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
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.