GitLab

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.

small tidyup for printing bindings with long identifiers: allow the
binder and its definition to go on separate lines

Print occ info in Core

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.

Dont print result type for Core case-expressions (too voluminous)

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 a long-standing indirection-zapping bug
	-----------------------------------------

	Merge to STABLE

Up to now we zap indirections as part of the occurence analyser.
But this is bogus.  The indirection zapper does the following:

	x_local = <expression>
	...bindings...
	x_exported = x_local

where x_exported is exported, and x_local is not, then we
replace it with this:

	x_exported = <expression>
	x_local = x_exported
	...bindings...

But this is plain wrong if x_exported has a RULE that mentions
something (f, say) in ...bindings.., because 'f' will then die.

After hacking a few solutions, I've eventually simply made the indirection
zapping into a separate pass (which is cleaner anyway), which wraps the
entire program back into a single Rec if the bad thing can happen.

On the way I've made indirection-zapping work in Recs too, which wasn't the
case before.

* Move the zapper from OccurAnal into SimplCore
* Tidy up the printing of pragmas (PprCore and friends)
* Add a new function Rules.addRules
* Merge rules in the indirection zapper (previously one set was discarded)

Remove redundant parens in pretty print

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

------------------------------------------
	Keep-alive set and Template Haskell quotes
	------------------------------------------

a) Template Haskell quotes should be able to mention top-leve
   things without resorting to lifting.  Example

	module Foo( foo ) where
	  f x = x
	  foo = [| f 4 |]

   Here the reference to 'f' is ok; no need to 'lift' it.
   The relevant changes are in TcExpr.tcId

b) However, we must take care not to discard the binding for f,
   so we add it to the 'keep-alive' set for the module.  I've
   now made this into (another) mutable bucket, tcg_keep, 
   in the TcGblEnv

c) That in turn led me to look at the handling of orphan rules;
   as a result I made IdCoreRule into its own data type, which
   has simle but non-local ramifications

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

Put parens around binders in debug prints

GC dead code and export list entries.

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

Cosmetics

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

A round of space-leak fixing.

  - re-instate zapping of the PersistentCompilerState at various
    points during the compilation cycle in HscMain.  This affects
    one-shot compilation only, since in this mode the information
    collected in the PCS is not required after creating the final
    interface file.

  - Unravel the recursive dependency between MkIface and
    CoreTidy/CoreToStg.  Previously the CafInfo for each binding was
    calculated by CoreToStg, and fed back into the IdInfo of the Ids
    generated by CoreTidy (an earlier pass).  MkIface then took this
    IdInfo and the bindings from CoreTidy to generate the interface;
    but it couldn't do this until *after* CoreToStg, because the CafInfo
    hadn't been calculated yet.  The result was that the CoreTidy
    output lived until after CoreToStg, and at the same time as the
    CorePrep and STG syntax, which is wasted space, not to mention
    the complexity and general ugliness in HscMain.

    So now we calculate CafInfo directly in CoreTidy.  The downside is
    that we have to predict what CorePrep is going to do to the
    bindings so we can tell what will turn into a CAF later, but it's
    no worse than before (it turned out that we were doing this
    prediction before in CoreToStg anyhow).

  - The typechecker lazilly typechecks unfoldings.  It turns out that
    this is a good idea from a performance perspective, but it also
    means that it must hang on to all the information it needs to
    do the typechecking.  Previously this meant holding on to the
    whole of the typechecker's environment, which includes all sorts
    of stuff which isn't necessary to typecheck unfoldings.  By paring
    down the environment captured by the lazy unfoldings, we can
    save quite a bit of space in the phases after typechecking.

-------------------------------------
      Add Core Notes and the {-# CORE #-} pragma
	-------------------------------------

This is an idea of Hal Daume's. The key point is that Notes in Core
are augmented thus:

  data Note
    = SCC CostCentre
    | ...
    | CoreNote String     -- NEW

These notes can be injected via a Haskell-source pragma:

   f x = ({-# CORE "foo" #-} show) ({-# CORE "bar" #-} x)

This wraps a (Note (CoreNote "foo")) around the 'show' variable,
and a similar note around the argument to 'show'.

These notes are basically ignored by GHC, but are emitted into
External Core, where they may convey useful information.

Exactly how code involving these notes is munged by the simplifier
isn't very well defined.  We'll see how it pans out.  Meanwhile
the impact on the rest of the compiler is minimal.

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

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

---------------------------------------
	Utterly expunge the tyGenInfo field of
			an IdInfo
	---------------------------------------

tyGenInfo was a relic of a previous version of Keith's usage
analyser.  It's just dead code, so I've nuked it.

FastString cleanup, stage 1.

The FastString type is no longer a mixture of hashed strings and
literal strings, it contains hashed strings only with O(1) comparison
(except for UnicodeStr, but that will also go away in due course).  To
create a literal instance of FastString, use FSLIT("..").

By far the most common use of the old literal version of FastString
was in the pattern

	  ptext SLIT("...")

this combination still works, although it doesn't go via FastString
any more.  The next stage will be to remove the need to use this
special combination at all, using a RULE.

To convert a FastString into an SDoc, now use 'ftext' instead of
'ptext'.

I've also removed all the FAST_STRING related macros from HsVersions.h
except for SLIT and FSLIT, just use the relevant functions from
FastString instead.

Tidier printing routines for Rules

Take the old strictness analyser out of #ifdef DEBUG and put it
instead in #ifdef OLD_STRICTNESS.  DEBUG was getting a bit slow.

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

Improved printing

Make the inclusion of the old strictness analyser, CPR analyser, and
the relevant IdInfo components, conditional on DEBUG.  This makes
IdInfo smaller by three fields in a non-DEBUG compiler, and reduces
the risk that the unused fields could harbour space leaks.

Eventually these passes will go away altogether.

- Pet peeve removal / code tidyup, replaced various sub-optimal
  uses of 'length' with something a bit better, i.e., replaced
  the following patterns

   *  length as `cmpOp` length bs
   *  length as `cmpOp` val   -- incl. uses where val == 1 and val == 0
   *  {take,drop,splitAt} (length as) bs
   *  length [ () | pat <- as ]

  with uses of misc Util functions.

  I'd be surprised if there's a noticeable reduction in running
  times as a result of these changes, but every little bit helps.

  [ The changes have been tested wrt testsuite/ - I'm seeing a couple
    of unexpected breakages coming from CorePrep, but I'm currently
    assuming that these are due to other recent changes. ]

- compMan/CompManager.lhs: restored 4.08 compilability + some code
  cleanup.

None of these changes are HEADworthy.

Cosmetica

------------------
		Simon's big commit
		------------------

This commit, which I don't think I can sensibly do piecemeal, consists
of the things I've been doing recently, mainly directed at making
Manuel, George, and Marcin happier with RULES.


Reogranise the simplifier
~~~~~~~~~~~~~~~~~~~~~~~~~
1. The simplifier's environment is now an explicit parameter.  This
makes it a bit easier to figure out where it is going.

2. Constructor arguments can now be arbitrary expressions, except
when the application is the RHS of a let(rec).  This makes it much
easier to match rules like

	RULES
	    "foo"  f (h x, g y) = f' x y

In the simplifier, it's Simplify.mkAtomicArgs that ANF-ises a
constructor application where necessary.  In the occurrence analyser,
there's a new piece of context info (OccEncl) to say whether a
constructor app is in a place where it should be in ANF.  (Unless
it knows this it'll give occurrence info which will inline the
argument back into the constructor app.)

3. I'm experimenting with doing the "float-past big lambda" transformation
in the full laziness pass, rather than mixed in with the simplifier (was
tryRhsTyLam).

4.  Arrange that
	case (coerce (S,T) (x,y)) of ...
will simplify.  Previous it didn't.
A local change to CoreUtils.exprIsConApp_maybe.

5. Do a better job in CoreUtils.exprEtaExpandArity when there's an
error function in one branch.


Phase numbers, RULES, and INLINE pragmas
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1.  Phase numbers decrease from N towards zero (instead of increasing).
This makes it easier to add new earlier phases, which is what users want
to do.

2.  RULES get their own phase number, N, and are disabled in phases before N.

e.g. 	{-# RULES "foo" [2] forall x y.  f (x,y) = f' x y #-}

Note the [2], which says "only active in phase 2 and later".

3.  INLINE and NOINLINE pragmas have a phase number to.  This is now treated
in just the same way as the phase number on RULE; that is, the Id is not inlined
in phases earlier than N.  In phase N and later the Id *may* be inlined, and
here is where INLINE and NOINLINE differ: INLNE makes the RHS look small, so
as soon as it *may* be inlined it probably *will* be inlined.

The syntax of the phase number on an INLINE/NOINLINE pragma has changed to be
like the RULES case (i.e. in square brackets).  This should also make sure
you examine all such phase numbers; many will need to change now the numbering
is reversed.

Inlining Ids is no longer affected at all by whether the Id appears on the
LHS of a rule.  Now it's up to the programmer to put a suitable INLINE/NOINLINE
pragma to stop it being inlined too early.


Implementation notes:

*  A new data type, BasicTypes.Activation says when a rule or inline pragma
is active.   Functions isAlwaysActive, isNeverActive, isActive, do the
obvious thing (all in BasicTypes).

* Slight change in the SimplifierSwitch data type, which led to a lot of
simplifier-specific code moving from CmdLineOpts to SimplMonad; a Good Thing.

* The InlinePragma in the IdInfo of an Id is now simply an Activation saying
when the Id can be inlined.  (It used to be a rather bizarre pair of a
Bool and a (Maybe Phase), so this is much much easier to understand.)

* The simplifier has a "mode" environment switch, replacing the old
black list.  Unfortunately the data type decl has to be in
CmdLineOpts, because it's an argument to the CoreDoSimplify switch

    data SimplifierMode = SimplGently | SimplPhase Int

Here "gently" means "no rules, no inlining".   All the crucial
inlining decisions are now collected together in SimplMonad
(preInlineUnconditionally, postInlineUnconditionally, activeInline,
activeRule).


Specialisation
~~~~~~~~~~~~~~
1.  Only dictionary *functions* are made INLINE, not dictionaries that
have no parameters.  (This inline-dictionary-function thing is Marcin's
idea and I'm still not sure whether it's a good idea.  But it's definitely
a Bad Idea when there are no arguments.)

2.  Be prepared to specialise an INLINE function: an easy fix in
Specialise.lhs

But there is still a problem, which is that the INLINE wins
at the call site, so we don't use the specialised version anyway.
I'm still unsure whether it makes sense to SPECIALISE something
you want to INLINE.





Random smaller things
~~~~~~~~~~~~~~~~~~~~~~

* builtinRules (there was only one, but may be more) in PrelRules are now
  incorporated.   They were being ignored before...

* OrdList.foldOL -->  OrdList.foldrOL, OrdList.foldlOL

* Some tidying up of the tidyOpenTyVar, tidyTyVar functions.  I've
  forgotten exactly what!

--------------------------
	Add a rule-check pass
	(special request by Manuel)
	--------------------------

	DO NOT merge with stable

The flag

	-frule-check foo

will report all sites at which RULES whose name starts with "foo.."
might apply, but in fact the arguments don't match so the rule
doesn't apply.

The pass is run right after all the core-to-core passes.  (Next thing
to do: make the core-to-core script external, so you can fiddle with
it.  Meanwhile, the core-to-core script is in
	DriverState.builCoreToDo
so you can move the CoreDoRuleCheck line around if you want.

The format of the report is experimental: Manuel, feel free to fiddle
with it.

Most of the code is in specialise/Rules.lhs


Incidental changes
~~~~~~~~~~~~~~~~~~
Change BuiltinRule so that the rule name is accessible
without actually successfully applying the rule.  This
change affects quite a few files in a trivial way.

--------------------------------
	First cut at the demand analyser
	--------------------------------

This demand analyser is intended to replace the strictness/absence
analyser, and the CPR analyser.

This commit adds it to the compiler, but in an entirely non-invasive
way.

	If you build the compiler without -DDEBUG,
	you won't get it at all.

	If you build the compiler with -DDEBUG,
	you'll get the demand analyser, but the existing
	strictness analyser etc are still there.  All the
	demand analyser does is to compare its output with
	the existing stuff and report differences.

There's no cross-module stuff for demand info yet.

The strictness/demand info is put the IdInfo as
	newStrictnessInfo
	newDemandInfo

Eventually we'll remove the old ones.

Simon

-------------------------------------------
	Towards generalising 'foreign' declarations
	-------------------------------------------

This is a first step towards generalising 'foreign' declarations to
handle langauges other than C.  Quite a lot of files are touched,
but nothing has really changed.  Everything should work exactly as
before.

	But please be on your guard for ccall-related bugs.

Main things

Basic data types: ForeignCall.lhs
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Remove absCSyn/CallConv.lhs

* Add prelude/ForeignCall.lhs.  This defines the ForeignCall
  type and its variants

* Define ForeignCall.Safety to say whether a call is unsafe
  or not (was just a boolean).  Lots of consequential chuffing.

* Remove all CCall stuff from PrimOp, and put it in ForeignCall


Take CCallOp out of the PrimOp type (where it was always a glitch)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Add IdInfo.FCallId variant to the type IdInfo.GlobalIdDetails,
	along with predicates Id.isFCallId, Id.isFCallId_maybe

* Add StgSyn.StgOp, to sum PrimOp with FCallOp, because it
  *is* useful to sum them together in Stg and AbsC land.  If
  nothing else, it minimises changes.


Also generally rename "CCall" stuff to "FCall" where it's generic
to all foreign calls.

Some rearrangements that Simon & I have been working on recently:

    - CoreSat is now CorePrep, and is a general "prepare-for-code-
      generation" pass.  It does cloning, saturation of constructors &
      primops, A-normal form, and a couple of other minor fiddlings.

    - CoreTidy no longer does cloning, and minor fiddlings.  It doesn't
      need the unique supply any more, so that's removed.

    - CoreToStg now collects CafInfo and the list of CafRefs for each
      binding.  The SRT pass is much simpler now.

    - IdInfo now has a CgInfo field for "code generator info".  It currently
      contains arity (the actual code gen arity which affects the calling
      convention as opposed to the ArityInfo which is a measure of how
      many arguments the Id can be applied to before it does any work), and
      CafInfo.

      Previously we overloaded the ArityInfo field to contain both
      codegen arity and simplifier arity.  Things are cleaner now.

    - CgInfo is collected by CoreToStg, and passed back into CoreTidy in
      a loop.  The compiler will complain rather than going into a black
      hole if the CgInfo is pulled on too early.

    - Worker info in an interface file now comes with arity info attached.
      Previously the main arity info was overloaded for this purpose, but
      it lead to a few hacks in the compiler, this tidies things up somewhat.

Bottom line: we removed several fragilities, and tidied up a number of
things.  Code size should be smaller, but we'll see...

--------------------
	A major hygiene pass
	--------------------

1. The main change here is to

	Move what was the "IdFlavour" out of IdInfo,
	and into the varDetails field of a Var

   It was a mess before, because the flavour was a permanent attribute
   of an Id, whereas the rest of the IdInfo was ephemeral.  It's
   all much tidier now.

   Main places to look:

	   Var.lhs	Defn of VarDetails
	   IdInfo.lhs	Defn of GlobalIdDetails

   The main remaining infelicity is that SpecPragmaIds are right down
   in Var.lhs, which seems unduly built-in for such an ephemeral thing.
   But that is no worse than before.


2. Tidy up the HscMain story a little.  Move mkModDetails from MkIface
   into CoreTidy (where it belongs more nicely)

   This was partly forced by (1) above, because I didn't want to make
   DictFun Ids into a separate kind of Id (which is how it was before).
   Not having them separate means we have to keep a list of them right
   through, rather than pull them out of the bindings at the end.

3. Add NameEnv as a separate module (to join NameSet).

4. Remove unnecessary {-# SOURCE #-} imports from FieldLabel.

Slightly more compact ppr for Core (makes it easier to read large functions).

merge rev. 1.49.2.1 from ghc-4-07-branch

Minor enhancements to printing machinery to aid debugging the BC generator.

When renaming, typechecking an expression from the user
interface, we may suck in declarations from interface
files (e.g. the Prelude).  This commit takes account of that.

To do so, I did some significant restructuring in TcModule,
with consequential changes and tidy ups elsewhere in the type
checker.  I think there should be fewer lines in total than before.