This has been a long-standing ToDo.

This changes the internal representation of TickBoxes,
from
        Note (TickBox "module" n)  <expr>
into

        case tick<module,n> of
          _ -> <expr>

tick has type :: #State #World, when the module and tick numbe
are stored inside IdInfo.

Binary tick boxes change from

         Note (BinaryTickBox "module" t f) <expr>

into

          btick<module,t,f> <expr>

btick has type :: Bool -> Bool, with the module and tick number
stored inside IdInfo.

This patch is a start on removing import lists and generally tidying
up the top of each module.  In addition to removing import lists:

   - Change DATA.IOREF -> Data.IORef etc.
   - Change List -> Data.List etc.
   - Remove $Id$
   - Update copyrights
   - Re-order imports to put non-GHC imports last
   - Remove some unused and duplicate imports

This large commit combines several interrelated changes:

  - IfaceSyn now contains actual Names rather than the special
    IfaceExtName type.  The binary interface file contains
    a symbol table of Names, where each entry is a (package,
    ModuleName, OccName) triple.  Names in the IfaceSyn point
    to entries in the symbol table.

    This reduces the size of interface files, which should
    hopefully improve performance (not measured yet).

    The toIfaceXXX functions now do not need to pass around
    a function from Name -> IfaceExtName, which makes that
    code simpler.

  - Names now do not point directly to their parents, and the
    nameParent operation has gone away.  It turned out to be hard to
    keep this information consistent in practice, and the parent info
    was only valid in some Names.  Instead we made the following
    changes:

    * ImportAvails contains a new field 
          imp_parent :: NameEnv AvailInfo
      which gives the family info for any Name in scope, and
      is used by the renamer when renaming export lists, amongst
      other things.  This info is thrown away after renaming.

    * The mi_ver_fn field of ModIface now maps to
      (OccName,Version) instead of just Version, where the
      OccName is the parent name.  This mapping is used when
      constructing the usage info for dependent modules.
      There may be entries in mi_ver_fn for things that are not in
      scope, whereas imp_parent only deals with in-scope things.

    * The md_exports field of ModDetails now contains
      [AvailInfo] rather than NameSet.  This gives us
      family info for the exported names of a module.

Also:

   - ifaceDeclSubBinders moved to IfaceSyn (seems like the
     right place for it).

   - heavily refactored renaming of import/export lists.

   - Unfortunately external core is now broken, as it relied on
     IfaceSyn.  It requires some attention.

Tue Sep 12 16:57:32 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
  * Type tags in import/export lists
  - To write something like GMapKey(type GMap, empty, lookup, insert)
  - Requires -findexed-types

I measured that this makes the comiler allocate a bit more, but it
might also make it faster and reduce residency.  The extra allocation
is probably just because we're not inlining enough somewhere, so I
think this change is a step in the right direction.

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.

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.

Comments

This big commit does several things at once (aeroplane hacking)
which change the format of interface files.  

	So you'll need to recompile your libraries!

1. The "stupid theta" of a newtype declaration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Retain the "stupid theta" in a newtype declaration.
For some reason this was being discarded, and putting it
back in meant changing TyCon and IfaceSyn slightly.
   

2. Overlap flags travel with the instance
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Arrange that the ability to support overlap and incoherence
is a property of the *instance declaration* rather than the
module that imports the instance decl.  This allows a library
writer to define overlapping instance decls without the
library client having to know.  

The implementation is that in an Instance we store the
overlap flag, and preseve that across interface files


3. Nuke the "instnce pool" and "rule pool"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A major tidy-up and simplification of the way that instances
and rules are sucked in from interface files.  Up till now
an instance decl has been held in a "pool" until its "gates" 
(a set of Names) are in play, when the instance is typechecked
and added to the InstEnv in the ExternalPackageState.  
This is complicated and error-prone; it's easy to suck in 
too few (and miss an instance) or too many (and thereby be
forced to suck in its type constructors, etc).

Now, as we load an instance from an interface files, we 
put it straight in the InstEnv... but the Instance we put in
the InstEnv has some Names (the "rough-match" names) that 
can be used on lookup to say "this Instance can't match".
The detailed dfun is only read lazily, and the rough-match
thing meansn it is'nt poked on until it has a chance of
being needed.

This simply continues the successful idea for Ids, whereby
they are loaded straightaway into the TypeEnv, but their
TyThing is a lazy thunk, not poked on until the thing is looked
up.

Just the same idea applies to Rules.

On the way, I made CoreRule and Instance into full-blown records
with lots of info, with the same kind of key status as TyCon or 
DataCon or Class.  And got rid of IdCoreRule altogether.   
It's all much more solid and uniform, but it meant touching
a *lot* of modules.


4. Allow instance decls in hs-boot files
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allowing instance decls in hs-boot files is jolly useful, becuase
in a big mutually-recursive bunch of data types, you want to give
the instances with the data type declarations.  To achieve this

* The hs-boot file makes a provisional name for the dict-fun, something
  like $fx9.

* When checking the "mother module", we check that the instance
  declarations line up (by type) and generate bindings for the 
  boot dfuns, such as
	$fx9 = $f2
  where $f2 is the dfun generated by the mother module

* In doing this I decided that it's cleaner to have DFunIds get their
  final External Name at birth.  To do that they need a stable OccName,
  so I have an integer-valued dfun-name-supply in the TcM monad.
  That keeps it simple.

This feature is hardly tested yet.


5. Tidy up tidying, and Iface file generation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main/TidyPgm now has two entry points:

  simpleTidyPgm is for hi-boot files, when typechecking only
  (not yet implemented), and potentially when compiling without -O.
  It ignores the bindings, and generates a nice small TypeEnv.

  optTidyPgm is the normal case: compiling with -O.  It generates a
  TypeEnv rich in IdInfo

MkIface.mkIface now only generates a ModIface.  A separate
procedure, MkIface.writeIfaceFile, writes the file out to disk.

Fix something that's been bugging me for a while: by default, -ddump-*
output doesn't include uniques when it outputs internal names, but in
most cases you need them because the output hasn't been tidied, so you
end up doing -dppr-debug which is overkill.

Now, -ddump-* prints uniques for internal names by default.  This
shouldn't affect anything else.

---------------------------------------------
Type signatures are no longer instantiated with skolem constants
	---------------------------------------------

	Merge to STABLE

Consider

  p :: a
  q :: b
  (p,q,r) = (r,r,p)

Here, 'a' and 'b' end up being the same, because they are both bound
to the type for 'r', which is just a meta type variable.  So 'a' and 'b'
can't be skolems.

Sigh.  This commit goes back to an earlier way of doing things, by
arranging that type signatures get instantiated with *meta* type
variables; then at the end we must check that they have not been
unified with types, nor with each other.

This is a real bore.  I had to do quite a bit of related fiddling around
to make error messages come out right.  Improved one or two.

Also a small unrelated fix to make
	:i (:+)
print with parens in ghci.  Sorry this got mixed up in the same commit.

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.

------------------------------------
	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).

Try to fix up the previous commit.  I'm not sure if this is quite
right, but it should be closer.

Oops: always qualify when printing code

-------------------------------
	Print built-in sytax right
	-------------------------------

Built-in syntax, like (:) and [], is not "in scope" via the GlobalRdrEnv
in the usual way.  When we print it out, we should also print it in unqualified
form, even though it's not in the environment.

I've finally bitten the (not very big) bullet, and added to Name the information
about whether or not a name is one of these built-in ones.  That entailed changing
the calls to mkWiredInName, but those are exactly the places where you have to
decide whether it's built-in or not, which is fine.


Built-in syntax => It's a syntactic form, not "in scope" (e.g. [])

Wired-in thing  => The thing (Id, TyCon) is fully known to the compiler,
		   not read from an interface file.
		   E.g. Bool, True, Int, Float, and many others

All built-in syntax is for wired-in things.

-------------------------------
	Add instance information to :i
 	Get rid of the DeclPool
	-------------------------------


1.  Add instance information to :info command.  GHCi now prints out
    which instances a type or class belongs to, when you use :i

2.  Tidy up printing of unqualified names in user output.
    Previously Outputable.PrintUnqualified was
	type PrintUnqualified = Name -> Bool
    but it's now
	type PrintUnqualified = ModuleName -> OccName -> Bool
    This turns out to be tidier even for Names, and it's now also usable
    when printing IfaceSyn stuff in GHCi, eliminating a grevious hack.

3.  On the way to doing this, Simon M had the great idea that we could
    get rid of the DeclPool holding pen, which held declarations read from
    interface files but not yet type-checked.   We do this by eagerly
    populating the TypeEnv with thunks what, when poked, do the type
    checking.   This is just a logical continuation of lazy import
    mechanism we've now had for some while.

The InstPool and RulePool still exist, but I plan to get rid of them in
the same way.  The new scheme does mean that more rules get sucked in than
before, because previously the TypeEnv was used to mean "this thing was needed"
and hence to control which rules were sucked in.  But now the TypeEnv is
populated more eagerly => more rules get sucked in.  However this problem
will go away when I get rid of the Inst and Rule pools.

I should have kept these changes separate, but I didn't.  Change (1)
affects mainly
	TcRnDriver, HscMain, CompMan, InteractiveUI
whereas change (3) is more wide ranging.

Merge backend-hacking-branch onto HEAD.  Yay!

----------------------------
        Re-do kind inference (again)
	----------------------------

   [WARNING: interface file binary representation has
   (as usual) changed slightly; recompile your libraries!]

Inspired by the lambda-cube, for some time GHC has used
	type Kind = Type
That is, kinds were represented by the same data type as types.

But GHC also supports unboxed types and unboxed tuples, and these
complicate the kind system by requiring a sub-kind relationship.
Notably, an unboxed tuple is acceptable as the *result* of a
function but not as an *argument*.  So we have the following setup:

		 ?
		/ \
	       /   \
	      ??   (#)
	     /  \
            *   #

where	*    [LiftedTypeKind]   means a lifted type
	#    [UnliftedTypeKind] means an unlifted type
	(#)  [UbxTupleKind]     means unboxed tuple
	??   [ArgTypeKind]      is the lub of *,#
	?    [OpenTypeKind]	means any type at all

In particular:

  error :: forall a:?. String -> a
  (->)  :: ?? -> ? -> *
  (\(x::t) -> ...)	Here t::?? (i.e. not unboxed tuple)

All this has beome rather difficult to accommodate with Kind=Type, so this
commit splits the two.

  * Kind is a distinct type, defined in types/Kind.lhs

  * IfaceType.IfaceKind disappears: we just re-use Kind.Kind

  * TcUnify.unifyKind is a distinct unifier for kinds

  * TyCon no longer needs KindCon and SuperKindCon variants

  * TcUnify.zapExpectedType takes an expected Kind now, so that
    in TcPat.tcMonoPatBndr we can express that the bound variable
    must have an argTypeKind (??).

The big change is really that kind inference is much more systematic and
well behaved.  In particular, a kind variable can unify only with a
"simple kind", which is built from * and (->).  This deals neatly
with awkward questions about how we can combine sub-kinding with type
inference.

Lots of small consequential changes, especially to the kind-checking
plumbing in TcTyClsDecls.  (We played a bit fast and loose before, and
now we have to be more honest, in particular about how kind inference
works for type synonyms.  They can have kinds like (* -> #), so

This cures two long-standing SourceForge bugs

* 753777 (tcfail115.hs), which used erroneously to pass,
  but crashed in the code generator
      type T a = Int -> (# Int, Int #)
      f :: T a -> T a
      f t = \x -> case t x of r -> r

* 753780 (tc167.hs), which used erroneously to fail
      f :: (->) Int# Int#


Still, the result is not entirely satisfactory.  In particular

* The error message from tcfail115 is pretty obscure

* SourceForge bug 807249 (Instance match failure on openTypeKind)
  is not fixed.  Alas.

------------------------------------
	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.

This commit does a long-overdue tidy-up

* Remove PprType (gets rid of one more bunch of hi-boot files)

* Put pretty-printing for types in TypeRep

* Make a specialised pretty-printer for Types, rather than
  converting to IfaceTypes and printing those

-------------------------
		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

-------------------------------------
  Big upheaval to the way that constructors are named
	-------------------------------------

This commit enshrines the new story for constructor names.  We could never
really get External Core to work nicely before, but now it does.

The story is laid out in detail in the Commentary
	ghc/docs/comm/the-beast/data-types.html
so I will not repeat it here.

	[Manuel: the commentary isn't being updated, apparently.]

However, the net effect is that in Core and in External Core, contructors look
like constructors, and the way things are printed is all consistent.

It is a fairly pervasive change (which is why it has been so long postponed),
but I hope the question is now finally closed.

All the libraries compile etc, and I've run many tests, but doubtless there will
be some dark corners.

Track locations for binders better (fixes several rn and mod failures)

Import wibbles

------------------------------------------
	1. New try and module and package dependencies
	2. OrigNameCache always contains final info
	------------------------------------------

These things nearly complete sorting out the incremental
linking problem that started us off!

1. This commit separates two kinds of information:

  (a) HscTypes.Dependencies:
	What (i)  home-package modules, and
	     (ii) other packages
      this module depends on, transitively.

      That is, to link the module, it should be enough
      to link the dependent modules and packages (plus
      any C stubs etc).

      Along with this info we record whether the dependent module
      is (a) a boot interface or (b) an orphan module.  So in
      fact (i) can contain non-home-package modules, namely the
      orphan ones in other packages (sigh).

  (b) HscTypes.Usage:
      What version of imported things were used to
      actually compile the module.  This info is used for
      recompilation control only.



2. The Finder now returns a correct Module (incl package indicator)
first time, so we can install the absolutely final Name in the
OrigNameCache when we first come across an occurrence of that name,
even if it's only an occurrence in an unfolding in some other interface
file.  This is much tidier.

As a result Module.lhs is much cleaner
	No DunnoYet
	No mkVanillaModule
ALl very joyful stuff.

--------------------------------------
	Make Template Haskell into the HEAD
	--------------------------------------

This massive commit transfers to the HEAD all the stuff that
Simon and Tim have been doing on Template Haskell.  The
meta-haskell-branch is no more!

WARNING: make sure that you

  * Update your links if you are using link trees.
    Some modules have been added, some have gone away.

  * Do 'make clean' in all library trees.
    The interface file format has changed, and you can
    get strange panics (sadly) if GHC tries to read old interface files:
    e.g.  ghc-5.05: panic! (the `impossible' happened, GHC version 5.05):
	  Binary.get(TyClDecl): ForeignType

  * You need to recompile the rts too; Linker.c has changed


However the libraries are almost unaltered; just a tiny change in
Base, and to the exports in Prelude.


NOTE: so far as TH itself is concerned, expression splices work
fine, but declaration splices are not complete.


		---------------
		The main change
		---------------

The main structural change: renaming and typechecking have to be
interleaved, because we can't rename stuff after a declaration splice
until after we've typechecked the stuff before (and the splice
itself).

* Combine the renamer and typecheker monads into one
	(TcRnMonad, TcRnTypes)
  These two replace TcMonad and RnMonad

* Give them a single 'driver' (TcRnDriver).  This driver
  replaces TcModule.lhs and Rename.lhs

* The haskell-src library package has a module
	Language/Haskell/THSyntax
  which defines the Haskell data type seen by the TH programmer.

* New modules:
	hsSyn/Convert.hs 	converts THSyntax -> HsSyn
	deSugar/DsMeta.hs 	converts HsSyn -> THSyntax

* New module typecheck/TcSplice type-checks Template Haskell splices.

		-------------
		Linking stuff
		-------------

* ByteCodeLink has been split into
	ByteCodeLink	(which links)
	ByteCodeAsm	(which assembles)

* New module ghci/ObjLink is the object-code linker.

* compMan/CmLink is removed entirely (was out of place)
  Ditto CmTypes (which was tiny)

* Linker.c initialises the linker when it is first used (no need to call
  initLinker any more).  Template Haskell makes it harder to know when
  and whether to initialise the linker.


	-------------------------------------
	Gathering the LIE in the type checker
	-------------------------------------

* Instead of explicitly gathering constraints in the LIE
	tcExpr :: RenamedExpr -> TcM (TypecheckedExpr, LIE)
  we now dump the constraints into a mutable varabiable carried
  by the monad, so we get
	tcExpr :: RenamedExpr -> TcM TypecheckedExpr

  Much less clutter in the code, and more efficient too.
  (Originally suggested by Mark Shields.)


		-----------------
		Remove "SysNames"
		-----------------

Because the renamer and the type checker were entirely separate,
we had to carry some rather tiresome implicit binders (or "SysNames")
along inside some of the HsDecl data structures.  They were both
tiresome and fragile.

Now that the typechecker and renamer are more intimately coupled,
we can eliminate SysNames (well, mostly... default methods still
carry something similar).

		-------------
		Clean up HsPat
		-------------

One big clean up is this: instead of having two HsPat types (InPat and
OutPat), they are now combined into one.  This is more consistent with
the way that HsExpr etc is handled; there are some 'Out' constructors
for the type checker output.

So:
	HsPat.InPat	--> HsPat.Pat
	HsPat.OutPat	--> HsPat.Pat
	No 'pat' type parameter in HsExpr, HsBinds, etc

	Constructor patterns are nicer now: they use
		HsPat.HsConDetails
	for the three cases of constructor patterns:
		prefix, infix, and record-bindings

	The *same* data type HsConDetails is used in the type
	declaration of the data type (HsDecls.TyData)

Lots of associated clean-up operations here and there.  Less code.
Everything is wonderful.

Type variables created by the typechecker didn't have the correct
NameSpace: they were in the Var namespace rather than the TyVar
namespace, which can lead to strange warnings about quantified type
variables being not mentioned in the type when DEBUG is on.

Name:
	- added mkSystemNameEncoded for use when the string
	  is already encoded (saves re-encoding the string every
	  time)

	- added mkSystemTvNameEncoded for making a type variable
	  name, as above

Var:
	- use mkSystemTvNameEncoded when making type variables

Id:
	- add mkSysLocalUnencoded for the (rare) case when
	  the string needs encoding

TcMType:
	- use mkSystemTvNameEncoded rather than mkSystemName for
	  making type variables

SetLevels:
	- use mkSysLocalUnencoded since the names generated here
	  need encoding.

Use Unique.getKey instead of Unique.u2i (they are the same function).

Import wibbles

Misc cleanup: remove the iface pretty-printing style, and clean up
bits of StringBuffer that aren't required any more.

------------------------
	Change
		GlobalName --> ExternalName
		LocalName  ->  InternalName
	------------------------

For a long time there's been terminological confusion between

	GlobalName vs LocalName	 (property of a Name)
	GlobalId vs LocalId	 (property of an Id)

I've now changed the terminology for Name to be

	ExternalName vs InternalName

I've also added quite a bit of documentation in the Commentary.

Binary Interface Files - stage 1
--------------------------------

This commit changes the default interface file format from text to
binary, in order to improve compilation performace.

To view an interface file, use 'ghc --show-iface Foo.hi'.

utils/Binary.hs is the basic Binary I/O library, based on the nhc98
binary I/O library but much stripped-down and working in terms of
bytes rather than bits, and with some special features for GHC: it
remembers which Module is being emitted to avoid dumping too many
qualified names, and it keeps track of a "dictionary" of FastStrings
so that we don't dump the same FastString more than once into the
binary file.  I'll make a generic version of this for the libraries at
some point.

main/BinIface.hs contains most of the Binary instances.  Some
instances are in the same module as the data type (RdrName, Name,
OccName in particular).  Most instances were generated using a
modified version of DrIFT, which I'll commit later.  However, editing
them by hand isn't hard (certainly easier than modifying
ParseIface.y).

The first thing in a binary interface is the interface version, so
nice error messages will be generated if the binary format changes and
you still have old interfaces lying around.  The version also now
includes the "way" as an extra sanity check.

Other changes
-------------

I don't like the way FastStrings contain both hashed strings (with
O(1) comparison) and literal C strings (with O(n) comparison).  So as
a first step to separating these I made serveral "literal" type
strings into hashed strings.  SLIT() still generates a literal, and
now FSLIT() generates a hashed string.  With DEBUG on, you'll get a
warning if you try to compare any SLIT()s with anything, and the
compiler will fall over if you try to dump any literal C strings into
an interface file (usually indicating a use of SLIT() which should be
FSLIT()).

mkSysLocal no longer re-encodes its FastString argument each time it
is called.

I also fixed the -pgm options so that the argument can now optionally
be separted from the option.

Bugfix: PrelNames declared Names for several comparison primops, eg.
eqCharName, eqIntName etc. but these had different uniques from the
real primop names.  I've moved these to PrimOps and defined them using
mkPrimOpIdName instead, and deleted some for which we don't have real
primops (Manuel: please check that things still work for you after
this change).

Make the OccName and SrcLoc fields of a Name strict, to eliminate
space leaks.  This doesn't hurt performance.

Fix codegen globalisation for -split-objs