• simonpj's avatar
    [project @ 2001-12-28 17:25:31 by simonpj] · ae969b47
    simonpj authored
    ---------------------
    	Dealing with deriving
    	---------------------
    
    I spent a ridiculously long time peering at a bug report whereby
    a 'deriving' clause sent GHC 5.02.1 into a loop.  It was all to
    do with allowing instances like
    
    	instance Foo a b => Baz (T a)
    
    (Notice the 'b' on the left which does not appear on the right.)
    
    I realised that it's hard for the deriving machinery to find a
    fixpoint when these sort of instance decls are around.  So I
    now constrain *derived* instance decls not to have this form;
    all the tyvars on the left must appear on the right.
    
    On the way I commoned up the previously-separate tcSimplify
    machinery for 'deriving' and 'default' decls with that for
    everything else.   As a result, quite a few files are touched.
    
    I hope I havn't broken anything.
    ae969b47
TcSimplify.lhs 64.5 KB