Why constraints on data are a bad thing?

Question

I know this question has been asked and answered lots of times but I still don\'t really understand why putting constraints on a data type is a bad thing.

For example, l

Answer 1

Constraints

The problem is that constraints are not a property of the data type, but of the algorithm/function that operates on them. Different functions might need different and unique constraints.

A Box example

As an example, let's assume we want to create a container called Box which contains only 2 values.

data Box a = Box a a

We want it to:

• be showable
• allow the sorting of the two elements via sort

Does it make sense to apply the constraint of both Ord and Show on the data type? No, because the data type in itself could be only shown or only sorted and therefore the constraints are related to its use, not it's definition.

instance (Show a) => Show (Box a) where
    show (Box a b) = concat ["'", show a, ", ", show b, "'"]

instance (Ord a) => Ord (Box a) where
    compare (Box a b) (Box c d) =
        let ca = compare a c
            cb = compare b d
        in if ca /= EQ then ca else cb

The Data.Map case

Data.Map's Ord constraints on the type is really needed only when we have > 1 elements in the container. Otherwise the container is usable even without an Ord key. For example, this algorithm:

transf :: Map NonOrd Int -> Map NonOrd Int
transf x = 
    if Map.null x
        then Map.singleton NonOrdA 1
        else x

Live demo

works just fine without the Ord constraint and always produce a non empty map.