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<b^d, where d is the value of > floatDigits x. encodeFloat performs the inverse of this transformation. > "
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<b^d, so that's one out of three. I suppose the problem is that quickcheck was developed only after the standard was written... David _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
