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    [project @ 2005-07-19 16:44:50 by simonpj] · a7ecdf96
    Simon Peyton Jones authored
    WARNING: this is a big commit.  You might want 
    	to wait a few days before updating, in case I've 
    	broken something.
    	However, if any of the changes are what you wanted,
    	please check it out and test!
    This commit does three main things:
    1. A re-organisation of the way that GHC handles bindings in HsSyn.
       This has been a bit of a mess for quite a while.  The key new
       types are
    	-- Bindings for a let or where clause
    	data HsLocalBinds id
    	  = HsValBinds (HsValBinds id)
    	  | HsIPBinds  (HsIPBinds id)
    	  | EmptyLocalBinds
    	-- Value bindings (not implicit parameters)
    	data HsValBinds id
    	  = ValBindsIn  -- Before typechecking
    		(LHsBinds id) [LSig id]	-- Not dependency analysed
    					-- Recursive by default
    	  | ValBindsOut	-- After typechecking
    		[(RecFlag, LHsBinds id)]-- Dependency analysed
    2. Implement Mark Jones's idea of increasing polymoprhism
       by using type signatures to cut the strongly-connected components
       of a recursive group.  As a consequence, GHC no longer insists
       on the contexts of the type signatures of a recursive group
       being identical.
       This drove a significant change: the renamer no longer does dependency
       analysis.  Instead, it attaches a free-variable set to each binding,
       so that the type checker can do the dep anal.  Reason: the typechecker
       needs to do *two* analyses:
    	one to find the true mutually-recursive groups
    		(which we need so we can build the right CoreSyn)
    	one to find the groups in which to typecheck, taking
    		account of type signatures
    3. Implement non-ground SPECIALISE pragmas, as promised, and as
       requested by Remi and Ross.  Certainly, this should fix the 
       current problem with GHC, namely that if you have
    	g :: Eq a => a -> b -> b
       then you can now specialise thus
    	SPECIALISE g :: Int -> b -> b
        (This didn't use to work.)
       However, it goes further than that.  For example:
    	f :: (Eq a, Ix b) => a -> b -> b
       then you can make a partial specialisation
    	SPECIALISE f :: (Eq a) => a -> Int -> Int
        In principle, you can specialise f to *any* type that is
        "less polymorphic" (in the sense of subsumption) than f's 
        actual type.  Such as
    	SPECIALISE f :: Eq a => [a] -> Int -> Int
        But I haven't tested that.
        I implemented this by doing the specialisation in the typechecker
        and desugarer, rather than leaving around the strange SpecPragmaIds,
        for the specialiser to find.  Indeed, SpecPragmaIds have vanished 
        altogether (hooray).
        Pragmas in general are handled more tidily.  There's a new
        data type HsBinds.Prag, which lives in an AbsBinds, and carries
        pragma info from the typechecker to the desugarer.
    Smaller things
    - The loop in the renamer goes via RnExpr, instead of RnSource.
      (That makes it more like the type checker.)
    - I fixed the thing that was causing 'check_tc' warnings to be