CODEWITHSUNDEEP
haskellhackage
Boris
Boris

Reputation: 5176

Source code for standard typeclass instance declarations

I was wondering about the Ord instance declaration for (a,b), and I wanted to do a quick lookup on hackage to confirm my intuition that the comparison is first on a and then, in case of equality, on b. Specifically I went here. Since hackage has links to the source code for data declarations and functions, I assumed that there would also be source code for instance declarations, but I can't find them. Is there a reason why they are not there, or have I just not looked hard enough? type Answer = Either Explanation Directions :)

Upvotes: 6

Views: 269

Answers (3)

Chris Kuklewicz
Chris Kuklewicz

Reputation: 8153

The Haskell 98 Report specifies this in section 10.1:

The class methods automatically introduced by derived instances of Eq and Ord are (==), (/=), compare, (<), (<=), (>), (>=), max, and min. The latter seven operators are defined so as to compare their arguments lexicographically with respect to the constructor set given, with earlier constructors in the datatype declaration counting as smaller than later ones.

Derived comparisons always traverse constructors from left to right.

...

All derived operations of class Eq and Ord are strict in both arguments.

Upvotes: 4

Don Stewart
Don Stewart

Reputation: 137937

The Ord instance for tuples is derived, according to the rules from the language specification, which goes back as far as Gofer.

instance (Eq a, Eq b) => Eq (a,b) where
    (x,y) == (u,v)  =  x==u && y==v

instance (Ord a, Ord b) => Ord (a,b) where
    (x,y) <= (u,v)  = x<u  ||  (x==u && y<=v)

Upvotes: 4

dave4420
dave4420

Reputation: 47042

I went looking in the Prelude, clicked on the source link for the Ord typeclass, scrolled down a little, and found that it's defined as

deriving instance (Ord a, Ord b) => Ord (a, b)

It's using the StandaloneDeriving extension. Basically it's generating the same code as if the type was defined as

data (a, b) = (a, b) deriving Ord

Upvotes: 4

Related Questions