GitLab

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.

Wibble to typerep (fixes crash I hope)

fix repType after changes to the representation of type synonyms.
This caused the stage2 compiler to crash, because various info tables
misrepresented the pointerhood of constructor arguments.

Two significant changes to the representation of types

1. Change the representation of type synonyms

    Up to now, type synonym applications have been held in
    *both* expanded *and* un-expanded form.  Unfortunately, this
    has exponential (!) behaviour when type synonyms are deeply
    nested.  E.g.
	    type P a b = (a,b)
	    f :: P a (P b (P c (P d e)))
    
    This showed up in a program of Joel Reymont, now immortalised
    as typecheck/should_compile/syn-perf.hs

    So now synonyms are held as ordinary TyConApps, and expanded
    only on demand.  

    SynNote has disappeared altogether, so the only remaining TyNote
    is a FTVNote.  I'm not sure if it's even useful.

2. Eta-reduce newtypes

    See the Note [Newtype eta] in TyCon.lhs
    
    If we have 
	    newtype T a b = MkT (S a b)
    
    then, in Core land, we would like S = T, even though the application
    of T is then not saturated. This commit eta-reduces T's RHS, and
    keeps that inside the TyCon (in nt_etad_rhs).  Result is that 
    coreEqType can be simpler, and has less need of expanding newtypes.

Add record syntax for GADTs
	~~~~~~~~~~~~~~~~~~~~~~~~~~~

Atrijus Tang wanted to add record syntax for GADTs and existential
types, so he and I worked on it a bit at ICFP.  This commit is the
result.  Now you can say

 data T a where
  T1 { x :: a }           	 :: T [a]
  T2 { x :: a, y :: Int } 	 :: T [a]
  forall b. Show b =>
 	T3 { naughty :: b, ok :: Int } :: T Int
  T4 :: Eq a => a -> b -> T (a,b)

Here the constructors are declared using record syntax.

Still to come after this commit:
  - User manual documentation
  - More regression tests
  - Some missing cases in the parser (e.g. T3 won't parse)
Autrijus is going to do these.


Here's a quick summary of the rules.  (Atrijus is going to write
proper documentation shortly.)

Defnition: a 'vanilla' constructor has a type of the form
	forall a1..an. t1 -> ... -> tm -> T a1 ... an
No existentials, no context, nothing.  A constructor declared with
Haskell-98 syntax is vanilla by construction.  A constructor declared
with GADT-style syntax is vanilla iff its type looks like the above.
(In the latter case, the order of the type variables does not matter.)

* You can mix record syntax and non-record syntax in a single decl

* All constructors that share a common field 'x' must have the
  same result type (T [a] in the example).

* You can use field names without restriction in record construction
  and record pattern matching.

* Record *update* only works for data types that only have 'vanilla'
  constructors.

* Consider the field 'naughty', which uses a type variable that does
  not appear in the result type ('b' in the example).  You can use the
  field 'naughty' in pattern matching and construction, but NO
  SELECTOR function is generated for 'naughty'.  [An attempt to use
  'naughty' as a selector function will elicit a helpful error
  message.]

* Data types declared in GADT syntax cannot have a context. So this
is illegal:
	data (Monad m) => T a where
		  ....

* Constructors in GADT syntax can have a context (t.g. T3, T4 above)
  and that context is stored in the constructor and made available
  when the constructor is pattern-matched on.  WARNING: not competely
  implemented yet, but that's the plan.



Implementation notes
~~~~~~~~~~~~~~~~~~~~
- Data constructors (even vanilla ones) no longer share the type
  variables of their parent type constructor.

- HsDecls.ConDecl has changed quite a bit

- TyCons don't record the field labels and type any more (doesn't
  make sense for existential fields)

- GlobalIdDetails records which selectors are 'naughty', and hence
  don't have real code.

Do 'tidying' on Kinds before printing them.  This avoids printing
stuff like 'k_43b' in user error messages.

To do this, I ended up adding an OccName to Kind.KindVar.  Even
then the implementation is a bit of hack (see comments with 
Type.tidyKind).  Still, it's a highly localised hack, whereas the
"right thing" entails making KindVar into a flavour of Var, which
seems like an uncomfortably big change.

   I think this change can merge to the stable branch

Add assertions (only)

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).

Flags cleanup.

Basically the purpose of this commit is to move more of the compiler's
global state into DynFlags, which is moving in the direction we need
to go for the GHC API which can have multiple active sessions
supported by a single GHC instance.

Before:

$ grep 'global_var' */*hs | wc -l
     78

After:

$ grep 'global_var' */*hs | wc -l
     27

Well, it's an improvement.  Most of what's left won't really affect
our ability to host multiple sessions.

Lots of static flags have become dynamic flags (yay!).  Notably lots
of flags that we used to think of as "driver" flags, like -I and -L,
are now dynamic.  The most notable static flags left behind are the
"way" flags, eg. -prof.  It would be nice to fix this, but it isn't
urgent.

On the way, lots of cleanup has happened.  Everything related to
static and dynamic flags lives in StaticFlags and DynFlags
respectively, and they share a common command-line parser library in
CmdLineParser.  The flags related to modes (--makde, --interactive
etc.) are now private to the front end: in fact private to Main
itself, for now.

Rename mkTvSubst to mkOpenTvSubst; add new mkTvSubst

-----------------------
	Fixup to hoistForAllTys
	-----------------------

* hoistForAllTys moves from TcHsType to TcType

hoistForAllTys was being too vigorous and breaking up type synonyms,
even when it was entirely unnecessary to do so.

Not only does this make error messsages less good, but it's actually
wrong for Haskell 98, because we are meant to report under-applied
type synonyms, and that check doesn't happen until after hoistForAllTys.
This led to a very obscure bug, immortalised as tcfail129.

------------------------
          GADTs and unification
  	------------------------

1. Adjustment to typechecking of pattern matching the call to
   gadtRefineTys in TcPat.  Now wobbly types are treated as wild
   cards in the unification process.

2. Add the WildCard possibility to the BindFlag in types/Unify.lhs

3. Some related refactoring of tcMatchTys etc.

Fix to the pre-Xmas simplifier changes, which should make 
everything work again.  I'd forgotten to attend to this
corner.  Still not properly tested I fear.

Also remove dead code from SimplEnv, and simplify the remainder (hooray).

---------------------------
          Refactor the simplifier
  	---------------------------

Driven by a GADT bug, I have refactored the simpifier, and the way GHC
treats substitutions.  I hope I have gotten it right.  Be cautious about updating.

* coreSyn/Subst.lhs has gone

* coreSyn/CoreSubst replaces it, except that it's quite a bit simpler

* simplCore/SimplEnv is added, and contains the simplifier-specific substitution
  stuff

Previously Subst was trying to be all things to all men, and that was making
it Too Complicated.

There may be a little more code now, but it's much easier to understand.

---------------------------------
     Improve handling of lexically scoped type variables
	---------------------------------

If we have

	f :: T a -> a
	f (x :: T b) = ...

then the lexically scoped variable 'b' should refer to the rigid
type variable 'a', without any intervening wobbliness.  Previously
the in-scope type variables were always mutable TyVars, which were
instantatiated to point to the type they were bound to; but since
the advent of GADTs the intervening mutable type variable is a bad
thing.

Hence
  * In the type environment, ATyVar now carries a type
  * The call to refineTyVars in tc_pat on SigPatIn
    finds the types by matching
  * Then tcExtendTyVarEnv3 extends the type envt appropriately

Rater a lot of huff and puff, but it's quite natural for ATyVar
to contain a type.

Various other small nomenclature changes along the way.

Fix bogon in type comparison

--------------------------------
	Deal properly with dual-renaming
	--------------------------------

When comparing types and terms, and during matching, we are faced
with 
	\x.e1	~   \y.e2

There are many pitfalls here, and GHC has never done the job properly.
Now, at last it does, using a new abstraction VarEnv.RnEnv2.  See
comments there for how it works.

There are lots of consequential changes to use the new stuff, especially
in 
	types/Type (type comparison), 
	types/Unify (matching on types)
	coreSyn/CoreUtils (equality on expressions), 
	specialise/Rules (matching).

I'm not 100% certain of that I've covered all the bases, so let me
know if something unexpected happens after you update.  Maybe wait until
a nightly build has worked ok first!

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

-----------------------------------
	Do simple checking on hi-boot files
	-----------------------------------

This commit arranges that, when compiling A.hs, we compare
the types we infer with those in A.hi-boot, if the latter 
exists.  (Or, more accurately, if anything A.hs imports in
turn imports A.hi-boot, directly or indirectly.)

This has been on the to-do list forever.

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

Merge backend-hacking-branch onto HEAD.  Yay!

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

Wibbles to exporting types abstractly

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

Wibbles to pretty printing of types

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

Use tcIsTyVarTy not isTyVarTy; and move isPredTy

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

Remove redundant where

--------------------------
  Fix an obscure but long-standing bug in Type.applyTys
	   --------------------------

The interesting case, which previously killed GHC 6.0, is this
	applyTys (forall a.a) [forall b.b, Int]
This really can happen, via dressing up polymorphic types with newtype
clothing.  Here's an example:
	newtype R = R (forall a. a->a)
	foo = case undefined :: R of

Test simplCore/should_compile/simpl0009 uses this as a test case.

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

-------------------------------------
	Remove all vestiges of usage analysis
	-------------------------------------

This commit removes a large blob of usage-analysis-related code, almost
all of which was commented out.

Sadly, it doesn't look as if Keith is going to have enough time to polish it
up, and in any case the actual performance benefits (so far as we can measure
them) turned out to be pretty modest (a few percent).

So, with regret, I'm chopping it all out.  It's still there in the repository
if anyone wants go hack on it.  And Tobias Gedell at Chalmers is implementing
a different analysis, via External Core.

------------------
  	   Fix a newtype-deriving bug
		------------------

The new newtype-deriving mechanism was erroneously using the
*representation type* of the newtype.  The rep type looks through all
ihtermediate newtypes, so that is wrong.  See Note [newtype
representation] in TcDeriv.lhs

deriving/should_run/drvrun013 now tests for this.

------------------------------------------
	Implement reification for Template Haskell
	------------------------------------------

This is entirely un-tested, but I don't think it'll break non-TH stuff.

Implements
	reifyDecl T :: Dec	-- Data type T
	reifyDecl C :: Dec	-- Class C
	reifyType f :: Typ	-- Function f

I hope.

Remove usage-analysis guff in applyTys

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

---------------------------------------------
	    Calculate the free vars of a type 'right'
	---------------------------------------------

	type C u a = u

Question: is 'a' free in 'C u a'?

I think the answer should be 'no'; see typecheck/should_compile/tc157.hs
for an example of why it matters.  This commit makes it so, and adds
comments to explain a dark corner in the zonking code.

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