This patch adds quasi-quotation, as described in
  "Nice to be Quoted: Quasiquoting for Haskell"
	(Geoffrey Mainland, Haskell Workshop 2007)
Implemented by Geoffrey and polished by Simon.

Overview
~~~~~~~~
The syntax for quasiquotation is very similar to the existing
Template haskell syntax:
	[$q| stuff |]
where 'q' is the "quoter".  This syntax differs from the paper, by using
a '$' rather than ':', to avoid clashing with parallel array comprehensions.
 
The "quoter" is a value of type Language.Haskell.TH.Quote.QuasiQuoter, which
contains two functions for quoting expressions and patterns, respectively.
 
     quote = Language.Haskell.TH.Quote.QuasiQuoter quoteExp quotePat
 
     quoteExp :: String -> Language.Haskell.TH.ExpQ
     quotePat :: String -> Language.Haskell.TH.PatQ

TEXT is passed unmodified to the quoter. The context of the
quasiquotation statement determines which of the two quoters is
called: if the quasiquotation occurs in an expression context,
quoteExp is called, and if it occurs in a pattern context, quotePat
is called.

The result of running the quoter on its arguments is spliced into
the program using Template Haskell's existing mechanisms for
splicing in code. Note that although Template Haskell does not
support pattern brackets, with this patch binding occurrences of
variables in patterns are supported. Quoters must also obey the same
stage restrictions as Template Haskell; in particular, in this
example quote may not be defined in the module where it is used as a
quasiquoter, but must be imported from another module.

Points to notice
~~~~~~~~~~~~~~~~
* The whole thing is enabled with the flag -XQuasiQuotes

* There is an accompanying patch to the template-haskell library. This
  involves one interface change:
	currentModule :: Q String
  is replaced by
	location :: Q Loc
  where Loc is a data type defined in TH.Syntax thus:
      data Loc
        = Loc { loc_filename :: String
	      , loc_package  :: String
	      , loc_module   :: String
	      , loc_start    :: CharPos
	      , loc_end      :: CharPos }

      type CharPos = (Int, Int)	-- Line and character position
 
  So you get a lot more info from 'location' than from 'currentModule'.
  The location you get is the location of the splice.
  
  This works in Template Haskell too of course, and lets a TH program
  generate much better error messages.

* There's also a new module in the template-haskell package called 
  Language.Haskell.TH.Quote, which contains support code for the
  quasi-quoting feature.

* Quasi-quote splices are run *in the renamer* because they can build 
  *patterns* and hence the renamer needs to see the output of running the
  splice.  This involved a bit of rejigging in the renamer, especially
  concerning the reporting of duplicate or shadowed names.

  (In fact I found and removed a few calls to checkDupNames in RnSource 
  that are redundant, becuase top-level duplicate decls are handled in
  RnNames.)

The type checker doesn't support lexically scoped type variables 
unless we are using the RelaxedPolyRec option.  Reasons: see
Note [Scoped tyvars] in TcBinds.

So I've changed DynFlags to add this implication, improved the 
documentation, and simplified the code in TcBinds somewhat.
(It's longer but only because of comments!)
 

Somehow we didn't have a separate flag for impredicativity; now we do.

Furthermore, Trac #2019 showed up a missing test for monotypes in data
constructor return types.  And I realised that we were even allowing
things like
	Num (forall a. a) => ...
which we definitely should not.  

This patch insists on monotypes in several places where we were (wrongly)
too liberal before.

Could be merged to 6.8 but no big deal.

  This patch implements generalised list comprehensions, as described in 
  the paper "Comprehensive comprehensions" (Peyton Jones & Wadler, Haskell
  Workshop 2007).  If you don't use the new comprehensions, nothing
  should change.
  
  The syntax is not exactly as in the paper; see the user manual entry 
  for details.
  
  You need an accompanying patch to the base library for this stuff 
  to work.
  
  The patch is the work of Max Bolingbroke [batterseapower@hotmail.com], 
  with some advice from Simon PJ.
  
  The related GHC Wiki page is 
    http://hackage.haskell.org/trac/ghc/wiki/SQLLikeComprehensions 

Merge to 6.8 branch

This patch implements three new features:
* view patterns (syntax: expression -> pat in a pattern)
* working versions of record wildcards and record puns
See the manual for detailed descriptions.

Other minor observable changes:
* There is a check prohibiting local fixity declarations
  when the variable being fixed is not defined in the same let
* The warn-unused-binds option now reports warnings for do and mdo stmts

Implementation notes: 

* The pattern renamer is now in its own module, RnPat, and the
implementation is now in a CPS style so that the correct context is
delivered to pattern expressions.

* These features required a fairly major upheaval to the renamer.
Whereas the old version used to collect up all the bindings from a let
(or top-level, or recursive do statement, ...) and put them into scope
before renaming anything, the new version does the collection as it
renames.  This allows us to do the right thing with record wildcard
patterns (which need to be expanded to see what names should be
collected), and it allows us to implement the desired semantics for view
patterns in lets.  This change had a bunch of domino effects brought on
by fiddling with the top-level renaming.

* Prior to this patch, there was a tricky bug in mkRecordSelId in HEAD,
which did not maintain the invariant necessary for loadDecl.  See note
[Tricky iface loop] for details.

Document the rules for pragmas that must occur at the top of a file.

Please merge this patch

This patch does a fairly major clean-up of the code that implements 'deriving.

* The big changes are in TcDeriv, which is dramatically cleaned up.
  In particular, there is a clear split into
	a) inference of instance contexts for deriving clauses
	b) generation of the derived code, given a context 
  Step (a) is skipped for standalone instance decls, which 
  have an explicitly provided context.

* The handling of "taggery", which is cooperative between TcDeriv and
  TcGenDeriv, is cleaned up a lot

* I have added documentation for standalone deriving (which was 
  previously wrong).

* The Haskell report is vague on exactly when a deriving clause should
  succeed.  Prodded by Conal I have loosened the rules slightly, thereyb
  making drv015 work again, and documented the rules in the user manual.

I believe this patch validates ok (once I've update the test suite)
and can go into the 6.8 branch.

This documentation was just duplicating what is in GHC.Prim now.

1. Record disambiguation (-fdisambiguate-record-fields)

In record construction and pattern matching (although not
in record updates) it is clear which field name is intended
even if there are several in scope.  This extension uses
the constructor to disambiguate.  Thus
	C { x=3 }
uses the 'x' field from constructor C (assuming there is one)
even if there are many x's in scope.


2. Record punning (-frecord-puns)

In a record construction or pattern match or update you can 
omit the "=" part, thus
	C { x, y }
This is just syntactic sugar for
	C { x=x, y=y }


3.  Dot-dot notation for records (-frecord-dot-dot)

In record construction or pattern match (but not update) 
you can use ".." to mean "all the remaining fields".  So

	C { x=v, .. }

means to fill in the remaining fields to give

	C { x=v, y=y }

(assuming C has fields x and y).  This might reasonably
considered very dodgy stuff.  For pattern-matching it brings
into scope a bunch of things that are not explictly mentioned;
and in record construction it just picks whatver 'y' is in
scope for the 'y' field.   Still, Lennart Augustsson really
wants it, and it's a feature that is extremely easy to explain.


Implementation
~~~~~~~~~~~~~~
I thought of using the "parent" field in the GlobalRdrEnv, but
that's really used for import/export and just isn't right for this.
For example, for import/export a field is a subordinate of the *type
constructor* whereas here we need to know what fields belong to a
particular *data* constructor.

The main thing is that we need to map a data constructor to its
fields, and we need to do so in the renamer.   For imported modules
it's easy: just look in the imported TypeEnv.  For the module being
compiled, we make a new field tcg_field_env in the TcGblEnv.
The important functions are
	RnEnv.lookupRecordBndr
	RnEnv.lookupConstructorFields

There is still a significant infelicity in the way the renamer
works on patterns, which I'll tackle next.


I also did quite a bit of refactoring in the representation of
record fields (mainly in HsPat).***END OF DESCRIPTION***

Place the long patch description above the ***END OF DESCRIPTION*** marker.
The first line of this file will be the patch name.


This patch contains the following changes:

M ./compiler/deSugar/Check.lhs -3 +5
M ./compiler/deSugar/Coverage.lhs -6 +7
M ./compiler/deSugar/DsExpr.lhs -6 +13
M ./compiler/deSugar/DsMeta.hs -8 +8
M ./compiler/deSugar/DsUtils.lhs -1 +1
M ./compiler/deSugar/MatchCon.lhs -2 +2
M ./compiler/hsSyn/Convert.lhs -3 +3
M ./compiler/hsSyn/HsDecls.lhs -9 +25
M ./compiler/hsSyn/HsExpr.lhs -13 +3
M ./compiler/hsSyn/HsPat.lhs -25 +63
M ./compiler/hsSyn/HsUtils.lhs -3 +3
M ./compiler/main/DynFlags.hs +6
M ./compiler/parser/Parser.y.pp -13 +17
M ./compiler/parser/RdrHsSyn.lhs -16 +18
M ./compiler/rename/RnBinds.lhs -2 +2
M ./compiler/rename/RnEnv.lhs -22 +82
M ./compiler/rename/RnExpr.lhs -34 +12
M ./compiler/rename/RnHsSyn.lhs -3 +2
M ./compiler/rename/RnSource.lhs -50 +78
M ./compiler/rename/RnTypes.lhs -50 +84
M ./compiler/typecheck/TcExpr.lhs -18 +18
M ./compiler/typecheck/TcHsSyn.lhs -20 +21
M ./compiler/typecheck/TcPat.lhs -8 +6
M ./compiler/typecheck/TcRnMonad.lhs -6 +15
M ./compiler/typecheck/TcRnTypes.lhs -2 +11
M ./compiler/typecheck/TcTyClsDecls.lhs -3 +4
M ./docs/users_guide/flags.xml +7
M ./docs/users_guide/glasgow_exts.xml +42

Two new -X flags, one for GADTs and one for relaxed polymorphic recursion

This also fixes a rather confusing error message that the Darcs folk 
tripped over.

We've often talked about having a separate flag for language extensions,
and now we have one. You can say

	-XImplicitParams
	-X=ImplicitParams
	-Ximplicit-params

as you like.  These replace the "-f" flags with similar names (though
the -f prefix will serve as a synonym for -X for a while).  

There's an optional "=", and the flag is normalised by removing hyphens
and lower-casing, so all the above variants mean the same thing.

The nomenclature is intended to match the LANGUAGE pramgas, which are
defined by Cabal.  So you can also say

	{-# LANGUAGE ImplicitParams #-}

But Cabal doesn't have as many language options as GHC does, so the -X
things are a superset of the LANGUAGE things.

The optional "=" applies to all flags that take an argument, so you can,
for example, say
	
	-pgmL=/etc/foo

I hope that's ok.  (It's an unforced change; just fitted in.)

I hope we'll add more -X flags, to replace the portmanteau -fglasgow-exts 
which does everything!  

I have updated the manual, but doubtless missed something.

See Trac #1426

- This adds the option to the flag reference and puts a stub pointing to the
  Haskell wiki page about type families into the section about type extensions.
- Once, the implementation has stabilised, the material from the wiki page 
  will be integreated into the user's manual.