ANNOUNCE: GHC 6.8.3 Release Candidate
Serge D. Mechveliani
mechvel at botik.ru
Sun Jun 1 13:34:06 EDT 2008
On Sun, Jun 01, 2008 at 03:34:00PM +0100, Ian Lynagh wrote:
> On Sun, Jun 01, 2008 at 05:39:49PM +0400, Serge D. Mechveliani wrote:
> >
> > This is why res and 1*res are not equivalent in Haskell-98 for
> > res :: Num a => a.
> >
> > Am I missing something?
>
> The library functions assume that class instances obey some unwritten
> laws; it's all a bit vague, but if your instances don't obey them then
> you might find that things go wrong when using library functions. For
> example, if your (*) isn't associative then (^) is going to give odd
> results, and if the type of the second argument to (^) doesn't do
> arithmetic in the normal way then very strange things could happen.
>
> Anyway, I've just tweaked the (^) definition again, so your code should
> work in 6.8.3.
Thank you.
This helps -- in that the public DoCon-2.11 test will run (I hope).
But I think that you have found a bug in the DoCon test program.
Thank you.
More precisely, here are my indeas.
1. Generally, for t = Num a => a, the expessions res :: t and
((fromInteger 1) :: t) * res
are not equivalent in Haskell-98,
and the compiler has not rigth to replace the former with the latter.
Right?
2. But I guess, our current questions are different:
(2.1) For t = Num a => a, has a Haskell implementation for (f^n) :: t
right to base on certain natural properties of the operation
fromInteger :: Integer -> t ?
(2.2) Has a reliable mathematical application right to base on that
for n > 0 the expression (f^n) :: Polynomial Integer
does not imply computing fromInteger 1 :: Polynomial Integer
?
Concerning (2.1): I do not know whether Haskell-98 allows to rely on
that f^3 is equivalent to (fromInteger 1)*(f^3).
But concerning (2.2), I think that (independently of the question (2.1))
a reliable mathematical application must not presume the above properties
of (^) and fromInteger.
Concerning f^3 :: UPol Integer in my test for DoCon:
1) fromInteger _ :: UPol _ is defined as
error "... use fromi instead".
2) (*) is defined via polMul ...
3) The expression f ^ 3 relies on the Haskell-98 library for (^).
The Haskell library defines (^) via (*) -- all right.
4) DoCon has the function `power' to use instead of (^),
and its library avoids (^).
But for n > 0, I considered f^n :: UPol _
as also correct. Because the Haskell library performs this via repeated
application of (*).
And I thought that if n > 0, then (fromInteger _ :: UPol _) will not
appear.
Maybe it does not appear in old GHC-s and does appear 6.8.3 ?
I was using expressions like [(f ^ n) :: UPol Integer | n <- [2 .. 9]]
in my _test programs_ for DoCon.
But this relies on a particular property of the Haskell library definition
for (f^n) :: a
-- on that if n > 0 then (fromInteger _ :: a) does not appear in this
computation.
Now, I think, either I need to hide the standard (^) and overload it
or to replace (^) with `power' in my examples too.
Regards,
-----------------
Serge Mechveliani
mechvel at botik.ru
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