I have the following function that sholud return only odd numbers from the List
oddsOnly :: Integral a => [a] -> [a]
oddsOnly [] = []
oddsOnly (x:xs)
|odd x = x : oddsOnly xs
|otherwise = oddsOnly xs
My question is about the aim of using
Integral a =>
Actually it is not posssible to implement such function with such type declaration
oddsOnly :: [a] -> [a]
As far as I know even and odd functions are in standart Prelude library so why simplified daclaration does not work?
Thanks in advance
why simplified daclaration does not work?
Because you can't tell if a string is odd or not. Or if a float is odd or not. In general, it's not possible to tell if a value of any type a is odd or not because the notion of being odd may not be defined for this type.
It is defined, however, for Integral types, so you must specify that a is Integral.
In terms of Haskell, odd works on Integrals only:
Prelude> :t odd
odd :: Integral a => a -> Bool
Thus, you must pass it a value of a type that is Integral.
The signature of odd and even is odd :: Integral a => a -> Bool and even :: Integral a => a -> Bool respectively. So these contain an Integral a type constraint [Haskell-wiki].
A type constraint has nothing to do from where it is defined. It specifies that the function only is defined for a subset of types. Only for types that are members of the Integral typeclass, you can use the function. Integral types are types like that specify a subset of ℤ, so Int, Int64, Word8, Integer, … Types. odd and even do not make much sense over Floats, Strings, Chars, etc. What would that mean?
By thus specifying the Integral a => type constraint, you specify that this function can only work with integer-like items.
Note that you can use filter :: (a -> Bool) -> [a] -> [a] here to retain only odd numbers:
oddsOnly :: Integral a => [a] -> [a]
oddsOnly = filter oddLet’s look at the type signature of odd: odd :: Integral a => a -> Bool. So to use odd with a parameter, that parameter needs to be Integral. This makes sense intuitively: as @ForceBru said in another answer, you can’t tell whether e.g. a float such as 1.5 or a string such as "hello" is odd, as the notion of oddness is only defined for integral values.