[Bug?] AST representation of type-synonyms with phantom parameters

[Alfonso Acosta]

Hi,

There seems to be a bug in the type representation of type synonyms
with phantom parameters

=== Phantom.hs ===
module Phantom where

-- non-phantom type synonym
type List a = [a]

type PhantomSyn a  = Int

data PhantomData a = PhantomData Int
==================

~/ghc/ghc/compiler/stage2$ ./ghc-inplace -fth --interactive /tmp/Phantom.hs
GHCi, version 6.9.20071025: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Phantom          ( /tmp/Phantom.hs, interpreted )
Ok, modules loaded: Phantom.
*Phantom> let {syn = undefined :: List a; phanSyn = undefined ::
PhantomSyn a; phanData = undefined :: PhantomData a}
*Phantom> :m +Language.Haskell.TH
*Phantom Language.Haskell.TH> let showType name = do {VarI _ t _ _ <-
reify name ; runIO $ putStrLn (pprint t);  [| 1 |]}
Loading package array-0.1 ... linking ... done.
Loading package packedstring-0.1 ... linking ... done.
Loading package containers-0.1 ... linking ... done.
Loading package pretty-1.0 ... linking ... done.
Loading package template-haskell ... linking ... done.
*Phantom Language.Haskell.TH> $(showType (mkName "syn"))
forall a_0 . Phantom.List a_0
forall a_0 . Phantom.List a_0
forall a_0 . Phantom.List a_0
forall a_0 . Phantom.List a_0
1
*Phantom Language.Haskell.TH> $(showType (mkName "phanSyn"))
Phantom.PhantomSyn GHC.Prim.Any
Phantom.PhantomSyn GHC.Prim.Any
Phantom.PhantomSyn GHC.Prim.Any
Phantom.PhantomSyn GHC.Prim.Any
1
*Phantom Language.Haskell.TH> $(showType (mkName "phanData"))
forall a_0 . Phantom.PhantomData a_0
forall a_0 . Phantom.PhantomData a_0
forall a_0 . Phantom.PhantomData a_0
forall a_0 . Phantom.PhantomData a_0
1


The output makes sense for "forall a_0 . Phantom.List a_0" and "forall
a_0 . Phantom.PhantomData a_0" but why "Phantom.PhantomSyn
GHC.Prim.Any"? (just  for information purposes, ghc 6.6 uses "()"
instead of "Any")

Shouldn't it be  "forall a_0 . Phantom.PhantomSyn a_0"?


Best Regards,

Fons


PS: BTW, $(showType 'phanSyn) works fine in ghci6.6 but gives a Stage
error in HEAD. That's why I used

mkName "phanSyn"

Is that another bug? (I couldn't find a related ticket in GHC's trac)
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[Alfonso Acosta]

It seems to be a general GHC-issue (due to the internal representation
of type-synonyms with phantom types)

For example:

*Phantom Language.Haskell.TH> let f = (\_-> 1) :: PhantomSyn a -> Int
*Phantom Language.Haskell.TH> f "a"

<interactive>:1:2:
    Couldn't match expected type `PhantomSyn GHC.Prim.Any'
           against inferred type `[Char]'
      Expected type: PhantomSyn GHC.Prim.Any
      Inferred type: [Char]
    In the first argument of `f', namely `"a"'
    In the expression: f "a"


Pay attention to the error, it says:

PhantomSyn GHC.Prim.Any

instead of

Expected type: PhantomSyn a

Again, should this be considered a bug?

I don't think the end-user should know anything about GHC.Prim.Any.
The error is a bit confusing.

However, I still think that independently of how does GHC represent
type synonyms with phantom parameters, TH should represent their type
variables as such, and not as "GHC.Prim.Any"
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[Ian Lynagh]

On Mon, Oct 29, 2007 at 02:27:37PM +0100, Alfonso Acosta wrote:
>
> Again, should this be considered a bug?

These all look like bugs to me. Please feel free to file them:
http://hackage.haskell.org/trac/ghc/wiki/ReportABug

> I don't think the end-user should know anything about GHC.Prim.Any.

That's correct, as I understand it.


Thanks
Ian

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[Alfonso Acosta]

The bug seems to be a general issue of GHC and I included it as such:

http://hackage.haskell.org/trac/ghc/ticket/1813

I also created a ticket for the lightweight quoting Stage error

http://hackage.haskell.org/trac/ghc/ticket/1814


On 10/29/07, Ian Lynagh <igloo@...> wrote:

> On Mon, Oct 29, 2007 at 02:27:37PM +0100, Alfonso Acosta wrote:
> >
> > Again, should this be considered a bug?
>
> These all look like bugs to me. Please feel free to file them:
> http://hackage.haskell.org/trac/ghc/wiki/ReportABug
>
> > I don't think the end-user should know anything about GHC.Prim.Any.
>
> That's correct, as I understand it.
>
>
> Thanks
> Ian
>
>

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