CODEWITHSUNDEEP
haskell
Fireapprentice
Fireapprentice

Reputation: 117

Meaning of => and (Ord a)

First of all, please be aware that I looked online (in stackoverflow and Hoogle) to see if I could find anything to this topic!

I have been working myself through a "learn yourself a haskell" and there are two things I have been stumbling upon quite often which I don't really understand. The use of => and (Ord a) in several important functions.

As an example: 

max :: (Ord a) => a -> a -> a

and is the use of "Ordering" the same as (Ord a), as in:

compareWith :: Int -> Ordering

Thank you for taking your time and helping me with this (probably) simpole question!

Upvotes: 1

Views: 1708

Answers (2)

vikingsteve
vikingsteve

Reputation: 40378

The part (Ord a) => is a type constraint, indicating a is an ordinal type (it can be ordered).

You can read max :: (Ord a) => a -> a -> a as:

where a is an ordinal type, "max" takes 2 arguments of type a and returns type a

Upvotes: 4

Chad Gilbert
Chad Gilbert

Reputation: 36375

The => symbol is used to build class constraints on a function. In the max example, it means that all parameters of type a need to implement the Ord typeclass.

Consider the implementation of max

max :: (Ord a) => a -> a -> a
max x y = if x < y then y else x

The only thing we know about x and y is that they have to satisfy the Ord typeclass. That in turn allows us to use the < function for comparison, whose signature is:

(<) :: Ord a => a -> a -> Bool

If you were to omit the Ord a constraint from the definition of max, then it wouldn't compile, because the body of max wouldn't be able to use the comparison function.

Upvotes: 8

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