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Generic number type

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1 Problem

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
GenericNumber
s you will encounter exactly the problems

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

A
GenericNumber
type would also negate the type safety that strongly typed numbers provide, putting the burden back on the programmer to make sure they are using numbers in a type-safe way. This can lead to subtle and hard-to-find bugs, for example, if some code ends up comparing two floating-point values for equality (usually a bad idea) without the programmer realizing it.

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 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

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 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

2.3 isSquare

It may seem irksome that
fromIntegral
is required in the function 
isSquare :: (Integral a) => a -> Bool
isSquare n = (floor . sqrt $ fromIntegral n) ^ 2 == n
With a
GenericNumber
type, one could instead write 
isSquare :: GenericNumber -> Bool
isSquare n = (floor . sqrt $ n) ^ 2 == n
but there is a subtle problem here: if the input happens to be represented internally by a non-integral type, this function will probably not work properly. This could be fixed by wrapping all occurrences of
n
by calls to
round
, but that's no easier (and less type-safe) than just including the call to
fromIntegral
in the first place. The point is that by using
GenericNumber
here, all opportunities for the type checker to warn you of problems is lost; now you, the programmer, must ensure that the underlying numeric types are always used correctly, which is made even harder by the fact that you can't inspect them.

3 See also

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Categories: FAQ | Mathematics | Idioms