[Haskell-cafe] SYB with class: Bug in Derive.hs module

Roman Cheplyaka roma at ro-che.info
Mon Sep 3 12:00:36 CEST 2012

There's a bug in syb-with-class reported by Alexey Rodriguez Yakushev in
2008 [1]. I can confirm that the bug is still there (syb-with-class-,
ghc 7.4.1).

[1]: http://www.haskell.org/pipermail/haskell-cafe/2008-March/041179.html

Here's an even simpler test case:

    {-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses,
        UndecidableInstances, TemplateHaskell, OverlappingInstances,
        DeriveDataTypeable #-}
    import Data.Generics.SYB.WithClass.Basics
    import Data.Generics.SYB.WithClass.Derive

    data Foo = Foo Foo | Bar
      deriving (Typeable, Show)

    deriveData [''Foo]

    f :: (Data NoCtx ast, Typeable ast) => ast -> TypeRep
    f = typeOf

    main = print $ f $ Foo Bar

The cause of this bug is a self-referencing instance created by

    instance (Data ctx Foo, Sat (ctx Foo)) => Data ctx Foo where ...

What's the proper way to fix it?

Roman I. Cheplyaka :: http://ro-che.info/

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