[Haskell-cafe] More strange errors when compiling code

16 Mar 2010

      Hi! I am getting more problems with compilation. The two files where the
problem occurs (a bit more of a general case of the sample of a few hours
ago) are:

-------------------------------------------------
--------------------Methods.hs-------------------
-------------------------------------------------
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances,
  UndecidableInstances, FlexibleContexts, EmptyDataDecls,
ScopedTypeVariables,
  TypeOperators, TypeSynonymInstances #-}

module Methods where

  import Records
  import References

  class Converter a rec_a where convert :: a -> rec_a

  data Method s a b = Method (s -> a -> (b,s))

  (<<-) :: forall rec_a s a b c n . (CNum n, HasField n (b->(c,a)) (rec_a),
Converter a rec_a) => (Reference s (rec_a)) -> n -> Method s b c
  (<<-) r n =
    Method(\s -> \(x :: b) ->
              let (v,s') = getter r s :: (rec_a,s)
                  m = v .! n :: (b -> (c,a))
                  (y,v') = m x :: (c, a)
                  v'' = convert v' :: rec_a
                  ((),s'') = setter r s' v''
              in (y,s''))

  (.!!) :: Method s a b -> a -> Reference s b
  (Method m) .!! x = from_constant (Constant (\s -> m s x))

-------------------------------------------------
---------------------Main.hs---------------------
-------------------------------------------------
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances,
  UndecidableInstances, FlexibleContexts, EmptyDataDecls,
ScopedTypeVariables,
  TypeOperators, TypeSynonymInstances #-}

--module Main where

import Records
import References
import ReferenceMonad
import Methods

val = firstLabel
incr = nextLabel val

instance Converter RecCounter (Counter RecCounter) where
  convert (RecCounter r) = r

type Counter k = (Integer :* (() -> ((),k)) :* EmptyRecord)
data RecCounter = RecCounter (Counter RecCounter)

mk_test :: Integer -> RecCounter
mk_test i =
    RecCounter(    val .= i
                .* incr .= (\() -> ((), mk_test (i+1)))
                .* EmptyRecord)

RecCounter test' = mk_test 0

res2 :: Reference (Counter RecCounter) Integer
res2 = do (this <<- incr :: Method (Counter RecCounter) () ()) .!! ()
          v <- (this <-- val)
          return v

count = fst (getter res2 test')

Whenever I try to compile I get the following error:

*Methods> :load Main.hs
[1 of 5] Compiling Records          ( Records.hs, interpreted )
[2 of 5] Compiling References       ( References.hs, interpreted )
[3 of 5] Compiling ReferenceMonad   ( ReferenceMonad.hs, interpreted )
[4 of 5] Compiling Methods          ( Methods.hs, interpreted )
[5 of 5] Compiling Main             ( Main.hs, interpreted )

Main.hs:55:11:
    No instances for (HasField
                        Z (() -> ((), a)) (AddField (() -> ((), RecCounter))
Emp
tyRecord),
                      Converter a (Counter RecCounter))
      arising from a use of `<<-' at Main.hs:55:11-23
    Possible fix:
      add an instance declaration for
      (HasField
         Z (() -> ((), a)) (AddField (() -> ((), RecCounter)) EmptyRecord),
       Converter a (Counter RecCounter))
    In the first argument of `(.!!)', namely
        `(this <<- incr :: Method (Counter RecCounter) () ())'
    In a stmt of a 'do' expression:
          (this <<- incr :: Method (Counter RecCounter) () ()) .!! ()
    In the expression:
        do (this <<- incr :: Method (Counter RecCounter) () ()) .!! ()
         v <- (this <-- val)
         return v
Failed, modules loaded: Methods, References, Records, ReferenceMonad.
*Methods>

Any hints?

PS:  just to give some perspective on the code: I am putting together a
system for expressing mutable objects through monads. I could provide the
rest of my code in case of need :)

-- 
Giuseppe Maggiore
Ph.D. Student (Languages and Games)
Microsoft Student Partner
Mobile: +393319040031
Office: +390412348444