[Haskell-cafe] Template Haskell sometimes sees hidden constructors

[Nicolas Frisby]

Whith 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.