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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.

Haddock fixes, "-- *" should be used with care...

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.)

----------------------------------------
     New Core invariant: keep case alternatives in sorted order
	----------------------------------------

We now keep the alternatives of a Case in the Core language in sorted
order.  Sorted, that is,
	by constructor tag	for DataAlt
	by literal		for LitAlt

The main reason is that it makes matching and equality testing more robust.
But in fact some lines of code vanished from SimplUtils.mkAlts.


WARNING: no change to interface file formats, but you'll need to recompile
your libraries so that they generate interface files that respect the
invariant.

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

arrow notation: allow arrow applications (f -< a) to take a non-empty
command stack, as suggested by Sebastian Boldt <Sebastian.Boldt@arcor.de>.

-----------------------------------------------------
  Fix a subtle loop in the context-reduction machinery
  ----------------------------------------------------

This bug was provoked by a recent change: when trying to prove
a constraint C, TcSimplify.reduce now adds C to the database before
trying to prove C, thus building recursive dictionaries.

Two bugs
a) If we add C's superclasses (which we were) we can now build a
   bogusly-recursive dictionary (see Note [SUPERCLASS-LOOP]).
   Solution: in reduce, add C only (via addIrred NoSCs) and then
   later use addWanted to add its definition plus SCs.

b) Since we can have recursive definitions, the superclass-loop
   handling machinery (findAllDeps) must carry its visited-set
   with it (which it was not doing before)


The main file is TcSimplify; but I modified a bunch of others to
take advantage of new function extendVarSetList

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.

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

fixes to desugaring of arrow notation:

* fix free variable calculation for if's

* various fixes for case

please merge to STABLE

Arrow notation: add a new (more primitive) form of command:

	cmd ::= ... | cmd aexp

analogous to ordinary application, and also represented using HsApp.
To avoid an overlap, the syntax for combining forms is changed to

	(|aexp cmd1 ... cmdn|)

Add extra functions operating on outsized tuples (used by the translation
of arrow notation).

Some random import trimming

fix bug with ExprStmt, plus some cleaning up.

----------------------------------------------
	Add support for Ross Paterson's arrow notation
	----------------------------------------------

Ross Paterson's ICFP'01 paper described syntax to support John Hughes's
"arrows", rather as do-notation supports monads.  Except that do-notation is
relatively modest -- you can write monads by hand without much trouble --
whereas arrow-notation is more-or-less essential for writing arrow programs.
It desugars to a massive pile of tuple construction and selection!

For some time, Ross has had a pre-processor for arrow notation, but the
resulting type error messages (reported in terms of the desugared code)
are impenetrable.  This commit integrates the syntax into GHC.  The
type error messages almost certainly still require tuning, but they should
be better than with the pre-processor.

Main syntactic changes (enabled with -farrows)

   exp ::= ... | proc pat -> cmd

   cmd ::= exp1 -<  exp2   |  exp1 >-  exp2
	|  exp1 -<< exp2   |  exp1 >>- exp2
	| \ pat1 .. patn -> cmd
	| let decls in cmd
	| if exp then cmd1 else cmd2
	| do { cstmt1 .. cstmtn ; cmd }
	| (| exp |) cmd1 .. cmdn
	| cmd1 qop cmd2
	| case exp of { calts }

   cstmt :: = let decls
	 |   pat <- cmd
	 |   rec { cstmt1 .. cstmtn }
	 |   cmd

New keywords and symbols:
	proc rec
	-<   >-   -<<   >>-
	(|  |)

The do-notation in cmds was not described in Ross's ICFP'01 paper; instead
it's in his chapter in The Fun of Programming (Plagrave 2003).

The four arrow-tail forms (-<) etc cover
  (a) which order the pices come in (-<  vs  >-), and
  (b) whether the locally bound variables can be used in the
		arrow part (-<  vs  -<<) .
In previous presentations, the higher-order-ness (b) was inferred,
but it makes a big difference to the typing required so it seems more
consistent to be explicit.

The 'rec' form is also available in do-notation:
  * you can use 'rec' in an ordinary do, with the obvious meaning
  * using 'mdo' just says "infer the minimal recs"


Still to do
~~~~~~~~~~~
Top priority is the user manual.

The implementation still lacks an implementation of
the case form of cmd.


Implementation notes
~~~~~~~~~~~~~~~~~~~~
Cmds are parsed, and indeed renamed, as expressions.  The type checker
distinguishes the two.