Template Haskell disallows use of promoted data constructor in same splice

[goldfire]

The following code fails to compile under GHC 7.5.20120413:

{-# LANGUAGE TemplateHaskell, DataKinds, PolyKinds #-}

$( [d|
  data Nat = Zero | Succ Nat
  data Proxy a = Proxy
  foo :: Proxy Zero
  foo = Proxy
  |])

The error is:

The exact Name `Zero_a3N3' is not in scope
  Probable cause: you used a unique name (NameU), perhaps via newName,
  in Template Haskell, but did not bind it
  If that's it, then -ddump-splices might be useful

Using -ddump-splices shows that the same internal name is used for the data constructor at declaration and usage.

The problem seems to be that a data constructor declared within a splice cannot be used as a promoted type in that same splice. Separating the splices works.

[simonpj@…]

commit 2a1190431218f838797be3c09084e22dc131c58a

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Wed Apr 25 12:55:41 2012 +0100

    Fix looking up of Exact RdrNames, fixes Trac #6005
    
    See Note [Splicing Exact names] in RnEnv.

 compiler/rename/RnEnv.lhs |   43 ++++++++++++++++++++++++++++++++++---------
 1 files changed, 34 insertions(+), 9 deletions(-)

[simonpj]

Thanks for pointing me to this. Fixed.

[goldfire]

Thanks for applying this fix. Unfortunately, the following case still fails:

$( [d|
  data Nat = Zero | Succ Nat deriving Show
  data Proxy a = Proxy
  foo :: Proxy Zero
  foo = Proxy
  |])

The only difference between this case and the original is the deriving Show.

[simonpj]

This turns out to be unrelated to data kinds etc. Instead it is simply to do with the difficulty of type-checking Template Haskell brackets, something that led me to propose a  complete overhaul (not yet done).

In this case we are generating the derived code for the bracket when we typecheck the bracket. This is silly because it's sure to type check; and it goes wrong too for reasons that are not intersting.

I have brutally simply suppressed all 'deriving' processing inside TH brackets. It'll all become irrelevant with the overhaul.

[simonpj@…]

commit d50cb3eda3912df0d75a8177f04783689b21dc9b

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Thu Apr 26 13:47:54 2012 +0100

    Do not generate derived instances in TH brackets
    
    See Note [Deriving inside TH brackets] in TcInstDcls
    Fixes Trac #6005 (again)

 compiler/typecheck/TcInstDcls.lhs |   23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)