# Possible bug/omission in Numeric library?

**Malcolm Wallace
**
Malcolm.Wallace@cs.york.ac.uk

*Wed, 28 Nov 2001 15:10:47 +0000*

There is something strange about the Haskell'98 Numeric library,
which I think could be considered a bug of sorts. There are functions
readDec, readOct, readHex :: (Integral a) => ReadS a
which read an integer from a string in base 10, 8, or 16, but there
are no corresponding show functions to convert an integer to a string
using base 8 or 16. The sole function given is
showInt :: (Integral a) => a -> ShowS
which shows a number in base 10 only. I think you'll agree that it
is odd that you can read a certain number format but cannot show it?
There are several obvious generalisations of showInt. One might be:
showInt :: (Integral a) => a -> a -> ShowS
showInt base n r
| n < 0 = error "Numeric.showInt: can't show negative numbers"
| otherwise =
let (n',d) = quotRem n base
r' = Char.intToDigit (fromIntegral d) : r
in if n' == 0 then r' else showInt base n' r'
which shows a number in any base up to 16. Another might be:
showInt :: (Integral a) => a -> (Int->Char) -> a -> ShowS
showInt base intToDig n r
| n < 0 = error "Numeric.showInt: can't show negative numbers"
| otherwise =
let (n',d) = quotRem n base
r' = intToDig (fromIntegral d) : r
in if n' == 0 then r' else showInt base intToDig n' r'
which more closely resembles the dual function readInt and could
be likewise accompanied by specific instances for the three most
common bases:
showDec, showOct, showHex :: (Integral a) => a -> ShowS
showDec = showInt 10 Char.intToDigit
showOct = showInt 8 Char.intToDigit
showHex = showInt 16 Char.intToDigit
However, I would guess that changing the type signature of the
current showInt function is unacceptable for Haskell'98. Maybe we
should consider adding the more general version under a new name
like showIntBase, together with show{Dec,Oct,Hex}? This would break
no existing code, and clear up the slight anomaly in the current
capability of the Haskell'98 library.
Regards,
Malcolm