bug in template haskell (with recursion?)

27 Jan 2010

      Hello All,

I'm trying to write a splice to make HList type signatures a bit more
concise, I don't know whether this is a good idea or not.

I wrote a small function to do '[(Type,Type)] -> Q Type' for this, and
got the following from GHC. Should I add this to the bug tracker?

Details:

THBug1.hs:
{-# LANGUAGE TemplateHaskell #-}

module THBug1 where

import Data.HList
import Language.Haskell.TH

mhlt :: [(Type,Type)] -> Q Type
mhlt x = [t| Record $(foldThing x)|]
  where
    foldThing ((f,t):xs) = [t|HCons (LVPair (Proxy $f) $t) $(foldThing xs)|]
    foldThing [] = [t|HNil|]

----

$ ghc -c THBug1.hs

THBug1.hs:12:61:
    GHC internal error: `foldThing' is not in scope during type
checking, but it passed the renamer
    tcg_type_env of environment: []
    In the expression: foldThing xs
    In the Template Haskell quotation
      [t| HCons (LVPair (Proxy $f) $t) $(foldThing xs) |]
    In the expression:
        [t| HCons (LVPair (Proxy $f) $t) $(foldThing xs) |]

I'm using GHC 6.12.1 from debian experimental:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.12.1

$ ghc-pkg list |grep template
    template-haskell-2.4.0.0

To get HList to install with GHC 6.12, I downloaded the latest version from
http://old-darcs.well-typed.com/HList/

and altered it slightly, updated package here:
http://launchpad.net/hssqlppp/prealpha/environment/+download/HList-0.2.tar.g...

the changes I made are:
diff -rN old-HList/Data/HList/GhcSyntax.hs new-HList/Data/HList/GhcSyntax.hs
2c2
< {-# LANGUAGE FlexibleContexts #-}
---
...
{-# LANGUAGE FlexibleContexts, TypeOperators #-}
diff -rN old-HList/Data/HList/Label2.hs new-HList/Data/HList/Label2.hs
1c1,2
< {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses,
UndecidableInstances, EmptyDataDecls #-}

...
{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances, EmptyDataDecls,
  ExistentialQuantification #-}
36c37
< data HNat x => Label x ns desc  -- labels are exclusively type-level entities

...
data Label x ns desc = HNat x => Label x ns desc  -- labels are exclusively type-level entities
diff -rN old-HList/Data/HList/Label3.hs new-HList/Data/HList/Label3.hs
1c1,2
< {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses,
UndecidableInstances, EmptyDataDecls #-}

...
{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances, EmptyDataDecls,
  ExistentialQuantification #-}
37c38
< data HNat x => Label x ns desc  -- labels are exclusively type-level entities

...
data Label x ns desc = HNat x => Label x ns desc  -- labels are exclusively type-level entities
(If it's not obvious, I have no idea what I'm doing..., but my small
amount of code which uses HList worked fine with this altered
package.)

Thanks,
Jake Wheat