1. 23 Jul, 2009 1 commit
    • simonpj@microsoft.com's avatar
      Add tuple sections as a new feature · 58521c72
      simonpj@microsoft.com authored
      This patch adds tuple sections, so that
      
      	(x,,z)  means   \y -> (x,y,z)
      
      Thanks for Max Bolinbroke for doing the hard work.
      
      In the end, instead of using two constructors in HsSyn, I used
      just one (still called ExplicitTuple) whose arguments can be
      	Present (LHsExpr id)
      or	Missing PostTcType
      
      While I was at it, I did a bit of refactoring too.
      58521c72
  2. 03 Jun, 2009 1 commit
    • simonpj@microsoft.com's avatar
      Allow RULES for seq, and exploit them · 90ce88a0
      simonpj@microsoft.com authored
      Roman found situations where he had
            case (f n) of _ -> e
      where he knew that f (which was strict in n) would terminate if n did.
      Notice that the result of (f n) is discarded. So it makes sense to
      transform to
            case n of _ -> e
      
      Rather than attempt some general analysis to support this, I've added
      enough support that you can do this using a rewrite rule:
      
        RULE "f/seq" forall n.  seq (f n) e = seq n e
      
      You write that rule.  When GHC sees a case expression that discards
      its result, it mentally transforms it to a call to 'seq' and looks for
      a RULE.  (This is done in Simplify.rebuildCase.)  As usual, the
      correctness of the rule is up to you.
      
      This patch implements the extra stuff.  I have not documented it explicitly
      in the user manual yet... let's see how useful it is first.
      
      The patch looks bigger than it is, because
        a) Comments; see esp MkId Note [seqId magic]
      
        b) Some refactoring.  Notably, I moved the special desugaring for
           seq from MkCore back into DsUtils where it properly belongs.
           (It's really a desugaring thing, not a CoreSyn invariant.)
      
        c) Annoyingly, in a RULE left-hand side we need to be careful that
           the magical desugaring done in MkId Note [seqId magic] item (c) 
           is *not* done on the LHS of a rule. Or rather, we arrange to 
           un-do it, in DsBinds.decomposeRuleLhs.
      
      90ce88a0
  3. 31 Mar, 2009 1 commit
  4. 30 Mar, 2009 1 commit
  5. 20 Sep, 2008 1 commit
    • simonpj@microsoft.com's avatar
      Tidy up the treatment of dead binders · 7e8cba32
      simonpj@microsoft.com authored
      This patch does a lot of tidying up of the way that dead variables are
      handled in Core.  Just the sort of thing to do on an aeroplane.
      
      * The tricky "binder-swap" optimisation is moved from the Simplifier
        to the Occurrence Analyser.  See Note [Binder swap] in OccurAnal.
        This is really a nice change.  It should reduce the number of
        simplifier iteratoins (slightly perhaps).  And it means that
        we can be much less pessimistic about zapping occurrence info
        on binders in a case expression.  
      
      * For example:
      	case x of y { (a,b) -> e }
        Previously, each time around, even if y,a,b were all dead, the
        Simplifier would pessimistically zap their OccInfo, so that we
        can't see they are dead any more.  As a result virtually no 
        case expression ended up with dead binders.  This wasn't Bad
        in itself, but it always felt wrong.
      
      * I added a check to CoreLint to check that a dead binder really
        isn't used.  That showed up a couple of bugs in CSE. (Only in
        this sense -- they didn't really matter.)
        
      * I've changed the PprCore printer to print "_" for a dead variable.
        (Use -dppr-debug to see it again.)  This reduces clutter quite a
        bit, and of course it's much more useful with the above change.
      
      * Another benefit of the binder-swap change is that I could get rid of
        the Simplifier hack (working, but hacky) in which the InScopeSet was
        used to map a variable to a *different* variable. That allowed me
        to remove VarEnv.modifyInScopeSet, and to simplify lookupInScopeSet
        so that it doesn't look for a fixpoint.  This fixes no bugs, but 
        is a useful cleanup.
      
      * Roman pointed out that Id.mkWildId is jolly dangerous, because
        of its fixed unique.  So I've 
      
           - localied it to MkCore, where it is private (not exported)
      
           - renamed it to 'mkWildBinder' to stress that you should only
             use it at binding sites, unless you really know what you are
             doing
      
           - provided a function MkCore.mkWildCase that emodies the most
             common use of mkWildId, and use that elsewhere
      
         So things are much better
      
      * A knock-on change is that I found a common pattern of localising
        a potentially global Id, and made a function for it: Id.localiseId
      7e8cba32
  6. 15 Sep, 2008 1 commit
  7. 31 Jul, 2008 1 commit
  8. 20 Jul, 2008 1 commit
  9. 01 Jul, 2008 1 commit
  10. 05 Jun, 2008 2 commits
    • simonpj@microsoft.com's avatar
    • simonpj@microsoft.com's avatar
      Add non-recursive let-bindings for types · 1b1190e0
      simonpj@microsoft.com authored
      This patch adds to Core the ability to say
      	let a = Int in <body>
      where 'a' is a type variable.  That is: a type-let.
      See Note [Type let] in CoreSyn.
      
      * The binding is always non-recursive
      * The simplifier immediately eliminates it by substitution 
      
      So in effect a type-let is just a delayed substitution.  This is convenient
      in a couple of places in the desugarer, one existing (see the call to
      CoreTyn.mkTyBind in DsUtils), and one that's in the next upcoming patch.
      
      The first use in the desugarer was previously encoded as
      	(/\a. <body>) Int
      rather that eagerly substituting, but that was horrid because Core Lint
      had do "know" that a=Int inside <body> else it would bleat.  Expressing
      it directly as a 'let' seems much nicer.
      
      1b1190e0
  11. 16 May, 2008 1 commit
    • simonpj@microsoft.com's avatar
      Improve the treatment of 'seq' (Trac #2273) · 4e36a8b1
      simonpj@microsoft.com authored
      Trac #2273 showed a case in which 'seq' didn't cure the space leak
      it was supposed to.  This patch does two things to help
      
      a) It removes a now-redundant special case in Simplify, which
         switched off the case-binder-swap in the early stages.  This
         isn't necessary any more because FloatOut has improved since
         the Simplify code was written.  And switching off the binder-swap
         is harmful for seq.
      
      However fix (a) is a bit fragile, so I did (b) too:
      
      b) Desugar 'seq' specially.  See Note [Desugaring seq (2)] in DsUtils
         This isn't very robust either, since it's defeated by abstraction, 
         but that's not something GHC can fix; the programmer should use
         a let! instead.
      4e36a8b1
  12. 12 Apr, 2008 1 commit
  13. 23 Mar, 2008 1 commit
  14. 28 Feb, 2008 1 commit
  15. 17 Jan, 2008 1 commit
  16. 20 Dec, 2007 1 commit
    • simonpj@microsoft.com's avatar
      Implement generalised list comprehensions · 67cb4091
      simonpj@microsoft.com authored
        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 
      67cb4091
  17. 10 Oct, 2007 1 commit
    • Dan Licata's avatar
      View patterns, record wildcards, and record puns · 6a05ec5e
      Dan Licata authored
      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.
      6a05ec5e
  18. 04 Sep, 2007 1 commit
  19. 03 Sep, 2007 1 commit
  20. 01 Sep, 2007 1 commit
  21. 21 Jun, 2007 1 commit
    • David Himmelstrup's avatar
      Add several new record features · 2eb04ca0
      David Himmelstrup authored
      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
      2eb04ca0
  22. 11 May, 2007 1 commit
  23. 02 May, 2007 1 commit
  24. 24 Apr, 2007 1 commit
    • Simon Marlow's avatar
      Breakpoints: get the names of the free variables right · 367b0590
      Simon Marlow authored
      Previously we relied on the names of the Ids attached to a tick being
      the same as the names of the original variables in the source code.
      Sometimes this worked, sometimes it didn't because the simplifier
      would inline away the Id.  So now we do this properly and retain the
      original OccNames from the source code for each breakpoint, and use
      these to construct the new Ids when we stop.
      
      Doing this involved moving the tracking of in-scope variables from the
      desugarer to the coverage pass.
      367b0590
  25. 17 Apr, 2007 1 commit
    • Simon Marlow's avatar
      Re-working of the breakpoint support · cdce6477
      Simon Marlow authored
      This is the result of Bernie Pope's internship work at MSR Cambridge,
      with some subsequent improvements by me.  The main plan was to
      
       (a) Reduce the overhead for breakpoints, so we could enable 
           the feature by default without incurrent a significant penalty
       (b) Scatter more breakpoint sites throughout the code
      
      Currently we can set a breakpoint on almost any subexpression, and the
      overhead is around 1.5x slower than normal GHCi.  I hope to be able to
      get this down further and/or allow breakpoints to be turned off.
      
      This patch also fixes up :print following the recent changes to
      constructor info tables.  (most of the :print tests now pass)
      
      We now support single-stepping, which just enables all breakpoints.
      
        :step <expr>     executes <expr> with single-stepping turned on
        :step            single-steps from the current breakpoint
      
      The mechanism is quite different to the previous implementation.  We
      share code with the HPC (haskell program coverage) implementation now.
      The coverage pass annotates source code with "tick" locations which
      are tracked by the coverage tool.  In GHCi, each "tick" becomes a
      potential breakpoint location.
      
      Previously breakpoints were compiled into code that magically invoked
      a nested instance of GHCi.  Now, a breakpoint causes the current
      thread to block and control is returned to GHCi.
      
      See the wiki page for more details and the current ToDo list:
      
        http://hackage.haskell.org/trac/ghc/wiki/NewGhciDebugger
      cdce6477
  26. 16 Mar, 2007 1 commit
    • simonpj@microsoft.com's avatar
      Desugar applications of 'seq' specially; fix Trac #1031 · 3bdbcf16
      simonpj@microsoft.com authored
      Merge to 6.6 branch.  Test case is dsrun014.
      
      Note [Desugaring seq]  cf Trac #1031
      ~~~~~~~~~~~~~~~~~~~~~
         f x y = x `seq` (y `seq` (# x,y #))
      
      The [CoreSyn let/app invariant] means that, other things being equal, because 
      the argument to the outer 'seq' has an unlifted type, we'll use call-by-value thus:
      
         f x y = case (y `seq` (# x,y #)) of v -> x `seq` v
      
      But that is bad for two reasons: 
        (a) we now evaluate y before x, and 
        (b) we can't bind v to an unboxed pair
      
      Seq is very, very special!  So we recognise it right here, and desugar to
      	case x of _ -> case y of _ -> (# x,y #)
      
      The special case would be valid for all calls to 'seq', but it's only *necessary*
      for ones whose second argument has an unlifted type. So we only catch the latter
      case here, to avoid unnecessary tests.
      3bdbcf16
  27. 05 Feb, 2007 1 commit
    • simonpj@microsoft.com's avatar
      Establish the CoreSyn let/app invariant · 7b01315d
      simonpj@microsoft.com authored
      This patch clears up a long-standing wart.   For some time it's been the
      case that 
      	the RHS of a non-recursive let can be unlifed iff 
      	the RHS is ok-for-speculation
      
      This patch extends the invariant to the argument of an App, and 
      establishes it by the smart constructors mkDsApp, mkDsApps in the desugarer.
      
      Once established, it should be maintained by the optimiser.
      
      This tides up some awkward cases, notably in exprIsHNF, and I think it
      fixes a outright strictness bug in Simplify.prepareRhs.
      7b01315d
  28. 13 Dec, 2006 1 commit
  29. 29 Nov, 2006 1 commit
    • andy@galois.com's avatar
      TickBox representation change · 8100cd43
      andy@galois.com authored
      This changes the internal representation of TickBoxes,
      from
              Note (TickBox "module" n)  <expr>
      into
      
              case tick<module,n> of
                _ -> <expr>
      
      tick has type :: #State #World, when the module and tick numbe
      are stored inside IdInfo.
      
      Binary tick boxes change from
      
               Note (BinaryTickBox "module" t f) <expr>
      
      into
      
                btick<module,t,f> <expr>
      
      btick has type :: Bool -> Bool, with the module and tick number
      stored inside IdInfo.
      8100cd43
  30. 24 Oct, 2006 1 commit
    • andy@galois.com's avatar
      Haskell Program Coverage · d5934bbb
      andy@galois.com authored
      This large checkin is the new ghc version of Haskell
      Program Coverage, an expression-level coverage tool for Haskell.
      
      Parts:
      
       - Hpc.[ch] - small runtime support for Hpc; reading/writing *.tix files.
       - Coverage.lhs - Annotates the HsSyn with coverage tickboxes.
        - New Note's in Core,
            - TickBox      -- ticked on entry to sub-expression
            - BinaryTickBox  -- ticked on exit to sub-expression, depending
      	       	     -- on the boolean result.
      
        - New Stg level TickBox (no BinaryTickBoxes, though) 
      
      You can run the coverage tool with -fhpc at compile time. 
      Main must be compiled with -fhpc. 
      				      
      d5934bbb
  31. 11 Oct, 2006 1 commit
    • Simon Marlow's avatar
      Module header tidyup, phase 1 · 49c98d14
      Simon Marlow authored
      This patch is a start on removing import lists and generally tidying
      up the top of each module.  In addition to removing import lists:
      
         - Change DATA.IOREF -> Data.IORef etc.
         - Change List -> Data.List etc.
         - Remove $Id$
         - Update copyrights
         - Re-order imports to put non-GHC imports last
         - Remove some unused and duplicate imports
      49c98d14
  32. 03 Oct, 2006 1 commit
  33. 20 Sep, 2006 1 commit
    • chak@cse.unsw.edu.au.'s avatar
      Indexed newtypes · 27897431
      chak@cse.unsw.edu.au. authored
      Mon Sep 18 19:24:27 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
        * Indexed newtypes
        Thu Aug 31 22:09:21 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
          * Indexed newtypes
          - This patch makes indexed newtypes work
          - Only lightly tested
          - We need to distinguish between open and closed newtypes in a number of 
            places, because looking through newtypes doesn't work easily for open ones.
      27897431
  34. 04 Aug, 2006 1 commit
  35. 07 Apr, 2006 1 commit
    • Simon Marlow's avatar
      Reorganisation of the source tree · 0065d5ab
      Simon Marlow authored
      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.
      0065d5ab
  36. 09 Feb, 2006 1 commit
    • simonpj@microsoft.com's avatar
      Fix desugaring of unboxed tuples · 3c245de9
      simonpj@microsoft.com authored
      This patch is a slightly-unsatisfactory fix to desugaring unboxed
      tuples; it fixes ds057 which has been failing for some time.
      
      Unsatisfactory because rather ad hoc -- but that applies to lots
      of the unboxed tuple stuff. 
      3c245de9
  37. 03 Feb, 2006 1 commit
  38. 02 Feb, 2006 1 commit
  39. 25 Jan, 2006 1 commit
    • simonpj@microsoft.com's avatar
      Simon's big boxy-type commit · ac10f840
      simonpj@microsoft.com authored
      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.
      ac10f840