[Haskell-cafe] Mutual recursive modules with GHC: Classes, instances

[Simon Peyton-Jones]

hi-boot files can't contain class declarations.  This should be checked,
but isn't, hence puzzling message.

GHC 6.4 has a better setup.   Actually, it still doesn't check for
absence of class decls, but it will shortly.

Simon

| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org
[mailto:haskell-cafe-bounces at haskell.org] On Behalf Of
| Henning Thielemann
| Sent: 04 May 2005 10:10
| To: Haskell Cafe
| Subject: [Haskell-cafe] Mutual recursive modules with GHC: Classes,
instances
| 
| 
| I tried to set up mutual recursive modules according to section
4.6.10. of
| the manual of GHC-6.2.2. I illustrate my problems with a simple
fictitious
| example.
| 
| ---- A.hi-boot ----
| 
| module A where
| 
| 
| class GHC.Num.Num a => C a where
|     answer :: a
| 
| f :: B.T -> GHC.Num.Int
| 
| 
| ---- A.hs ----
| 
| module A where
| 
| import B
| 
| class Num a => C a where
|     answer :: a
| 
| instance C Int where
|     answer = 42
| 
| f :: B.T -> Int
| f (B.Cons x) = x
| 
| 
| ---- B.hs ----
| 
| module B where
| 
| import {-# SOURCE #-} A
| 
| 
| data T = Cons Int
| 
| newAnswer :: Int
| newAnswer = A.answer + 1
| 
| --------------
| 
| 
| $ ghc --make A.hs B.hs
| Chasing modules from: A.hs,B.hs
| Compiling B                ( B.hs, B.o )
| 
| B.hs:1:
|      Failed to find interface decl for `A.$dmanswer'
|      from module `A'
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