Re: [Haskell-beginners] Haskell type definitions

Sorry, that's "implement Num and Ord", not "implement Ord and Eq"! On Wed, Apr 10, 2013 at 5:39 PM, Patrick Mylund Nielsen < haskell@patrickmylund.com> wrote:

Yes, it is a type signature. It's "function name :: optional type constraints => function of type a -> function of type b -> type c"

take' :: Num i => i -> [a] -> [a] = take' is a function whose first argument is of type i, where the i type must be an instance of the Num typeclass. The function also takes a list of values of type a (any specific type), and returns a list of values of the same type.

take' :: (Num i, Ord i) => i -> [a] -> [a] = take' is a function whose first argument is of type i, where the i type must be an instance of both the Num and Ord typeclasses. The function also takes a list of values of type a (any one type), and returns a list of values of the same type.

So the tuple behind the => doesn't mean that one of the arguments is a tuple, or that the constraints apply to the arguments positionally, but rather that the specified type, e.g. i, must satisfy some constraints--in this case implement Ord and Eq so that you can check if it's below 0 (Ord) and use it as a number (Num.) take' :: i -> [a] -> [a] would have been valid too, but you wouldn't know if you can compare against i, or use it as a number.

If you wanted to sort the [a], then indeed, you would need to make it take' :: (Num i, Ord i, Ord a), since sort from Data.List has a type signature of sort :: Ord a => [a] -> [a]

On Wed, Apr 10, 2013 at 11:32 AM, Angus Comber wrote:

...

I am learning Haskell using Learn You a Haskell. On page 54 is an implementation of take like so:

take' :: (Num i, Ord i) => i -> [a] -> [a] take' n _ | n <= 0 = [] take' _ [] = [] take' n (x:xs) = x : take' (n-1) xs

I understand all the code apart from the first line.

The :: part I understand as meaning this is a type definition?

(Num i, Ord i) is a tuple. The first element of the tuple has to be numeric, fair enough. the second param has to be able to be ordered. The parameter is the same - both are i. This means that the types have to be the same?

Why is it not (Num i, Ord j)? Isn't the 2nd tuple element referring to the list? Which could be of any type?

What does => signify?

i -> [a] -> [a] means first parameter is numeric? 2nd param is any type list, 3rd param is any type list. So this is saying first param numeric, 2nd param a list of any type and it returns a list of any type. Well that is understandable I suppose.

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