showFloat for higher precision types produces strange results for some values

I've written a library which is a quda-double type a la the QD C/C++ package, and showFloat does not behave correctly for numbers with such high precision.

My type has ~212 bits of precision, and when using showFloat from Numeric, I get strange results for integer values:

show (1 :: QDouble) = "0.00000000000000000000000000000000000000000000001e47"
show (1.1 :: QDouble) = "1.1"
show (1000 :: QDouble) = "0.00000000000000000000000000000000000000000000001e50"
-- These seems to suggest it happens for any number with only a
-- few high bits set to 1 in the result of decodeFloat
show (1.125 :: QDouble) = "0.00000000000000000000000000000000000000000000001125e47"
show (1.625 :: QDouble) = "0.00000000000000000000000000000000000000000000001625e47"

The problem seems to be related to the result of floatDigits, which starts causing problem when it's larger than 56 (floatDigits x, show x):

(56,"1.0")
(57,"01.0")
(60,"001.0")
(212,"0.00000000000000000000000000000000000000000000001e47")

My fix has been to use a modified version of showFloat from Numeric by changing the floatToDigits function to include a fix for times when large numbers of zeros are produced:

  fixup2 (xs,k) = let (zs,ys) = span (==0) xs in (ys,k-length zs)
 in
 fixup2 (map fromIntegral (reverse rds), k)

This fixes the symptom but not the issue itself (though it seems like a reasonable thing to have any result returned by floatToDigits.

I have attached as minimal test case as I could come up with. Using floatToDigits from Numeric causes the strange behaviour, while floatToDigits' included in the test case does not.

Trac metadata

Trac field	Value
Version	7.8.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Prelude
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