[Haskell-cafe]
Mutual recursive modules with GHC: Classes, instances
Henning Thielemann
lemming at henning-thielemann.de
Wed May 4 05:10:25 EDT 2005
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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