# Generic number type

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m (Generic numeric type moved to Generic number type) |

## Revision as of 12:18, 20 June 2007

## Contents |

## 1 Problem

Question:

Can I have a generic numeric data type in Haskell which coversInteger

Rational

Double

Answer: In principle you can define a type like

data GenericNumber = Integer Integer | Rational Rational | Double Double

Num

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 usingGenericNumber

that all scripting language users have encountered so far (or ignored :-).

## 2 Solutions

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.

### 2.1 average

You may find it cumbersome to write

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

genericNumberLength

Data.List.genericLength

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

### 2.2 ratios

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 literals1

3

floor

Integral

Rational

%

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

## 3 See also

- Suggestions for implementing a generic number type: http://www.haskell.org/pipermail/haskell-cafe/2007-June/027092.html