GitLab

Thu Aug  3 19:29:38 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
  * Added error checks & fixed bugs

Tue Aug  1 14:10:39 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
  * Revised kind signatures

Mon Jul 31 17:20:56 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
  * Cleanup (re type function parsing)

Fri Jul 28 21:52:46 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
  * Parser support for assoc synonyms

Wed Jul 26 17:46:55 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
  * Migrate cvs diff from fptools-assoc branch
  - Syntactic support for associated types
  - Renamer support for associated types
  - ATs are only allowed with -fglasgow-exts
  - Handle ATs in the type and class declaration kinding knot-tying exercise

See #815

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

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

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

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

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

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

Internal changes:

  - There are two module-related types:

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

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

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

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

Docs to follow.

Bulat pointed out that in Template Haskell
   $x
is allowed instead of 
   $(x)
in expressions, but not at the top level of modules.

This commit fixes the omission.  Now you can say

	f x = x
 	$h
	data T = T

and the $h will run Template Haskell just as you'd expect.

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 commit adds bang-patterns, 
	enabled by -fglasgow-exts or -fbang-patterns
	diabled by -fno-bang-patterns

The idea is described here
	http://haskell.galois.com/cgi-bin/haskell-prime/trac.cgi/wiki/BangPatterns

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

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

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

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

Please test, and let me know of any hiccups


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

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

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

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


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

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

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

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

1) The main invariants:

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

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

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

2) Scoping

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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


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

	f x :: a = x	-- NO

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

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

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

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

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

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

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

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

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

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

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

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

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

Add support for UTF-8 source files

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

Additionally, the following synonyms for keywords are now recognised:

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

there are probably more things we could add here.

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

Internally, there were some pretty big changes:

  - FastStrings are now stored in UTF-8

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

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

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

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

Better error reporting for newtypes with too many constructors,
or too many fields.  Instead of yielding a parse error, we
parse it like a data type declaration, and give a comprehensible
error message later.

A suggestion from Jan-Willem.

Better TH -> HsSyn conversion

	Merge to stable (attempt)

This commit monad-ises the TH syntax -> HS syntax conversion.
This means that error messages can be reported in a more civilised
way.  It also ensures that the entire structure is converted eagerly.
That means that any exceptions buried inside it are triggered 
during conversion, and caught by the exception handler in TcSplice.
Before, they could be triggered later, and looked like comiler
crashes.

Wibble to: "Add a new pragma: SPECIALISE INLINE"

I messed up the way that NOINLINE is parsed; this commit fixes it.

Add a new pragma: SPECIALISE INLINE

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

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

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

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

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


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

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.

remove old comment

From: Autrijus Tang <autrijus@autrijus.org>

Arrange that a 'deriving' clause works for a GADT-syntax
data type delaration, provided it declares a Haskell-98-style
data type (i.e. no existentials or GADT stuff).

This just allows you to use a different syntax for data type
declarations without losing 'deriving'. A couple of people requested
this, and it's really easy to do.

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.

Make it so that you can deprecate a data constructor.
Previously {-# DEPRECATED T "no" #-} referred only to the type
or class T.  Now it refers to the data constructor T as well,
just like in fixity declarations.  

There's no way to deprecate the data constructor T without also
deprecating the type T, alas.  Same problem in fixity decls.
Main problem is coming up with a suitable concrete syntax to do
so.

We could consider merging this to the STABLE branch.

NB: Sven, the manual fixes are not XML-valideated!  I'm at home.

This commit combines three overlapping things:

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

2.  Tidy up HsSyn.  In particular:

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

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

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


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

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

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.

Wibbles to infix operators; please merge

Comments

Add parser support for infix type-variable operators

---------------------------------------------
	Make type synonyms uniform with data types
	so far as infix operators are concerned
	---------------------------------------------

	Merge to STABLE


This allows

	type (a :+: b) c d = ...

which was prevented before by accident.

I've also documented the fact that classes can be infix;
and arranged that class constraints in types can be in infix form.
	f :: (a :=: b) => ....

--------------------------------------------
          Replace hi-boot files with hs-boot files
  	--------------------------------------------

This major commit completely re-organises the way that recursive modules
are dealt with.

  * It should have NO EFFECT if you do not use recursive modules

  * It is a BREAKING CHANGE if you do

====== Warning: .hi-file format has changed, so if you are
======		updating into an existing HEAD build, you'll
======		need to make clean and re-make


The details:  [documentation still to be done]

* Recursive loops are now broken with Foo.hs-boot (or Foo.lhs-boot),
  not Foo.hi-boot

* An hs-boot files is a proper source file.  It is compiled just like
  a regular Haskell source file:
	ghc Foo.hs		generates Foo.hi, Foo.o
	ghc Foo.hs-boot		generates Foo.hi-boot, Foo.o-boot

* hs-boot files are precisely a subset of Haskell. In particular:
	- they have the same import, export, and scoping rules
	- errors (such as kind errors) in hs-boot files are checked
  You do *not* need to mention the "original" name of something in
  an hs-boot file, any more than you do in any other Haskell module.

* The Foo.hi-boot file generated by compiling Foo.hs-boot is a machine-
  generated interface file, in precisely the same format as Foo.hi

* When compiling Foo.hs, its exports are checked for compatibility with
  Foo.hi-boot (previously generated by compiling Foo.hs-boot)

* The dependency analyser (ghc -M) knows about Foo.hs-boot files, and
  generates appropriate dependencies.  For regular source files it
  generates
	Foo.o : Foo.hs
	Foo.o : Baz.hi		-- Foo.hs imports Baz
	Foo.o : Bog.hi-boot	-- Foo.hs source-imports Bog

  For a hs-boot file it generates similar dependencies
	Bog.o-boot : Bog.hs-boot
	Bog.o-boot : Nib.hi	-- Bog.hs-boto imports Nib

* ghc -M is also enhanced to use the compilation manager dependency
  chasing, so that
	ghc -M Main
  will usually do the job.  No need to enumerate all the source files.

* The -c flag is no longer a "compiler mode". It simply means "omit the
  link step", and synonymous with -no-link.

------------------------
    Reorganisation of hi-boot files
  	------------------------

The main point of this commit is to arrange that in the Compilation
Manager's dependendency graph, hi-boot files are proper nodes. This
is important to make sure that we compile everything in the right
order.  It's a step towards hs-boot files.

* The fundamental change is that CompManager.ModSummary has a new
  field, ms_boot :: IsBootInterface

  I also tided up CompManager a bit.  No change to the Basic Plan.

  ModSummary is now exported abstractly from CompManager (was concrete)

* Hi-boot files now have import declarations.  The idea is they are
  compulsory, so that the dependency analyser can find them

* I changed an invariant: the Compilation Manager used to ensure that
  hscMain was given a HomePackageTable only for the modules 'below' the
  one being compiled.  This was really only important for instances and
  rules, and it was a bit inconvenient.  So I moved the filter to the
  compiler itself: see HscTypes.hptInstances and hptRules.

* Module Packages.hs now defines
    data PackageIdH
    = HomePackage 		-- The "home" package is the package
 				-- curently being compiled
    | ExtPackage PackageId	-- An "external" package is any other package

   It was just a Maybe type before, so this makes it a bit clearer.

* I tried to add a bit better location info to the IfM monad, so that
  errors in interfaces come with a slightly more helpful error message.
  See the if_loc field in TcRnTypes --- and follow-on consequences

* Changed Either to Maybes.MaybeErr in a couple of places (more perspicuous)

HEADS UP!  You now need to use an up to date Happy from CVS to build
GHC.  Happy version 1.15 will be released shortly.

Replace the slow hacked up String-based GetImports with one based on
the real Haskell parser.  This requires a new addition to Happy to
support parsing partial files.  We now avoid reading each source file
off the disk twice: once to get its module name and imports, and again
to parse it.  Instead we just slurp it once, and cache the StringBuffer.

This should result in improved startup times for ghc --make,
especially when there are lots of source files.

Allow trailing semicolon in GADT constructor list