TH, tuples and "impossible happened" problem

[Václav Haisman]

Hi,
I have a little problem with Template Haskell. I have been trying make a
template function that would allow me to read any element of tuple
without explicit knowledge of how wide the tuple is. I have done some
code but recently I have been stopped by GHC's "impossible happened":


Compiler output:

Chasing modules from: MyTHUse.hs
Compiling MyTH             ( ./MyTH.hs, ./MyTH.o )
Compiling Main             ( MyTHUse.hs, MyTHUse.o )
Loading package base-1.0 ... linking ... done.
Loading package haskell98-1.0 ... linking ... done.
Loading package template-haskell-1.0 ... linking ... done.
ghc: panic! (the `impossible' happened, GHC version 6.4.1):
        tyThingDataCon Type constructor `DataziTuple.Z3T{(w) tc 49}'

Please report it as a compiler bug to glasgow-haskell-bugs at haskell.org,
or http://sourceforge.net/projects/ghc/.

--------------------------------------------------------------------------
MyTH.hs:

module MyTH where

import Language.Haskell.TH
import Language.Haskell.TH.Syntax
import Language.Haskell.TH.Ppr

-- TI data_constructor param_count
data TupleInfo = TI Name Int

--
getTupleInfo :: Name -> Q TupleInfo
getTupleInfo var_name =
    do
    info <- reify var_name
    case info
         of
         vi@(VarI _ typ _ _) ->
             case typ
                  of
                  appt@(AppT _ _) -> return $ traverseAppT 0 appt
                  _ -> error "Not a tuple-like type."
         _ -> error "Not a variable."
    where
    traverseAppT n (AppT t1 _) = traverseAppT (n+1) t1
    traverseAppT n (ConT name) = TI name (n+1)

--
tupleGet :: Name -> Int -> Q Exp
tupleGet var_name tuple_index =
    do
    (TI constr_name width) <- getTupleInfo var_name
    param_name <- newName "tuple_elem_"
    sub_patterns <- genPattern 1 width tuple_index param_name
    lambda <-
        lamE [conP constr_name (map return sub_patterns)]
                 (varE param_name)
    return lambda
    where
    genPattern counter n param_index param_name
               | counter == (n+1) = return []
               | counter == param_index =
                   do
                   p <- varP param_name
                   rest <- genPattern (counter+1) n param_index param_name
                   return (p:rest)
               | otherwise =
                   do
                   wp <- wildP
                   rest <- genPattern (counter+1) n param_index param_name
                   return (wp:rest)

------------------------------------------------------------------
MyTHUse.hs:

import MyTH
import System.IO
import Language.Haskell.TH
import Language.Haskell.TH.Syntax
import Data.Tuple

x = (1,2,3)

get1Of3 = $(tupleGet 'x 1)


--
Vaclav Haisman


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