[Haskell-cafe] Template Haskell sometimes sees hidden constructors

[Nicolas Frisby]

A quick follow-up:

1) I had a typo: it should say "N4 is like N1 with a phantom type variable".

2) In my larger code base, the constructor that is visible to TH when
I think it shouldn't be is part of a type that is alpha-equivalent to
N3. It's odd that N3 doesn't exhibit the leakiness here but an
alpha-equivalent type does exhibit it in my larger program.

On Fri, May 27, 2011 at 12:04 PM, Nicolas Frisby
<nicolas.frisby at gmail.com> wrote:
> With the three modules at the end of this email, I get some
> interesting results. Note that none of the constructors are exported,
> yet Template Haskell can see (and splice in variable occurrences of!)
> T, C2, W1, and W4.
>
> If you load Dump into GHCi, you get to see the Info that TH provides
> when you reify each of the data types. For T, T2, N1, and N4, their
> construct is visible in the Info even though M doesn't export it.
>
> As a consequence, you can load Unhide with no errors. Thus c = C, c2 =
> C2, w1 = N1, and w4 = N4, even though those constructors were not
> supposed to be imported.
>
> I couldn't find any mention of this on the GHC Trac for Template
> Haskell or for a general search of "reify".
>
>  * http://j.mp/l9Ztjz (Description contains "reify")
>  * http://j.mp/mprUmq (Component = Template Haskell)
>  * Disclaimer: I didn't take the time to inspect this one
> http://hackage.haskell.org/trac/ghc/ticket/4946
>
> T is isomorphic to (), T2 is like T with a phantom type argument, N1
> is a newtype wrapping an Int, and N4 is like N3 with a phantom type
> variable. This seems too inconsistent to be an intended behavior. Am I
> missing something? Thanks.
>
> ==> M.hs <==
> module M (T(), T1(), T2(), T3(), T4(), N1(), N3(), N4()) where
>
> data T = C
> data T1 = C1 Int
> data T2 a = C2
> data T3 a = C3 a
> data T4 a = C4 Int
> newtype N1 = W1 Int
> newtype N3 a = W3 a
> newtype N4 a = W4 Int
>
> ==> Dump.hs <==
> {-# LANGUAGE TemplateHaskell #-}
>
> module Dump where
>
> import Language.Haskell.TH
> import M
>
> dumpT, dumpT1, dumpT2, dumpT3, dumpT4, dumpN1, dumpN3, dumpN4 :: ()
> dumpT = $(reify ''T >>= fail . show)
> dumpT1 = $(reify ''T1 >>= fail . show)
> dumpT2 = $(reify ''T2 >>= fail . show)
> dumpT3 = $(reify ''T3 >>= fail . show)
> dumpT4 = $(reify ''T4 >>= fail . show)
> dumpN1 = $(reify ''N1 >>= fail . show)
> dumpN3 = $(reify ''N3 >>= fail . show)
> dumpN4 = $(reify ''N4 >>= fail . show)
>
> ==> Unhide.hs <==
> {-# LANGUAGE TemplateHaskell #-}
>
> module Unhide where
>
> import Language.Haskell.TH
> import M
>
> c :: T
> c = $((\(TyConI (DataD _ _ _ [NormalC n _] _)) -> ConE n) `fmap` reify ''T)
> c2 :: T2 a
> c2 = $((\(TyConI (DataD _ _ _ [NormalC n _] _)) -> ConE n) `fmap` reify ''T2)
> w1 :: Int -> N1
> w1 = $((\(TyConI (NewtypeD _ _ _ (NormalC n _) _)) -> ConE n) `fmap` reify ''N1)
> w4 :: Int -> N4 a
> w4 = $((\(TyConI (NewtypeD _ _ _ (NormalC n _) _)) -> ConE n) `fmap` reify ''N4)
>
>
>
> ----- for convenience, this is what I get when I load Dump in ghci
>
> Dump.hs:9:11:
>    TyConI (DataD [] M.T [] [NormalC M.C []] [])
>    In the expression: $(reify 'T >>= fail . show)
>    In an equation for `dumpT': dumpT = $(reify 'T >>= fail . show)
>
> Dump.hs:10:12:
>    TyConI (DataD [] M.T1 [] [] [])
>    In the expression: $(reify 'T1 >>= fail . show)
>    In an equation for `dumpT1': dumpT1 = $(reify 'T1 >>= fail . show)
>
> Dump.hs:11:12:
>    TyConI (DataD [] M.T2 [PlainTV a_1627390697] [NormalC M.C2 []] [])
>    In the expression: $(reify 'T2 >>= fail . show)
>    In an equation for `dumpT2': dumpT2 = $(reify 'T2 >>= fail . show)
>
> Dump.hs:12:12:
>    TyConI (DataD [] M.T3 [PlainTV a_1627390696] [] [])
>    In the expression: $(reify 'T3 >>= fail . show)
>    In an equation for `dumpT3': dumpT3 = $(reify 'T3 >>= fail . show)
>
> Dump.hs:13:12:
>    TyConI (DataD [] M.T4 [PlainTV a_1627390695] [] [])
>    In the expression: $(reify 'T4 >>= fail . show)
>    In an equation for `dumpT4': dumpT4 = $(reify 'T4 >>= fail . show)
>
> Dump.hs:14:12:
>    TyConI (NewtypeD [] M.N1 [] (NormalC M.W1 [(NotStrict,ConT
> GHC.Types.Int)]) [])
>    In the expression: $(reify 'N1 >>= fail . show)
>    In an equation for `dumpN1': dumpN1 = $(reify 'N1 >>= fail . show)
>
> Dump.hs:15:12:
>    TyConI (DataD [] M.N3 [PlainTV a_1627390694] [] [])
>    In the expression: $(reify 'N3 >>= fail . show)
>    In an equation for `dumpN3': dumpN3 = $(reify 'N3 >>= fail . show)
>
> Dump.hs:16:12:
>    TyConI (NewtypeD [] M.N4 [PlainTV a_1627390693] (NormalC M.W4
> [(NotStrict,ConT GHC.Types.Int)]) [])
>    In the expression: $(reify 'N4 >>= fail . show)
>    In an equation for `dumpN4': dumpN4 = $(reify 'N4 >>= fail . show)
> Failed, modules loaded: M.
>