812
user13425812

Reputation: 31

How to do multiple things in a Haskell function?

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 :)

Upvotes: 0

Views: 662

Answers (2)

Rein Henrichs
Rein Henrichs

Reputation: 15605

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

Upvotes: 4

Thomas Meyer
Thomas Meyer

Reputation: 394

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

Upvotes: 1

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