Is the Haskell type class 'Real' a misnomer?

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Category:Languages

I am trying to understand the numeric type class hierarchy in Haskell. The basic numeric type is

class Num a where ... 

As a side note, according to some of my sources (slides), it should actually be

class Eq a => Num a where ... 

but I cannot find this in the Prelude.

Now, I am interested in the Real type class

class (Num a, Ord a) => Real a where   -- the rational equivalent of its real argument with full precision   toRational :: a -> Rational 

I guess that Real refers to the fact that types that are instances of Real are not complex. But my understanding of a Real number from Mathematics is that it can be Rational and Irrational, so there is no equivalent of toRational for all of them. Of course, irrational numbers can't be used in computers anyways...

Thanks!

 


Yes, the name Real is a rather misleading name for the class of types that can be converted to a rational.

Indeed, we have instances like Real Integer, Real IntPtr, Real CBool which can be very surprising.

The standard numeric classes are generally regarded as being a bit weird, both in their names and in their overall design.

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