Template Haskell: Cycle in type synonym declarations

[igloo]

type-level-numbers-0.1 doesn't build with 7.0.1 RC 1. Here's the essence:

{-# LANGUAGE TemplateHaskell #-}
module Q where

data Z

type N0 = $( [t| Z |] )
type N1 = $( [t| Z |] )

$ ghc --make m.hs
[1 of 1] Compiling Q                ( m.hs, m.o )

m.hs:7:1:
    Cycle in type synonym declarations:
      m.hs:7:1-23: type N0 = $([t| Z |])
      m.hs:8:1-23: type N1 = $([t| Z |])

[simonpj]

We won't do this for 7.0; it's not a regression. It'll be part of the TH improvements described in  http://hackage.haskell.org/trac/ghc/blog/Template%20Haskell%20Proposal

[jakewheat]

Hello, I am getting the same error with the following code. This code compiles fine in ghc 6.12.1 - I'm not sure if this is the same bug or not.

ThStuff?.hs:

{-# LANGUAGE TemplateHaskell #-}
module ThStuff (makeRecord) where

import Data.HList
import Language.Haskell.TH

makeRecord :: [(Name,Name)] -> Q Type
makeRecord lvs =
  [t| Record $(ps) |]
  where
    ps = sequence (map mkPair lvs) >>= foldIt
    mkPair (l,v) = [t| LVPair (Proxy $(conT l)) $(conT v) |]

foldIt :: [Type] -> Q Type
foldIt = \tt -> case tt of
                  [] -> [t|HNil|]
                  (t:ts) -> [t| HCons $(return t) $(foldIt ts)|]

Types.hs:

{-# LANGUAGE TemplateHaskell,EmptyDataDecls,DeriveDataTypeable,FlexibleContexts #-}
module Types where

import Data.HList.MakeLabels

$(label "size")

Test.hs:

{-# LANGUAGE TemplateHaskell #-}
import Data.HList
import ThStuff
import Data.HList.Label4 ()
import Types

-- this definition only works in ghc 6.12.1

type TestRow = $(makeRecord [(''Size, ''Int)])

-- using this definition works in both ghc 6.12.1 and ghc 7.0.2

--type TestRow = Record (HCons (LVPair (Proxy Size) Int)
--                             HNil)

main :: IO ()
main = do
  let t :: TestRow
      t = size .=. 1
          .*. emptyRecord
  print t

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.0.2
$ ghc --make Test.hs
[1 of 3] Compiling Types            ( Types.hs, Types.o )
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Loading package pretty-1.0.1.2 ... linking ... done.
Loading package array-0.3.0.2 ... linking ... done.
Loading package containers-0.4.0.0 ... linking ... done.
Loading package template-haskell ... linking ... done.
Loading package HList-0.2.3 ... linking ... done.
[2 of 3] Compiling ThStuff          ( ThStuff.hs, ThStuff.o )
[3 of 3] Compiling Main             ( Test.hs, Test.o )

Test.hs:10:1:
    Cycle in type synonym declarations:
      Test.hs:10:1-46: type TestRow = $(makeRecord [('Size, 'Int)])

I am using the GHC from here:  http://haskell.org/ghc/download_ghc_7_0_2#x86_64linux on Debian testing

[simonpj]

Yes, same problem. The trouble is that type splices currently throw up their hands and conservatively say "I might refer to any other type". Solution is  http://hackage.haskell.org/trac/ghc/blog/Template%20Haskell%20Proposal. Just gotta do it. But several other things are queued up first, alas.

Simon

[simonpj]

No time to do this before 7.4. Sorry