I want to create Haskell function with different return value from its parameter, example:
I want function isOdd 3 return value either True or False.
I've tried
isOdd :: Integer -> Bool
isOdd x = (if x mod 2 == 0 False else True)
but it returns an error, can anybody help me? Also, is there a tutorial about functions in Haskell? I can't find a good tutorial about function in haskell.
isOdd :: Integer -> Bool
isOdd x = (if x mod 2 == 0 False else True)
You don't need the parens:
isOdd :: Integer -> Bool
isOdd x = if x mod 2 == 0 False else True
You missed out then:
isOdd :: Integer -> Bool
isOdd x = if x mod 2 == 0 then False else True
As you are using mod as an operator, you must surround it with backticks:
isOdd :: Integer -> Bool
isOdd x = if x `mod` 2 == 0 then False else True
That works.
Furthermore, you can write if blah then False else True more simply as not (blah):
isOdd :: Integer -> Bool
isOdd x = not (x `mod` 2 == 0)
Even more simply:
isOdd :: Integer -> Bool
isOdd x = x `mod` 2 /= 0
Please note: this is practically the same as the standard odd function.
Which tutorials have you tried?
Learn You A Haskell has a chapter introducing functions.
The problem here is that mod isn't infix. Additionally, in Haskell if statements work like this
if cond then expr1 else expr2
Notice the then.
You should use mod like this mod x 2. However you can make it infix like this:
x `mod` 2
On a side not
x `mod` 2 /= 0
Is much easier to read than the whole if statement.
As far as tutorials: Learn You a Haskell for Great Good is a good start. For a more in depth coverage Real World Haskell is excellent.
If you just want to find a function then Hoogle is your friend.
func args the backticks are a special trick for functions with two arguments that let you write arg1 'func' arg2 just for clarity. There is no difference between mod x 2 and x 'mod' 2 except the second one is perhaps easier to understand