CODEWITHSUNDEEP
haskell
haskellHQ
haskellHQ

Reputation: 1047

Is Num special in this way?

I can create a Num a => a like this:

foo :: Num a => a
foo = 2

Similarly for any other number class:

foo :: Fractional a => a
foo = 2.0

However, I can’t think of a way to create something of type Eq a => a, Ord a => a, or anything non-number (without using undefined).

It seems to me that numbers are special in this way.

Are they?

Upvotes: 0

Views: 91

Answers (3)

Daniel Wagner
Daniel Wagner

Reputation: 153212

There are many more examples:

def :: Default a => a
mempty :: Monoid a => a
maxBound :: Bounded a => a
toEnum 0 :: Enum a => a
read "" :: Read a => a
fromString "" :: IsString a => a

This is just off the top of my head. I'm sure there's many more.

Upvotes: 5

Lazersmoke
Lazersmoke

Reputation: 1741

Num is special in that it has special syntax for writing literals. This syntax uses the function fromInteger :: Num a => Integer -> a internally, which is defined in Num. The compiler parses what you have written into an Integer, and gives that to fromInteger to get the type you see.

The reason you can't do that with for example Eq a => a is there is no function in Eq that returns something of that type.

If you really need a value like that, you can use something like data Equatable = forall e. Eq e => MkEquatable e using the ExistentialQuantification extension, but that is probably not what you want to do.

Here is an example of a type class for which you can create a value with type MyClass a => a: gist

Upvotes: 1

amalloy
amalloy

Reputation: 92117

Num isn't "special" in the sense that it is the only thing that it behaves this way. But it does have some characteristics that make it possible. Consider the Bounded typeclass instead. It is perfectly possible to define a similar function:

top :: Bounded a => a
top = maxBound

This is possible because Bounded, like Num but unlike Eq, provides as part of its class definition a way to create values of its type.

Upvotes: 6

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