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This patch pushes through one fundamental change: a module is now
identified by the pair of its package and module name, whereas
previously it was identified by its module name alone.  This means
that now a program can contain multiple modules with the same name, as
long as they belong to different packages.

This is a language change - the Haskell report says nothing about
packages, but it is now necessary to understand packages in order to
understand GHC's module system.  For example, a type T from module M
in package P is different from a type T from module M in package Q.
Previously this wasn't an issue because there could only be a single
module M in the program.

The "module restriction" on combining packages has therefore been
lifted, and a program can contain multiple versions of the same
package.

Note that none of the proposed syntax changes have yet been
implemented, but the architecture is geared towards supporting import
declarations qualified by package name, and that is probably the next
step.

It is now necessary to specify the package name when compiling a
package, using the -package-name flag (which has been un-deprecated).
Fortunately Cabal still uses -package-name.

Certain packages are "wired in".  Currently the wired-in packages are:
base, haskell98, template-haskell and rts, and are always referred to
by these versionless names.  Other packages are referred to with full
package IDs (eg. "network-1.0").  This is because the compiler needs
to refer to entities in the wired-in packages, and we didn't want to
bake the version of these packages into the comiler.  It's conceivable
that someone might want to upgrade the base package independently of
GHC.

Internal changes:

  - There are two module-related types:

        ModuleName      just a FastString, the name of a module
        Module          a pair of a PackageId and ModuleName

    A mapping from ModuleName can be a UniqFM, but a mapping from Module
    must be a FiniteMap (we provide it as ModuleEnv).

  - The "HomeModules" type that was passed around the compiler is now
    gone, replaced in most cases by the current package name which is
    contained in DynFlags.  We can tell whether a Module comes from the
    current package by comparing its package name against the current
    package.

  - While I was here, I changed PrintUnqual to be a little more useful:
    it now returns the ModuleName that the identifier should be qualified
    with according to the current scope, rather than its original
    module.  Also, PrintUnqual tells whether to qualify module names with
    package names (currently unused).

Docs to follow.

Most of the other users of the fptools build system have migrated to
Cabal, and with the move to darcs we can now flatten the source tree
without losing history, so here goes.

The main change is that the ghc/ subdir is gone, and most of what it
contained is now at the top level.  The build system now makes no
pretense at being multi-project, it is just the GHC build system.

No doubt this will break many things, and there will be a period of
instability while we fix the dependencies.  A straightforward build
should work, but I haven't yet fixed binary/source distributions.
Changes to the Building Guide will follow, too.

We must record the type of a TuplePat after typechecking, just like a ConPatOut,
so that desugaring works correctly for GADTs. See comments with the declaration
of HsPat.TuplePat, and test gadt15

This very large commit adds impredicativity to GHC, plus
numerous other small things.
  
*** WARNING: I have compiled all the libraries, and
***	     a stage-2 compiler, and everything seems
***	     fine.  But don't grab this patch if you 
***	     can't tolerate a hiccup if something is
***	     broken.
  
The big picture is this:

a) GHC handles impredicative polymorphism, as described in the
   "Boxy types: type inference for higher-rank types and
   impredicativity" paper

b) GHC handles GADTs in the new simplified (and very sligtly less
   epxrssive) way described in the
   "Simple unification-based type inference for GADTs" paper

  
But there are lots of smaller changes, and since it was pre-Darcs
they are not individually recorded.
  
Some things to watch out for:
  
c)   The story on lexically-scoped type variables has changed, as per
     my email.  I append the story below for completeness, but I 
     am still not happy with it, and it may change again.  In particular,
     the new story does not allow a pattern-bound scoped type variable
     to be wobbly, so (\(x::[a]) -> ...) is usually rejected.  This is
     more restrictive than before, and we might loosen up again.
  
d)   A consequence of adding impredicativity is that GHC is a bit less
     gung ho about converting automatically between
  	(ty1 -> forall a. ty2)    and    (forall a. ty1 -> ty2)
     In particular, you may need to eta-expand some functions to make
     typechecking work again.
   
     Furthermore, functions are now invariant in their argument types,
     rather than being contravariant.  Again, the main consequence is
     that you may occasionally need to eta-expand function arguments when
     using higher-rank polymorphism.
  

Please test, and let me know of any hiccups


Scoped type variables in GHC
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	January 2006

0) Terminology.
   
   A *pattern binding* is of the form
	pat = rhs

   A *function binding* is of the form
	f pat1 .. patn = rhs

   A binding of the formm
	var = rhs
   is treated as a (degenerate) *function binding*.


   A *declaration type signature* is a separate type signature for a
   let-bound or where-bound variable:
	f :: Int -> Int

   A *pattern type signature* is a signature in a pattern: 
	\(x::a) -> x
	f (x::a) = x

   A *result type signature* is a signature on the result of a
   function definition:
	f :: forall a. [a] -> a
	head (x:xs) :: a = x

   The form
	x :: a = rhs
   is treated as a (degnerate) function binding with a result
   type signature, not as a pattern binding.

1) The main invariants:

     A) A lexically-scoped type variable always names a (rigid)
 	type variable (not an arbitrary type).  THIS IS A CHANGE.
        Previously, a scoped type variable named an arbitrary *type*.

     B) A type signature always describes a rigid type (since
	its free (scoped) type variables name rigid type variables).
	This is also a change, a consequence of (A).

     C) Distinct lexically-scoped type variables name distinct
	rigid type variables.  This choice is open; 

2) Scoping

2(a) If a declaration type signature has an explicit forall, those type
   variables are brought into scope in the right hand side of the 
   corresponding binding (plus, for function bindings, the patterns on
   the LHS).  
	f :: forall a. a -> [a]
	f (x::a) = [x :: a, x]
   Both occurences of 'a' in the second line are bound by 
   the 'forall a' in the first line

   A declaration type signature *without* an explicit top-level forall
   is implicitly quantified over all the type variables that are
   mentioned in the type but not already in scope.  GHC's current
   rule is that this implicit quantification does *not* bring into scope
   any new scoped type variables.
	f :: a -> a
	f x = ...('a' is not in scope here)...
   This gives compatibility with Haskell 98

2(b) A pattern type signature implicitly brings into scope any type
   variables mentioned in the type that are not already into scope.
   These are called *pattern-bound type variables*.
	g :: a -> a -> [a]
	g (x::a) (y::a) = [y :: a, x]
   The pattern type signature (x::a) brings 'a' into scope.
   The 'a' in the pattern (y::a) is bound, as is the occurrence on 
   the RHS.  

   A pattern type siganture is the only way you can bring existentials 
   into scope.
	data T where
	  MkT :: forall a. a -> (a->Int) -> T

	f x = case x of
		MkT (x::a) f -> f (x::a)

2a) QUESTION
	class C a where
	  op :: forall b. b->a->a

	instance C (T p q) where
	  op = <rhs>
    Clearly p,q are in scope in <rhs>, but is 'b'?  Not at the moment.
    Nor can you add a type signature for op in the instance decl.
    You'd have to say this:
	instance C (T p q) where
	  op = let op' :: forall b. ...
	           op' = <rhs>
	       in op'

3) A pattern-bound type variable is allowed only if the pattern's
   expected type is rigid.  Otherwise we don't know exactly *which*
   skolem the scoped type variable should be bound to, and that means
   we can't do GADT refinement.  This is invariant (A), and it is a 
   big change from the current situation.

	f (x::a) = x	-- NO; pattern type is wobbly
	
	g1 :: b -> b
	g1 (x::b) = x	-- YES, because the pattern type is rigid

	g2 :: b -> b
	g2 (x::c) = x	-- YES, same reason

	h :: forall b. b -> b
	h (x::b) = x	-- YES, but the inner b is bound

	k :: forall b. b -> b
	k (x::c) = x	-- NO, it can't be both b and c

3a) You cannot give different names for the same type variable in the same scope
    (Invariant (C)):

	f1 :: p -> p -> p		-- NO; because 'a' and 'b' would be
	f1 (x::a) (y::b) = (x::a)	--     bound to the same type variable

	f2 :: p -> p -> p		-- OK; 'a' is bound to the type variable
	f2 (x::a) (y::a) = (x::a)	--     over which f2 is quantified
					-- NB: 'p' is not lexically scoped

	f3 :: forall p. p -> p -> p	-- NO: 'p' is now scoped, and is bound to
	f3 (x::a) (y::a) = (x::a)	--     to the same type varialble as 'a'

	f4 :: forall p. p -> p -> p	-- OK: 'p' is now scoped, and its occurences
	f4 (x::p) (y::p) = (x::p)	--     in the patterns are bound by the forall


3b) You can give a different name to the same type variable in different
    disjoint scopes, just as you can (if you want) give diferent names to 
    the same value parameter

	g :: a -> Bool -> Maybe a
	g (x::p) True  = Just x  :: Maybe p
	g (y::q) False = Nothing :: Maybe q

3c) Scoped type variables respect alpha renaming. For example, 
    function f2 from (3a) above could also be written:
	f2' :: p -> p -> p
	f2' (x::b) (y::b) = x::b
   where the scoped type variable is called 'b' instead of 'a'.


4) Result type signatures obey the same rules as pattern types signatures.
   In particular, they can bind a type variable only if the result type is rigid

	f x :: a = x	-- NO

	g :: b -> b
	g x :: b = x	-- YES; binds b in rhs

5) A *pattern type signature* in a *pattern binding* cannot bind a 
   scoped type variable

	(x::a, y) = ...		-- Legal only if 'a' is already in scope

   Reason: in type checking, the "expected type" of the LHS pattern is
   always wobbly, so we can't bind a rigid type variable.  (The exception
   would be for an existential type variable, but existentials are not
   allowed in pattern bindings either.)
 
   Even this is illegal
	f :: forall a. a -> a
	f x = let ((y::b)::a, z) = ... 
	      in 
   Here it looks as if 'b' might get a rigid binding; but you can't bind
   it to the same skolem as a.

6) Explicitly-forall'd type variables in the *declaration type signature(s)*
   for a *pattern binding* do not scope AT ALL.

	x :: forall a. a->a	  -- NO; the forall a does 
	Just (x::a->a) = Just id  --     not scope at all

	y :: forall a. a->a
	Just y = Just (id :: a->a)  -- NO; same reason

   THIS IS A CHANGE, but one I bet that very few people will notice.
   Here's why:

	strange :: forall b. (b->b,b->b)
	strange = (id,id)

	x1 :: forall a. a->a
	y1 :: forall b. b->b
	(x1,y1) = strange

    This is legal Haskell 98 (modulo the forall). If both 'a' and 'b'
    both scoped over the RHS, they'd get unified and so cannot stand
    for distinct type variables. One could *imagine* allowing this:
   
	x2 :: forall a. a->a
	y2 :: forall a. a->a
	(x2,y2) = strange

    using the very same type variable 'a' in both signatures, so that
    a single 'a' scopes over the RHS.  That seems defensible, but odd,
    because though there are two type signatures, they introduce just
    *one* scoped type variable, a.

7) Possible extension.  We might consider allowing
	\(x :: [ _ ]) -> <expr>
    where "_" is a wild card, to mean "x has type list of something", without
    naming the something.

Add support for UTF-8 source files

GHC finally has support for full Unicode in source files.  Source
files are now assumed to be UTF-8 encoded, and the full range of
Unicode characters can be used, with classifications recognised using
the implementation from Data.Char.  This incedentally means that only
the stage2 compiler will recognise Unicode in source files, because I
was too lazy to port the unicode classifier code into libcompat.

Additionally, the following synonyms for keywords are now recognised:

  forall symbol 	(U+2200)	forall
  right arrow   	(U+2192)	->
  left arrow   		(U+2190)	<-
  horizontal ellipsis 	(U+22EF)	..

there are probably more things we could add here.

This will break some source files if Latin-1 characters are being used.
In most cases this should result in a UTF-8 decoding error.  Later on
if we want to support more encodings (perhaps with a pragma to specify
the encoding), I plan to do it by recoding into UTF-8 before parsing.

Internally, there were some pretty big changes:

  - FastStrings are now stored in UTF-8

  - Z-encoding has been moved right to the back end.  Previously we
    used to Z-encode every identifier on the way in for simplicity,
    and only decode when we needed to show something to the user.
    Instead, we now keep every string in its UTF-8 encoding, and
    Z-encode right before printing it out.  To avoid Z-encoding the
    same string multiple times, the Z-encoding is cached inside the
    FastString the first time it is requested.

    This speeds up the compiler - I've measured some definite
    improvement in parsing at least, and I expect compilations overall
    to be faster too.  It also cleans up a lot of cruft from the
    OccName interface.  Z-encoding is nicely hidden inside the
    Outputable instance for Names & OccNames now.

  - StringBuffers are UTF-8 too, and are now represented as
    ForeignPtrs.

  - I've put together some test cases, not by any means exhaustive,
    but there are some interesting UTF-8 decoding error cases that
    aren't obvious.  Also, take a look at unicode001.hs for a demo.

Marginally improve the error message on a failure in DsMeta

Add a new pragma: SPECIALISE INLINE

This amounts to adding an INLINE pragma to the specialised version
of the function.  You can add phase stuff too (SPECIALISE INLINE [2]),
and NOINLINE instead of INLINE.

The reason for doing this is to support inlining of type-directed
recursive functions.  The main example is this:

  -- non-uniform array type
  data Arr e where
    ArrInt  :: !Int -> ByteArray#       -> Arr Int
    ArrPair :: !Int -> Arr e1 -> Arr e2 -> Arr (e1, e2)

  (!:) :: Arr e -> Int -> e
  {-# SPECIALISE INLINE (!:) :: Arr Int -> Int -> Int #-}
  {-# SPECIALISE INLINE (!:) :: Arr (a, b) -> Int -> (a, b) #-}
  ArrInt  _ ba    !: (I# i) = I# (indexIntArray# ba i)
  ArrPair _ a1 a2 !: i      = (a1 !: i, a2 !: i)

If we use (!:) at a particular array type, we want to inline (:!),
which is recursive, until all the type specialisation is done.


On the way I did a bit of renaming and tidying of the way that
pragmas are carried, so quite a lot of files are touched in a
fairly trivial way.

Buglets in GADT record-syntax stuff, which killed the weekend builds

Wibble; fixes TH failures

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 
  emitted.

Fix TH handling of guards

Wibbles arising from HsSyn changes

This commit combines three overlapping things:

1.  Make rebindable syntax work for do-notation. The idea
    here is that, in particular, (>>=) can have a type that
    has class constraints on its argument types, e.g.
       (>>=) :: (Foo m, Baz a) => m a -> (a -> m b) -> m b
    The consequence is that a BindStmt and ExprStmt must have
    individual evidence attached -- previously it was one
    batch of evidence for the entire Do
    
    Sadly, we can't do this for MDo, because we use bind at
    a polymorphic type (to tie the knot), so we still use one
    blob of evidence (now in the HsStmtContext) for MDo.
    
    For arrow syntax, the evidence is in the HsCmd.
    
    For list comprehensions, it's all built-in anyway.
    
    So the evidence on a BindStmt is only used for ordinary
    do-notation.

2.  Tidy up HsSyn.  In particular:

	- Eliminate a few "Out" forms, which we can manage
	without (e.g. 

	- It ought to be the case that the type checker only
	decorates the syntax tree, but doesn't change one
	construct into another.  That wasn't true for NPat,
	LitPat, NPlusKPat, so I've fixed that.

	- Eliminate ResultStmts from Stmt.  They always had
	to be the last Stmt, which led to awkward pattern
	matching in some places; and the benefits didn't seem
	to outweigh the costs.  Now each construct that uses
	[Stmt] has a result expression too (e.g. GRHS).


3.  Make 'deriving( Ix )' generate a binding for unsafeIndex,
    rather than for index.  This is loads more efficient.

    (This item only affects TcGenDeriv, but some of point (2)
    also affects TcGenDeriv, so it has to be in one commit.)

Tweaks to get the GHC sources through Haddock.  Doesn't quite work
yet, because Haddock complains about the recursive modules.  Haddock
needs to understand SOURCE imports (it can probably just ignore them
as a first attempt).

---------------------------------
          Template Haskell: names again
  	---------------------------------

On 2 Dec 04 I made this commit (1.58 in Convert.lhs)

    Fix a Template Haskell bug that meant that top-level names created
    with newName were not made properly unique.

But that just introduced a new bug!  THe trouble is that names created by
newName are NameUs; but I was *also* using NameU for names of free varaibles,
such as the 'x' in the quoted code here
	f x = $( g [| \y -> (x,y) |])

But when converting to HsSyn, the x and y must be treated diffferently.
The 'x' must convert to an Exact RdrName, so that it binds to the 'x' that's
in the type environment; but the 'y' must generate a nice unique RdrName.

So this commit adds NameL for the lexically-scoped bindings like 'x'.

Further integration with the new package story.  GHC now supports
pretty much everything in the package proposal.

  - GHC now works in terms of PackageIds (<pkg>-<version>) rather than
    just package names.  You can still specify package names without
    versions on the command line, as long as the name is unambiguous.

  - GHC understands hidden/exposed modules in a package, and will refuse
    to import a hidden module.  Also, the hidden/eposed status of packages
    is taken into account.

  - I had to remove the old package syntax from ghc-pkg, backwards
    compatibility isn't really practical.

  - All the package.conf.in files have been rewritten in the new syntax,
    and contain a complete list of modules in the package.  I've set all
    the versions to 1.0 for now - please check your package(s) and fix the
    version number & other info appropriately.

  - New options:

	-hide-package P    sets the expose flag on package P to False
	-ignore-package P  unregisters P for this compilation

	For comparison, -package P sets the expose flag on package P
        to True, and also causes P to be linked in eagerly.

        -package-name is no longer officially supported.  Unofficially, it's
	a synonym for -ignore-package, which has more or less the same effect
	as -package-name used to.

	Note that a package may be hidden and yet still be linked into
	the program, by virtue of being a dependency of some other package.
	To completely remove a package from the compiler's internal database,
        use -ignore-package.

	The compiler will complain if any two packages in the
        transitive closure of exposed packages contain the same
        module.

	You *must* use -ignore-package P when compiling modules for
        package P, if package P (or an older version of P) is already
        registered.  The compiler will helpfully complain if you don't.
	The fptools build system does this.

   - Note: the Cabal library won't work yet.  It still thinks GHC uses
     the old package config syntax.

Internal changes/cleanups:

   - The ModuleName type has gone away.  Modules are now just (a
     newtype of) FastStrings, and don't contain any package information.
     All the package-related knowledge is in DynFlags, which is passed
     down to where it is needed.

   - DynFlags manipulation has been cleaned up somewhat: there are no
     global variables holding DynFlags any more, instead the DynFlags
     are passed around properly.

   - There are a few less global variables in GHC.  Lots more are
     scheduled for removal.

   - -i is now a dynamic flag, as are all the package-related flags (but
     using them in {-# OPTIONS #-} is Officially Not Recommended).

   - make -j now appears to work under fptools/libraries/.  Probably
     wouldn't take much to get it working for a whole build.

Implement FunDeps for TH.

Implement TH ForallC constructor.

------------------------------------
	Simplify the treatment of newtypes
	Complete hi-boot file consistency checking
	------------------------------------

In the representation of types, newtypes used to have a special constructor
all to themselves, very like TyConApp, called NewTcApp.    The trouble is
that means we have to *know* when a newtype is a newtype, and in an hi-boot
context we may not -- the data type might be declared as
	data T
in the hi-boot file, but as
	newtype T = ...
in the source file.  In GHCi, which accumulates stuff from multiple compiles,
this makes a difference.

So I've nuked NewTcApp.  Newtypes are represented using TyConApps again. This
turned out to reduce the total amount of code, and simplify the Type data type,
which is all to the good.


This commit also fixes a few things in the hi-boot consistency checking
stuff.

------------------------------------
	Add Generalised Algebraic Data Types
	------------------------------------

This rather big commit adds support for GADTs.  For example,

    data Term a where
 	  Lit :: Int -> Term Int
	  App :: Term (a->b) -> Term a -> Term b
	  If  :: Term Bool -> Term a -> Term a
	  ..etc..

    eval :: Term a -> a
    eval (Lit i) = i
    eval (App a b) = eval a (eval b)
    eval (If p q r) | eval p    = eval q
    		    | otherwise = eval r


Lots and lots of of related changes throughout the compiler to make
this fit nicely.

One important change, only loosely related to GADTs, is that skolem
constants in the typechecker are genuinely immutable and constant, so
we often get better error messages from the type checker.  See
TcType.TcTyVarDetails.

There's a new module types/Unify.lhs, which has purely-functional
unification and matching for Type. This is used both in the typechecker
(for type refinement of GADTs) and in Core Lint (also for type refinement).

Template Haskell improvements

a) Make '() and '[] work.
b) Add tupleTypeName, tupleDataName
b) Try to improve error message for (lack of) existential data constructors in TH

-------------------------------
	Fix a grevious bug in DsMeta
	which caused a seg fault
	-------------------------------

The bug was an incorrectly declared type for one of the Template
Haskell construction functions in DsMeta (repRecCon, repRecUpd)
and some associated jiggery pokery.

-dcore-lint showed it up nicely, because the desugarer generated
ill-typed code.

DsMeta PrelNames TH.Lib

Add missing functions to TH export list (mostly spotted by Duncan Coutts).

Update TH test output.

Add TH support for patterns with type signatures, and test for same
(requested by Isaac Jones).

Add TH support for pattern guards, and tests for same
(requested by Isaac Jones).

Add infix patterns to TH datatypes.

Added Lift instances for 2- to 7-tuples (requested by Duncan Coutts).

Buglet in desugaring TH syntax

Remove the entirely-redundant location from the argument of 
constructor HsPredTy,
    so that we have
	HsPredTy HsType
    rather than
	HsPredTy LHsType

Wibble to last commit

Extend the "newtype deriving" feature a little bit more
(at the request of Wolfgang Jeltsch)

Here's the example:
    class C a b
    instance C [a] Char
    newtype T = T Char deriving( C [a] )

Perfectly sensible, and no reason it should not work.
Fixing this required me to generalise the abstract syntax of
a 'deriving' item, hence the non-local effects.

Add support for foreign imports inside quasi-quotes.
Gave TH a few more uniques to play with and fixed a typo.

------------------------
	More newtype clearing up
	------------------------

* Change the representation of TyCons so that it accurately reflects
	* data     (0 or more constrs)
	* newtype  (1 constr)
	* abstract (unknown)
  Replaces DataConDetails and AlgTyConFlavour with AlgTyConRhs

* Add IfaceSyn.IfaceConDecls, a kind of stripped-down analogue
  of AlgTyConRhs

* Move NewOrData from BasicTypes to HsDecl (it's now an HsSyn thing)

* Arrange that Type.newTypeRep and splitRecNewType_maybe unwrap just
  one layer of new-type-ness, leaving the caller to recurse.

  This still leaves typeRep and repType in Type.lhs; these functions
  are still vaguely disturbing and probably should get some attention.

Lots of knock-on changes.  Fixes bug in ds054.

Split Template Haskell out to its own package and update docs and tests.

Buglet in desugaring instance declarations

Split the pretty-printer out - pprint will now give you pretty much
anything you are likely to want to pretty-print as a String. For a Doc
you need (to_HPJ_Doc . ppr), which could be made nicer if it is widely
used.

Also took the opportunity to do a bit of module renaming and fixed the
odd typo here and there.

--------------------
	Towards type splices
	--------------------

Starts the move to supporting type splices, by making
HsExpr.HsSplice a separate type of its own, and adding
HsSpliceTy constructor to HsType.

Add accurate source location annotations to HsSyn
-------------------------------------------------

Every syntactic entity in HsSyn is now annotated with a SrcSpan, which
details the exact beginning and end points of that entity in the
original source file.  All honest compilers should do this, and it was
about time GHC did the right thing.

The most obvious benefit is that we now have much more accurate error
messages; when running GHC inside emacs for example, the cursor will
jump to the exact location of an error, not just a line somewhere
nearby.  We haven't put a huge amount of effort into making sure all
the error messages are accurate yet, so there could be some tweaking
still needed, although the majority of messages I've seen have been
spot-on.

Error messages now contain a column number in addition to the line
number, eg.

   read001.hs:25:10: Variable not in scope: `+#'

To get the full text span info, use the new option -ferror-spans.  eg.

   read001.hs:25:10-11: Variable not in scope: `+#'

I'm not sure whether we should do this by default.  Emacs won't
understand the new error format, for one thing.

In a more elaborate editor setting (eg. Visual Studio), we can arrange
to actually highlight the subexpression containing an error.  Eventually
this information will be used so we can find elements in the abstract
syntax corresponding to text locations, for performing high-level editor
functions (eg. "tell me the type of this expression I just highlighted").

Performance of the compiler doesn't seem to be adversely affected.
Parsing is still quicker than in 6.0.1, for example.

Implementation:

This was an excrutiatingly painful change to make: both Simon P.J. and
myself have been working on it for the last three weeks or so.  The
basic changes are:

 - a new datatype SrcSpan, which represents a beginning and end position
   in a source file.

 - To reduce the pain as much as possible, we also defined:

      data Located e = L SrcSpan e

 - Every datatype in HsSyn has an equivalent Located version.  eg.

      type LHsExpr id = Located (HsExpr id)

   and pretty much everywhere we used to use HsExpr we now use
   LHsExpr.  Believe me, we thought about this long and hard, and
   all the other options were worse :-)


Additional changes/cleanups we made at the same time:

  - The abstract syntax for bindings is now less arcane.  MonoBinds
    and HsBinds with their built-in list constructors have gone away,
    replaced by HsBindGroup and HsBind (see HsSyn/HsBinds.lhs).

  - The various HsSyn type synonyms have now gone away (eg. RdrNameHsExpr,
    RenamedHsExpr, and TypecheckedHsExpr are now HsExpr RdrName,
    HsExpr Name, and HsExpr Id respectively).

  - Utilities over HsSyn are now collected in a new module HsUtils.
    More stuff still needs to be moved in here.

  - MachChar now has a real Char instead of an Int.  All GHC versions that
    can compile GHC now support 32-bit Chars, so this was a simplification.

------------------------------------
	Major increment for Template Haskell
	------------------------------------

1.  New abstract data type "Name" which appears where String used to be.
    E.g. 	data Exp = VarE Name | ...

2.  New syntax 'x and ''T, for quoting Names.  It's rather like [| x |]
    and [t| T |] respectively, except that

	a) it's non-monadic:  'x :: Name
	b) you get a Name not an Exp or Type

3.  reify is an ordinary function
	reify :: Name -> Q Info
    New data type Info which tells what TH knows about Name

4.  Local variables work properly.  So this works now (crashed before):
	f x = $( [| x |] )

5.  THSyntax is split up into three modules:

  Language.Haskell.TH		TH "clients" import this

  Language.Haskell.TH.THSyntax	data type declarations and internal stuff

  Language.Haskell.TH.THLib	Support library code (all re-exported
				by TH), including smart constructors and
				pretty printer

6.  Error reporting and recovery are in (not yet well tested)

	report :: Bool {- True <=> fatal -} -> String -> Q ()
	recover :: Q a -> Q a -> Q a

7.  Can find current module

	currentModule :: Q String


Much other cleaning up, needless to say.

Beginnings of VarBr

1.  When desugaring a [d| ... |], do not gensym the method names of
    an instance declaration, else we get very confused!

2.  We don't need to make fresh Names in DsMeta.mkGenSym, because
    the existing Name will do, I believe.  We'll do the genSym when
    we *run* the splice.  (This stuff makes my head hurt.)

    Assuming this is so, we can simplify the mkGenSym stuff still more.

Updating TH; not finished

Wibble for HsForAllTy

-------------------------
		GHC heart/lung transplant
		-------------------------

This major commit changes the way that GHC deals with importing
types and functions defined in other modules, during renaming and
typechecking.  On the way I've changed or cleaned up numerous other
things, including many that I probably fail to mention here.

Major benefit: GHC should suck in many fewer interface files when
compiling (esp with -O).  (You can see this with -ddump-rn-stats.)

It's also some 1500 lines of code shorter than before.

**	So expect bugs!  I can do a 3-stage bootstrap, and run
**	the test suite, but you may be doing stuff I havn't tested.
** 	Don't update if you are relying on a working HEAD.


In particular, (a) External Core and (b) GHCi are very little tested.

	But please, please DO test this version!


	------------------------
		Big things
	------------------------

Interface files, version control, and importing declarations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* There is a totally new data type for stuff that lives in interface files:
	Original names			IfaceType.IfaceExtName
	Types				IfaceType.IfaceType
	Declarations (type,class,id)	IfaceSyn.IfaceDecl
	Unfoldings			IfaceSyn.IfaceExpr
  (Previously we used HsSyn for type/class decls, and UfExpr for unfoldings.)
  The new data types are in iface/IfaceType and iface/IfaceSyn.  They are
  all instances of Binary, so they can be written into interface files.
  Previous engronkulation concering the binary instance of RdrName has
  gone away -- RdrName is not an instance of Binary any more.  Nor does
  Binary.lhs need to know about the ``current module'' which it used to,
  which made it specialised to GHC.

  A good feature of this is that the type checker for source code doesn't
  need to worry about the possibility that we might be typechecking interface
  file stuff.  Nor does it need to do renaming; we can typecheck direct from
  IfaceSyn, saving a whole pass (module TcIface)

* Stuff from interface files is sucked in *lazily*, rather than being eagerly
  sucked in by the renamer. Instead, we use unsafeInterleaveIO to capture
  a thunk for the unfolding of an imported function (say).  If that unfolding
  is every pulled on, TcIface will scramble over the unfolding, which may
  in turn pull in the interface files of things mentioned in the unfolding.

  The External Package State is held in a mutable variable so that it
  can be side-effected by this lazy-sucking-in process (which may happen
  way later, e.g. when the simplifier runs).   In effect, the EPS is a kind
  of lazy memo table, filled in as we suck things in.  Or you could think
  of it as a global symbol table, populated on demand.

* This lazy sucking is very cool, but it can lead to truly awful bugs. The
  intent is that updates to the symbol table happen atomically, but very bad
  things happen if you read the variable for the table, and then force a
  thunk which updates the table.  Updates can get lost that way. I regret
  this subtlety.

  One example of the way it showed up is that the top level of TidyPgm
  (which updates the global name cache) to be much more disciplined about
  those updates, since TidyPgm may itself force thunks which allocate new
  names.

* Version numbering in interface files has changed completely, fixing
  one major bug with ghc --make.  Previously, the version of A.f changed
  only if A.f's type and unfolding was textually different.  That missed
  changes to things that A.f's unfolding mentions; which was fixed by
  eagerly sucking in all of those things, and listing them in the module's
  usage list.  But that didn't work with --make, because they might have
  been already sucked in.

  Now, A.f's version changes if anything reachable from A.f (via interface
  files) changes.  A module with unchanged source code needs recompiling
  only if the versions of any of its free variables changes. [This isn't
  quite right for dictionary functions and rules, which aren't mentioned
  explicitly in the source.  There are extensive comments in module MkIface,
  where all version-handling stuff is done.]

* We don't need equality on HsDecls any more (because they aren't used in
  interface files).  Instead we have a specialised equality for IfaceSyn
  (eqIfDecl etc), which uses IfaceEq instead of Bool as its result type.
  See notes in IfaceSyn.

* The horrid bit of the renamer that tried to predict what instance decls
  would be needed has gone entirely.  Instead, the type checker simply
  sucks in whatever instance decls it needs, when it needs them.  Easy!

  Similarly, no need for 'implicitModuleFVs' and 'implicitTemplateHaskellFVs'
  etc.  Hooray!


Types and type checking
~~~~~~~~~~~~~~~~~~~~~~~
* Kind-checking of types is far far tidier (new module TcHsTypes replaces
  the badly-named TcMonoType).  Strangely, this was one of my
  original goals, because the kind check for types is the Right Place to
  do type splicing, but it just didn't fit there before.

* There's a new representation for newtypes in TypeRep.lhs.  Previously
  they were represented using "SourceTypes" which was a funny compromise.
  Now they have their own constructor in the Type datatype.  SourceType
  has turned back into PredType, which is what it used to be.

* Instance decl overlap checking done lazily.  Consider
	instance C Int b
	instance C a Int
  These were rejected before as overlapping, because when seeking
  (C Int Int) one couldn't tell which to use.  But there's no problem when
  seeking (C Bool Int); it can only be the second.

  So instead of checking for overlap when adding a new instance declaration,
  we check for overlap when looking up an Inst.  If we find more than one
  matching instance, we see if any of the candidates dominates the others
  (in the sense of being a substitution instance of all the others);
  and only if not do we report an error.



	------------------------
	     Medium things
	------------------------

* The TcRn monad is generalised a bit further.  It's now based on utils/IOEnv.lhs,
  the IO monad with an environment.  The desugarer uses the monad too,
  so that anything it needs can get faulted in nicely.

* Reduce the number of wired-in things; in particular Word and Integer
  are no longer wired in.  The latter required HsLit.HsInteger to get a
  Type argument.  The 'derivable type classes' data types (:+:, :*: etc)
  are not wired in any more either (see stuff about derivable type classes
  below).

* The PersistentComilerState is now held in a mutable variable
  in the HscEnv.  Previously (a) it was passed to and then returned by
  many top-level functions, which was painful; (b) it was invariably
  accompanied by the HscEnv.  This change tidies up top-level plumbing
  without changing anything important.

* Derivable type classes are treated much more like 'deriving' clauses.
  Previously, the Ids for the to/from functions lived inside the TyCon,
  but now the TyCon simply records their existence (with a simple boolean).
  Anyone who wants to use them must look them up in the environment.

  This in turn makes it easy to generate the to/from functions (done
  in types/Generics) using HsSyn (like TcGenDeriv for ordinary derivings)
  instead of CoreSyn, which in turn means that (a) we don't have to figure
  out all the type arguments etc; and (b) it'll be type-checked for us.
  Generally, the task of generating the code has become easier, which is
  good for Manuel, who wants to make it more sophisticated.

* A Name now says what its "parent" is. For example, the parent of a data
  constructor is its type constructor; the parent of a class op is its
  class.  This relationship corresponds exactly to the Avail data type;
  there may be other places we can exploit it.  (I made the change so that
  version comparison in interface files would be a bit easier; but in
  fact it tided up other things here and there (see calls to
  Name.nameParent).  For example, the declaration pool, of declararations
  read from interface files, but not yet used, is now keyed only by the 'main'
  name of the declaration, not the subordinate names.

* New types OccEnv and OccSet, with the usual operations.
  OccNames can be efficiently compared, because they have uniques, thanks
  to the hashing implementation of FastStrings.

* The GlobalRdrEnv is now keyed by OccName rather than RdrName.  Not only
  does this halve the size of the env (because we don't need both qualified
  and unqualified versions in the env), but it's also more efficient because
  we can use a UniqFM instead of a FiniteMap.

  Consequential changes to Provenance, which has moved to RdrName.

* External Core remains a bit of a hack, as it was before, done with a mixture
  of HsDecls (so that recursiveness and argument variance is still inferred),
  and IfaceExprs (for value declarations).  It's not thoroughly tested.


	------------------------
	     Minor things
	------------------------

* DataCon fields dcWorkId, dcWrapId combined into a single field
  dcIds, that is explicit about whether the data con is a newtype or not.
  MkId.mkDataConWorkId and mkDataConWrapId are similarly combined into
  MkId.mkDataConIds

* Choosing the boxing strategy is done for *source* type decls only, and
  hence is now in TcTyDecls, not DataCon.

* WiredIn names are distinguished by their n_sort field, not by their location,
  which was rather strange

* Define Maybes.mapCatMaybes :: (a -> Maybe b) -> [a] -> [b]
  and use it here and there

* Much better pretty-printing of interface files (--show-iface)

Many, many other small things.


	------------------------
	     File changes
	------------------------
* New iface/ subdirectory
* Much of RnEnv has moved to iface/IfaceEnv
* MkIface and BinIface have moved from main/ to iface/
* types/Variance has been absorbed into typecheck/TcTyDecls
* RnHiFiles and RnIfaces have vanished entirely.  Their
  work is done by iface/LoadIface
* hsSyn/HsCore has gone, replaced by iface/IfaceSyn
* typecheck/TcIfaceSig has gone, replaced by iface/TcIface
* typecheck/TcMonoType has been renamed to typecheck/TcHsType
* basicTypes/Var.hi-boot and basicTypes/Generics.hi-boot have gone altogether