CODEWITHSUNDEEP
haskell
Alexey Orlov
Alexey Orlov

Reputation: 2804

Haskell: negate a value of type Ordering

Is there a way to negate or invert a value of type Ordering? Something like this:

inv = not compare

Upvotes: 5

Views: 1445

Answers (2)

wiegert
wiegert

Reputation: 11

Yes it is contained in Data.Ord and is a type called Down.

I cite:

"If a has an Ord instance associated with it then comparing two values thus wrapped will give you the opposite of their normal sort order."

Reference: https://hackage.haskell.org/package/base-4.10.1.0/docs/Data-Ord.html

Upvotes: 0

David Fletcher
David Fletcher

Reputation: 2818

If you are getting the Ordering from compare, then you can change it to flip compare:

> flip compare 3 4
GT

Or you can wrap the values in Down from Data.Ord, a newtype that exists to reverse the ordering of things:

> compare (Down 3) (Down 4)
GT

If you can't or don't want to change the place that the Ordering comes from, you'll need a function to invert it and I don't know of a built-in one. You can write the obvious pattern match, or since Ordering is itself ordered:

inv :: Ordering -> Ordering
inv = compare EQ

Upvotes: 16

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