Re: [Haskell-cafe] 1/0

On Mon, Jun 16, 2008 at 05:39:39PM -0700, Don Stewart wrote:

...

decodeFloat really ought to be a partial function, and this should be a crashing bug, if the standard libraries were better-written.

It's a bug in the H98 report then:

Section 6.4.6:

"The function decodeFloat applied to a real floating-point number returns the significand expressed as an Integer and an appropriately scaled exponent (an Int). If decodeFloat x yields (m,n), then x is equal in value to mb^n, where b is the floating-point radix, and furthermore, either m and n are both zero or else b^d-1<=m

Yes, it is a bug in the report, that it doesn't fully specify the behavior this function when given a NaN or infinity. There's also a bug in the standard libraries, which is that they don't conform to the report. let x = 0/0 let (m,n) = decodeFloat x Prelude> (m,n) (-6755399441055744,972) Prelude> let x = 0/0 Prelude> x NaN Prelude> let d = floatDigits x Prelude> let (m,n) = decodeFloat x Prelude> let x' = (fromInteger m :: Double)*2^n Prelude> x' -Infinity Prelude> 2^d-1<=m False Prelude> m<2^d True So for the ghc decodeFloat, when operating on a NaN, the specifications of decodeFloat are almost completely unsatisfied. On the plus side, at least it's true that m