auto-derivation of Enum is disallowed with recursive imports

[heatsink]

A data type with one single-parameter constructor can normally be declared deriving(Enum) when compiling with -fglasgow-exts. However, when the data type is also declared in a hs-boot file, this is not accepted. The following test case demonstrates the problem. The same thing happens if the 'deriving' clause is also present in the hs-boot file.

{- file X.hs-boot -}
module X where
newtype X = X Int

{- file X.hs -}
module X where
import {-# SOURCE #-} X
newtype X = X Int deriving(Enum)

ghc -c -fglasgow-exts X.hs-boot X.hs

The error occurs in X.hs: Can't make a derived instance of `Enum X' (`X' has non-nullary constructors)

[simonpj]

Are you sure you are using -fglasgow-exts. I have just compiled your example with GHC 6.6, thus

bash-3.1$ ghc -c -fglasgow-exts X.hs-boot X.hs
bash-3.1$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.6
bash-3.1$ 

And all is happy. Without -fglasgow-exts it fails, because in Haskell 98 you can't derive Enum for a newtype.

Can you double check your test case and send and exact log of what happened? Remove all .o and .hi and .hi-boot files first.

Simon

[heatsink]

command line log of compile error

[heatsink]

The log file was generated with the OS X Intel binary distribution of GHC 6.6.

[heatsink]

Correction. The log file was generated with the OS X PPC binary distribution of GHC 6.6.

[igloo]

I can reproduce this with 6.6, HEAD and 6.6-branch on amd64/Linux.

[simonpj]

The error message is certainly misleading. It now says:

    Can't make a derived instance of `Enum X'
      (even with cunning newtype deriving:
       the newtype may be recursive)
    In the newtype declaration for `X'

The problem is this. Generally, the "newtype deriving" mechanism is cautious about recursive newtypes. Consider

newtype A = MkA [A]

We do not want to generate the instance

instance Eq [A] => Eq A

In this case the newtype patently isn't recursive; but in that case why is it being declared in a hs-boot file? Generally GHC treats all hs-boot file types as recursive, because generally it can't see their definitions.

The whole deal about recursive types is an unsatisfactory aspect of GHC. I'm going to treat this one as fixed for 6.6.1 (via the error message improvement). But I'll leave it open at low priority for later versions.

Simon

[morabbin]

This is now fine when compiled with flag -XGeneralizedNewtypeDeriving; close as fixed?

[igloo]

Yes, looks good, thanks. I added a couple of tests.