[Haskell-beginners] truncate results depend on strict/lazy

[David McBride]

Whenever I have to deal with precision issues I try to remove floats and
doubles from the equation entirely.

> toRational (0.12 :: Float)
16106127 % 134217728
> toRational (0.12 :: Double)
1080863910568919 % 9007199254740992>

truncate $ (toRational (0.12 :: Float)* 100)
11
truncate $ (toRational (0.12 :: Double)* 100)
11

Wrong, but at least it is consistently wrong and should not be interpreted
differently regardless of optimizations.




On Tue, Sep 10, 2013 at 1:14 PM, Bryan Vicknair <bryanvick at gmail.com> wrote:

> On Mon, Sep 09, 2013 at 08:28:48PM -0400, Brandon Allbery wrote:
> > On Mon, Sep 9, 2013 at 8:03 PM, Bryan Vicknair <bryanvick at gmail.com>
> wrote:
> >
> > > Whatever the behavior of truncate is, given the same input it should
> give
> > > the
> > > same output.
> > >
> >
> > An ideal, but unlikely when floating point is involved; optimization can
> > result in altered evaluation order or removal (or even addition in some
> > cases) of load/stores which can modify the internal representation. Note
> > that ghci is completely unoptimized.
>
> Thanks everyone for the help.  I think I'll write my own parsing code
> instead
> of using reads and truncate.  I really don't like floats.  The whole reason
> this parsing code exists is so that the DB can store a simple integer
> instead
> of a float, but I still need to show the value as a float to the users.
>
> This is scary though.  This is the first leak I've found in the referential
> transparency abstraction in Haskell (besides unsafePerformIO).  And the
> problem
> is, if I don't have referential transparency for 100% of pure functions,
> then I
> have to be suspicious of every one.  I can use some heuristics to narrow
> down
> the suspects, such as questioning floating point functions first, but I
> just
> had my bubble burst nonetheless.
>
> Is there any resource that compiles all the potential exceptions to the
> Haskell
> abstract machine?
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