[Haskell-cafe] Re: Polyvariadic functions operating with a monoid

10 Oct 2010

      OK, upon further investigation, the problem is that GHC cannot in
general infer the return type of polyToMonoid despite the hint it is
given (the type signature of the first parameter).

If I write:

main = putStrLn $ show $ unwrap $ ((polyToMonoid [""] True (Just
(5::Int))) :: WMonoid [String])

or

main = putStrLn $ show $ unwrap $ ((polyToMonoid (0::Int) (1::Int)
(2::Int) (3::Int)) :: WMonoid Int)

the code compiles and returns the expected result.

Kevin

On Oct 10, 8:58 am, Kevin Jardine  wrote:
...
And in fact in both cases, it appears that GHC is trying to derive the
*wrong* instances of PolyVariadic.
It should be deriving:
PolyVariadic Int (WMonoid Int)
not
PolyVariadic Int (WMonoid m)
and
PolyVariadic [String] (WMonoid [String])
not
PolyVariadic [String] (WMonoid String)
specifically, GHC is attempting to derive PolyVariadic with the wrong
version of WMonoid in each case.
I'm using GHC 6.12.3
Perhaps the new GHC 7 type system would work better?
Kevin
On Oct 10, 8:26 am, Kevin Jardine  wrote:
...
Hi Brandon,
...
True, when I replace [] with [""], I get a different error message:
...
 No instance for (PolyVariadic [[Char]] (WMonoid String))
...
which now looks a bit like the Int example. In both cases, GHC appears
to be unable to derive the appropriate instance of PolyVariadic. Why
this is so, but worked for Oleg's specific example. is still not clear
to me.
...
Kevin
...
On Oct 9, 11:51 pm, Brandon S Allbery KF8NH 
wrote:
...
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On 10/9/10 10:25 , Kevin Jardine wrote:
...
...
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instance Show a => Monoidable a [String] where
    toMonoid a = [show a]
...
...
...
main = putStrLn $ unwrap $ polyToMonoid [] True () (Just (5::Int))
...
...
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fails to compile.
...
...
...
Why would that be? My understanding is that all lists are
automatically monoids.
...
...
I *think* the problem here is that Oleg specifically pointed out that the
first parameter to polyToMonoid must specify the type of the monoid.  []
tells you it's a list, therefore a monoid, but it doesn't say enough to
allow the [String] instance to be chosen.  (No, the fact that you only
declared an instance for [String] isn't really enough.)
...
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- --
brandon s. allbery     [linux,solaris,freebsd,perl]      allb...@kf8nh.com
system administrator  [openafs,heimdal,too many hats]  allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon university      KF8NH
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