Problems with specialization

[Marcin 'Qrczak' Kowalczyk]

There is something like this:

    class MArray a e m
    
    instance MArray IOArray  e    IO -- Instance with polymorphic elements
    
    instance MArray IOUArray Char IO -- Instances with concrete elements
    instance MArray IOUArray Int  IO -- ...
    instance MArray IOUArray Word IO -- ...
    
    foo :: (MArray a e IO, Ix ix) => a ix e -> ix -> IO e
    
    -- Specializations for ix = Int:
    {-# SPECIALIZE foo ::
        IOArray  Int e    -> Int -> IO e,
        IOUArray Int Char -> Int -> IO Char,
        IOUArray Int Int  -> Int -> IO Int,
        IOUArray Int Word -> Int -> IO Char
        #-}

GHC silently drops the first specialization. Documentation says that
there are no partial specializations: all constrained type variables
must be concretized.

This is a pity, because the IOArray variant will not use the element
type anyway.



Ok, I understand that you can't have everything. So I tried the
following workaround to remove the element type from the constraint:

    foo' :: Ix ix => IOArray ix e -> ix -> IO e
    foo' = foo
    {-# SPECIALIZE foo' :: IOArray Int e -> Int -> IO e #-}

Unfortunately ghc ignores the SPECIALIZE pragma when the definition
is too simple! Actually foo above must be nontrivial too, otherwise
all specializations are ignored.

Copy & paste of the definition just to get specialization would
be silly.

Any ideas?

-- 
 __("<  Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/
 \__/
  ^^                      SYGNATURA ZASTĘPCZA
QRCZAK