Generic number type

From HaskellWiki
Revision as of 12:20, 20 June 2007 by Lemming (talk | contribs) (link to Converting numbers)

Jump to: navigation, search


Question: Can I have a generic numeric data type in Haskell which covers Integer, Rational, Double and so on, like it is done in scripting languages like Perl and MatLab?

Answer: In principle you can define a type like

data GenericNumber =
    Integer Integer
  | Rational Rational
  | Double Double

and define appropriate instances for Num class et. al. However you will find that it is difficult to implement these methods in a way that is appropriate for each use case. There is simply no type that can emulate the others. Floating point numbers are imprecise - a/b*b=a does not hold in general. Rationals are precise but pi and sqrt 2 are not rational. That is, when using GenericNumbers you will encounter exactly the problems that all scripting language users have encountered so far (or ignored :-).


It is strongly advised to carefully check whether a GenericNumber is indeed useful for your application. So let's revisit some examples and their idiomatic solutions in plain Haskell 98.


You may find it cumbersome to manually convert integers to fractional number types like in

average :: Fractional a => [a] -> a
average xs = sum xs / fromIntegral (length xs)

and you may prefer

average :: [GenericNumber] -> GenericNumber
average xs = sum xs / genericNumberLength xs

with an appropriate implementation of genericNumberLength. However, there is already Data.List.genericLength and you can write

average :: Fractional a => [a] -> a
average xs = sum xs / genericlength xs


You find it easy to write

1 / 3 :: Rational

but uncomfortable that

1 / floor pi :: Rational

does not work. The first example works, because the numeric literals 1 and 3 are interpreted as rationals itself. The second example fails, because floor always returns an Integral number type, where Rational is not an instance. You should use % instead. This constructs a fraction out of two integers:

1 % 3 :: Rational
1 % floor pi :: Rational

See also