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

10 Oct 2010

      For example, the notation can be reduced to:

poly([String],True () (Just (5::Int)))

using:

#define poly(TYPE,VALUES) ((polyToMonoid (mempty :: TYPE) VALUES) ::
TYPE)

which I think is as concise as it can get.

Kevin

On Oct 10, 1:47 pm, Kevin Jardine  wrote:
...
It is interesting to see that the dummy parameters can actually be
replaced by:
mempty :: [String]
mempty :: String
mempty: Int
in my three examples and the code still compiles and gives the
expected results.
This suggests that a further simplification might be possible (ideally
in straight Haskell, but if not then with CPP or Template Haskell).
Kevin
On Oct 10, 1:28 pm, Kevin Jardine  wrote:
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For anyone who's interested, the code I have now is:
...
{-# LANGUAGE TypeSynonymInstances, FlexibleInstances,
MultiParamTypeClasses #-}
module PolyTest where
...
import Data.Monoid
...
class Monoid m => Monoidable a m where
    toMonoid :: a -> m
...
squish :: Monoidable a m => m -> a -> m
squish m a = (m `mappend` (toMonoid a))
...
class Monoid m => PolyVariadic m r where
    polyToMonoid :: m -> r
...
instance Monoid m => PolyVariadic m m where
    polyToMonoid acc = acc
...
instance (Monoidable a m, PolyVariadic m r) => PolyVariadic m (a->r)
where
    polyToMonoid acc = \a -> polyToMonoid (squish acc a)
...
and three example uses are:
...
-- [String] example
instance Show a => Monoidable a [String] where
    toMonoid a = [show a]
...
testStringList = putStrLn $ show $ ((polyToMonoid [""] True () (Just
(5::Int))) :: [String])
...
-- String example
instance Show a => Monoidable a String where
    toMonoid a = show a
...
testString = putStrLn $ ((polyToMonoid "" True () (Just (5::Int))) ::
String)
...
-- sum example
...
instance Monoid Int where
    mappend = (+)
    mempty = 0
...
instance Monoidable Int Int where
    toMonoid = id
...
testSum = putStrLn $ show $ ((polyToMonoid (0::Int) (1::Int) (2::Int)
(3::Int)) :: Int)
...
main = do
    testStringList
    testString
    testSum
...
$ runhaskell PolyTest.hs
["","True","()","Just 5"]
True()Just 5
6
...
This removes the unwrap and I don't mind the need for the outer type
cast.
...
I do wonder if there is a need for the first (dummy) parameter to
communicate the type as well as this seems redundant given the outer
type cast but I can't find a way to remove it.
...
It appears that GHC needs to be told the type both coming and going so
to speak for this to work consistently.
...
Any suggestions for improvement welcome!
...
Kevin
...
On Oct 10, 11:12 am, Kevin Jardine  wrote:
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...
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:
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...
...
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:
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...
...
...
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.
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> Why would that be? My understanding is that all lists are
> automatically monoids.
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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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