1. 31 Oct, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-10-31 15:22:53 by simonpj] · 61bfd5dd
      simonpj authored
      ------------------------------------------
      	Improved handling of scoped type variables
      	------------------------------------------
      
      The main effect of this commit is to allow scoped type variables
      in pattern bindings, thus
      
      	(x::a, y::b) = e
      
      This was illegal, but now it's ok.  a and b have the same scope
      as x and y.
      
      
      On the way I beefed up the info inside a type variable
      (TcType.TyVarDetails; c.f. IdInfo.GlobalIdDetails) which
      helps to improve error messages. Hence the wide ranging changes.
      Pity about the extra loop from Var to TcType, but can't be helped.
      61bfd5dd
  2. 25 Oct, 2001 2 commits
    • simonpj's avatar
      [project @ 2001-10-25 14:30:43 by simonpj] · d5f94cc1
      simonpj authored
      -------------------------------------------------------
        Correct an error in the handling of implicit parameters
        -------------------------------------------------------
      
      	MERGE WITH STABLE BRANCH UNLESS HARD TO DO
      
      Mark Shields discovered a bug in the way that implicit parameters
      are dealt with by the type checker.  It's all a bit subtle, and
      is extensively documented in TcSimplify.lhs.
      
      This commit makes the code both simpler and more correct.  It subtly
      changes the way in which type signatures are treated, but not in a way
      anyone would notice: see notes with "Question 2: type signatures"
      in TcSimplify.lhs.
      d5f94cc1
    • simonpj's avatar
      [project @ 2001-10-25 09:58:39 by simonpj] · 2007c7e6
      simonpj authored
      Cosmetica
      2007c7e6
  3. 17 Oct, 2001 1 commit
  4. 28 Aug, 2001 1 commit
  5. 20 Aug, 2001 1 commit
  6. 25 Jul, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-07-25 15:55:30 by simonpj] · 7fde87b3
      simonpj authored
      -----------------------------------------
      	Fix a bug in the monomorphism restriction
      	------------------------------------------
      
      Thanks for Koen for reporting this bug.
      
      In tcSimplifyRestricted, I wrongly called tcSimpifyToDicts,
      whereas actually we have to simplfy further than simply to
      a dictionary.
      
      The test for this is in typecheck/should_compile/tc132.hs
      7fde87b3
  7. 23 Jul, 2001 1 commit
  8. 17 Jul, 2001 1 commit
  9. 12 Jul, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-07-12 16:21:22 by simonpj] · ab46fd8e
      simonpj authored
      --------------------------------------------
      	Fix another bug in the squash-newtypes story.
      	--------------------------------------------
      
      [This one was spotted by Marcin, and is now enshrined in test tc130.]
      
      The desugarer straddles the boundary between the type checker and
      Core, so it sometimes needs to look through newtypes/implicit parameters
      and sometimes not.  This is really a bit painful, but I can't think of
      a better way to do it.
      
      The only simple way to fix things was to pass a bit more type
      information in the HsExpr type, from the type checker to the desugarer.
      That led to the non-local changes you can see.
      
      On the way I fixed one other thing.  In various HsSyn constructors
      there is a Type that is bogus (bottom) before the type checker, and
      filled in with a real type by the type checker.  In one place it was
      a (Maybe Type) which was Nothing before, and (Just ty) afterwards.
      I've defined a type synonym HsTypes.PostTcType for this, and a named
      bottom value HsTypes.placeHolderType to use when you want the bottom
      value.
      ab46fd8e
  10. 25 Jun, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-06-25 08:01:16 by simonpj] · a12bed53
      simonpj authored
      ----------------------------------
      	Fix a predicate-simplification bug
      	----------------------------------
      
      Fixes a bug pointed out by Marcin
      
          data R = R {f :: Int}
          foo:: (?x :: Int) => R -> R
          foo r = r {f = ?x}
      
          Test.hs:4:
      	Could not deduce `?x :: Int' from the context ()
      	arising from use of implicit parameter `?x' at Test.hs:4
      	In the record update: r {f = ?x}
      	In the definition of `foo': r {f = ?x}
      
      The predicate simplifier was declining to 'inherit' an
      implicit parameter.  This is right for a let-binding, but
      wrong for an expression binding.  For example, a simple
      expression type signature:
      
      		(?x + 1) :: Int
      
      This was rejected because the ?x constraint could not be
      floated out -- but that's wrong for expressions.
      a12bed53
  11. 03 May, 2001 3 commits
    • simonpj's avatar
      [project @ 2001-05-03 12:33:50 by simonpj] · bbc670f4
      simonpj authored
      ****	MERGE WITH 5.00 BRANCH     ********
      
      	--------------------------------
      	Monomorphism restriction for implicit parameters
      	--------------------------------
      
      This commit tidies up the way in which monomorphic bindings
      are dealt with, incidentally fixing a bug to do with implicit
      parameters.
      
      The tradeoffs concerning monomorphism and implicit paramters are
      now documented in TcSimplify.lhs, and all the strategic choices
      are made there (rather than in TcBinds where they were before).
      
      I've continued with choice (B) -- which Jeff first implemented --
      because that's what Hugs does, lacking any other consensus.
      bbc670f4
    • simonpj's avatar
      [project @ 2001-05-03 09:32:48 by simonpj] · b473b6c2
      simonpj authored
      ------------------------------------------------
      	Dramatically improve the error messages arising
      	from failed unifications triggered by 'improvement'
      	------------------------------------------------
      
      A bit more plumbing in FunDeps, and consequential wibbles elsewhere
      
      Changes this:
      
          Couldn't match `Int' against `[(String, Int)]'
      	Expected type: Int
      	Inferred type: [(String, Int)]
      
      to this:
      
          Foo.hs:8:
      	Couldn't match `Int' against `[(String, Int)]'
      	    Expected type: Int
      	    Inferred type: [(String, Int)]
      	When using functional dependencies to combine
      	  ?env :: Int, arising from a type signature at Foo.hs:7
      	  ?env :: [(String, Int)],
      	    arising from use of implicit parameter `?env' at Foo.hs:8
      	When generalising the types for ident
      b473b6c2
    • simonpj's avatar
      [project @ 2001-05-03 08:13:25 by simonpj] · cd7dc9b1
      simonpj authored
      ****	MERGE WITH 5.00 BRANCH     ********
      
      	--------------------------------
      	Fix a bad implicit parameter bug
      	--------------------------------
      
      TcSimplify.tcSimplifyIPs was just completely wrong; it wasn't
      doing improvement properly nor binding values properly. Sigh.
      
      To make this work nicely I added
      	Inst.instName :: Inst -> Name
      cd7dc9b1
  12. 30 Apr, 2001 1 commit
  13. 12 Apr, 2001 1 commit
    • lewie's avatar
      [project @ 2001-04-12 21:29:43 by lewie] · ebf2c802
      lewie authored
      Don't use the same simplify code for both restricted and unrestricted
      bindings.  In particular, a restricted binding shouldn't try to capture
      implicit params.
      ebf2c802
  14. 05 Apr, 2001 1 commit
  15. 13 Mar, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-03-13 14:58:25 by simonpj] · 788faebb
      simonpj authored
      ----------------
      	Nuke ClassContext
      	----------------
      
      This commit tidies up a long-standing inconsistency in GHC.
      The context of a class or instance decl used to be restricted
      to predicates of the form
      	C t1 .. tn
      with
      	type ClassContext = [(Class,[Type])]
      
      but everywhere else in the compiler we used
      
      	type ThetaType = [PredType]
      where PredType can be any sort of constraint (= predicate).
      
      The inconsistency actually led to a crash, when compiling
      	class (?x::Int) => C a where {}
      
      I've tidied all this up by nuking ClassContext altogether, and using
      PredType throughout.  Lots of modified files, but all in
      more-or-less trivial ways.
      
      I've also added a check that the context of a class or instance
      decl doesn't include a non-inheritable predicate like (?x::Int).
      
      Other things
      
       * rename constructor 'Class' from type TypeRep.Pred to 'ClassP'
         (makes it easier to grep for)
      
       * rename constructor HsPClass  => HsClassP
      		      HsPIParam => HsIParam
      788faebb
  16. 28 Feb, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-02-28 17:17:55 by simonpj] · 56d75e0b
      simonpj authored
      Improve rule matching
      
      When doing constraint simplification on the LHS of a rule,
      we *don't* want to do superclass commoning up.  Consider
      
      	fromIntegral :: (Integral a, Num b) => a -> b
      	{-# RULES "foo"  fromIntegral = id :: Int -> Int #-}
      
      Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
      want to get
      
      	forall dIntegralInt.
      	fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
      
      because the scsel (super class selection) will mess up matching.
      Instead we want
      
      	forall dIntegralInt, dNumInt.
      	fromIntegral Int Int dIntegralInt dNumInt = id Int
      
      
      TcSimplify.tcSimplifyToDicts is the relevant function, but I had
      to generalise the main simplification loop a little (adding the
      type WantSCs).
      56d75e0b
  17. 26 Feb, 2001 1 commit
    • simonmar's avatar
      [project @ 2001-02-26 15:06:57 by simonmar] · 1c62b517
      simonmar authored
      Implement do-style bindings on the GHCi command line.
      
      The syntax for a command-line is exactly that of a do statement, with
      the following meanings:
      
        - `pat <- expr'
          performs expr, and binds each of the variables in pat.
      
        - `let pat = expr; ...'
          binds each of the variables in pat, doesn't do any evaluation
      
        - `expr'
          behaves as `it <- expr' if expr is IO-typed, or `let it = expr'
          followed by `print it' otherwise.
      1c62b517
  18. 20 Feb, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-02-20 09:42:50 by simonpj] · 22ffc06a
      simonpj authored
      Typechecking [TcModule, TcBinds, TcHsSyn, TcInstDcls, TcSimplify]
      ~~~~~~~~~~~~
      * Fix a bug in TcSimplify that broke functional dependencies.
        Interleaving unification and context reduction is trickier 
        than I thought.  Comments in the code amplify.  
      
      * Fix a functional-dependency bug, that meant that this pgm:
      	class C a b | a -> b where f :: a -> b
      	
      	g :: (C a b, Eq b) => a -> Bool
      	g x = f x == f x
        gave an ambiguity error report.  I'm afraid I've forgotten
        what the problem was.
      
      
      * Correct the implementation of the monomorphism restriction,
        in TcBinds.generalise.  This fixes Marcin's bug report:
      	test1 :: Eq a => a -> b -> b
      	test1 x y = y
      
      	test2 = test1 (3::Int)
        Previously we were erroneously inferring test2 :: () -> ()
      
      * Make the "unf_env" that is looped round in TcModule go round
        in a big loop, not just round tcImports.  This matters when
        we have mutually recursive modules, so that the Ids bound in
        the source code may appear in the imports.  Sigh.  But no big
        deal.
      
        It does mean that you have to be careful not to call isLocalId,
        isDataConId etc, because they consult the IdInfo of an Id, which 
        in turn may be determined by the loop-tied unf_env.
      22ffc06a
  19. 30 Jan, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-01-30 09:53:11 by simonpj] · ade2eac4
      simonpj authored
      More on functional dependencies
      
      My last commit allowed this:
      
      	instance C a b => C [a] [b] where ...
      
      if we have
      
      	class C a b | a -> b
      
      This commit completes the change, by making the 
      improvement stages improve only the 'shape' of the second
      argument of C.  
      
      I also had to change the iteration in TcSimplify -- see
      the comments in TcSimplify.inferLoop.
      ade2eac4
  20. 29 Jan, 2001 1 commit
  21. 26 Jan, 2001 1 commit
  22. 25 Jan, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-01-25 17:54:24 by simonpj] · 4e342297
      simonpj authored
      ------------------------------------
      	   Mainly FunDeps (23 Jan 01)
      	------------------------------------
      
      This commit re-engineers the handling of functional dependencies.
      A functional dependency is no longer an Inst; instead, the necessary
      dependencies are snaffled out of their Class when necessary.
      
      As part of this exercise I found that I had to re-work how to do generalisation
      in a binding group.  There is rather exhaustive documentation on the new Plan
      at the top of TcSimplify.
      
      	******************
      	WARNING: I have compiled all the libraries with this new compiler
      		 and all looks well, but I have not run many programs.
      		 Things may break.  Let me know if so.
      	******************
      
      The main changes are these:
      
      1.  typecheck/TcBinds and TcSimplify have a lot of changes due to the
          new generalisation and context reduction story.  There are extensive
          comments at the start of TcSimplify
      
      2.  typecheck/TcImprove is removed altogether.  Instead, improvement is
          interleaved with context reduction (until a fixpoint is reached).
          All this is done in TcSimplify.
      
      3.  types/FunDeps has new exports
      	* 'improve' does improvement, returning a list of equations
      	* 'grow' and 'oclose' close a list of type variables wrt a set of
      	  PredTypes, but in slightly different ways.  Comments in file.
      
      4.  I improved the way in which we check that main::IO t.  It's tidier now.
      
      In addition
      
      *   typecheck/TcMatches:
      	a) Tidy up, introducing a common function tcCheckExistentialPat
      
      	b) Improve the typechecking of parallel list comprehensions,
      	   which wasn't quite right before.  (see comments with tcStmts)
      
      	WARNING: (b) is untested!  Jeff, you might want to check.
      
      *   Numerous other incidental changes in the typechecker
      
      *   Manuel found that rules don't fire well when you have partial applications
          from overloading.  For example, we may get
      
      	f a (d::Ord a) = let m_g = g a d
      			 in
      			 \y :: a -> ...(m_g (h y))...
      
          The 'method' m_g doesn't get inlined because (g a d) might be a redex.
          Yet a rule that looks like
      		g a d (h y) = ...
          won't fire because that doesn't show up.  One way out would be to make
          the rule matcher a bit less paranoid about duplicating work, but instead
          I've added a flag
      			-fno-method-sharing
          which controls whether we generate things like m_g in the first place.
          It's not clear that they are a win in the first place.
      
          The flag is actually consulted in Inst.tcInstId
      4e342297
  23. 03 Jan, 2001 1 commit
  24. 14 Nov, 2000 1 commit
  25. 13 Nov, 2000 1 commit
  26. 10 Nov, 2000 1 commit
    • simonpj's avatar
      [project @ 2000-11-10 15:12:50 by simonpj] · f23ba2b2
      simonpj authored
      1.	Outputable.PprStyle now carries a bit more information
      	In particular, the printing style tells whether to print
      	a name in unqualified form.  This used to be embedded in
      	a Name, but since Names now outlive a single compilation unit,
      	that's no longer appropriate.
      
      	So now the print-unqualified predicate is passed in the printing
      	style, not embedded in the Name.
      
         2.	I tidied up HscMain a little.  Many of the showPass messages
      	have migraged into the repective pass drivers
      f23ba2b2
  27. 23 Oct, 2000 1 commit
  28. 17 Oct, 2000 1 commit
  29. 16 Oct, 2000 1 commit
  30. 12 Oct, 2000 1 commit
  31. 03 Oct, 2000 1 commit
    • simonpj's avatar
      [project @ 2000-10-03 08:43:00 by simonpj] · 710e2074
      simonpj authored
      --------------------------------------
      	Adding generics		SLPJ Oct 2000
      	--------------------------------------
      
      This big commit adds Hinze/PJ-style generic class definitions, based
      on work by Andrei Serjantov.  For example:
      
        class Bin a where
          toBin   :: a -> [Int]
          fromBin :: [Int] -> (a, [Int])
      
          toBin {| Unit |}    Unit	  = []
          toBin {| a :+: b |} (Inl x)   = 0 : toBin x
          toBin {| a :+: b |} (Inr y)   = 1 : toBin y
          toBin {| a :*: b |} (x :*: y) = toBin x ++ toBin y
      
      
          fromBin {| Unit |}    bs      = (Unit, bs)
          fromBin {| a :+: b |} (0:bs)  = (Inl x, bs')    where (x,bs') = fromBin bs
          fromBin {| a :+: b |} (1:bs)  = (Inr y, bs')    where (y,bs') = fromBin bs
          fromBin {| a :*: b |} bs  	  = (x :*: y, bs'') where (x,bs' ) = fromBin bs
      							  (y,bs'') = fromBin bs'
      
      Now we can say simply
      
        instance Bin a => Bin [a]
      
      and the compiler will derive the appropriate code automatically.
      
      		(About 9k lines of diffs.  Ha!)
      
      
      Generic related things
      ~~~~~~~~~~~~~~~~~~~~~~
      
      * basicTypes/BasicTypes: The EP type (embedding-projection pairs)
      
      * types/TyCon:
      	An extra field in an algebraic tycon (genInfo)
      
      * types/Class, and hsSyn/HsBinds:
      	Each class op (or ClassOpSig) carries information about whether
      	it  	a) has no default method
      		b) has a polymorphic default method
      		c) has a generic default method
      	There's a new data type for this: Class.DefMeth
      
      * types/Generics:
      	A new module containing good chunk of the generic-related code
      	It has a .hi-boot file (alas).
      
      * typecheck/TcInstDcls, typecheck/TcClassDcl:
      	Most of the rest of the generics-related code
      
      * hsSyn/HsTypes:
      	New infix type form to allow types of the form
      		data a :+: b = Inl a | Inr b
      
      * parser/Parser.y, Lex.lhs, rename/ParseIface.y:
      	Deal with the new syntax
      
      * prelude/TysPrim, TysWiredIn:
      	Need to generate generic stuff for the wired-in TyCons
      
      * rename/RnSource RnBinds:
      	A rather gruesome hack to deal with scoping of type variables
      	from a generic patterns.  Details commented in the ClassDecl
      	case of RnSource.rnDecl.
      
      	Of course, there are many minor renamer consequences of the
      	other changes above.
      
      * lib/std/PrelBase.lhs
      	Data type declarations for Unit, :+:, :*:
      
      
      Slightly unrelated housekeeping
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      * hsSyn/HsDecls:
      	ClassDecls now carry the Names for their implied declarations
      	(superclass selectors, tycon, etc) in a list, rather than
      	laid out one by one.  This simplifies code between the parser
      	and the type checker.
      
      * prelude/PrelNames, TysWiredIn:
      	All the RdrNames are now together in PrelNames.
      
      * utils/ListSetOps:
      	Add finite mappings based on equality and association lists (Assoc a b)
      	Move stuff from List.lhs that is related
      710e2074
  32. 22 Sep, 2000 1 commit
    • simonpj's avatar
      [project @ 2000-09-22 15:56:12 by simonpj] · 1bba522f
      simonpj authored
      --------------------------------------------------
      	Tidying up HsLit, and making it possible to define
      		your own numeric library
      
      		Simon PJ 22 Sept 00
      	--------------------------------------------------
      
      ** NOTE: I did these changes on the aeroplane.  They should compile,
      	 and the Prelude still compiles OK, but it's entirely 
      	 possible that I've broken something
      
      The original reason for this many-file but rather shallow
      commit is that it's impossible in Haskell to write your own
      numeric library.  Why?  Because when you say '1' you get 
      (Prelude.fromInteger 1), regardless of what you hide from the
      Prelude, or import from other libraries you have written.  So the
      idea is to extend the -fno-implicit-prelude flag so that 
      in addition to no importing the Prelude, you can rebind 
      	fromInteger	-- Applied to literal constants
      	fromRational	-- Ditto
      	negate		-- Invoked by the syntax (-x)
      	the (-) used when desugaring n+k patterns
      
      After toying with other designs, I eventually settled on a simple,
      crude one: rather than adding a new flag, I just extended the
      semantics of -fno-implicit-prelude so that uses of fromInteger,
      fromRational and negate are all bound to "whatever is in scope" 
      rather than "the fixed Prelude functions".  So if you say
      
      	{-# OPTIONS -fno-implicit-prelude #-}
      	module M where
       	import MyPrelude( fromInteger )
      
      	x = 3
      
      the literal 3 will use whatever (unqualified) "fromInteger" is in scope,
      in this case the one gotten from MyPrelude.
      
      
      On the way, though, I studied how HsLit worked, and did a substantial tidy
      up, deleting quite a lot of code along the way.  In particular.
      
      * HsBasic.lhs is renamed HsLit.lhs.  It defines the HsLit type.
      
      * There are now two HsLit types, both defined in HsLit.
      	HsLit for non-overloaded literals (like 'x')
      	HsOverLit for overloaded literals (like 1 and 2.3)
      
      * HsOverLit completely replaces Inst.OverloadedLit, which disappears.
        An HsExpr can now be an HsOverLit as well as an HsLit.
      
      * HsOverLit carries the Name of the fromInteger/fromRational operation,
        so that the renamer can help with looking up the unqualified name 
        when -fno-implicit-prelude is on.  Ditto the HsExpr for negation.
        It's all very tidy now.
      
      * RdrHsSyn contains the stuff that handles -fno-implicit-prelude
        (see esp RdrHsSyn.prelQual).  RdrHsSyn also contains all the "smart constructors"
        used by the parser when building HsSyn.  See for example RdrHsSyn.mkNegApp
        (previously the renamer (!) did the business of turning (- 3#) into -3#).
      
      * I tidied up the handling of "special ids" in the parser.  There's much
        less duplication now.
      
      * Move Sven's Horner stuff to the desugarer, where it belongs.  
        There's now a nice function DsUtils.mkIntegerLit which brings together
        related code from no fewer than three separate places into one single
        place.  Nice!
      
      * A nice tidy-up in MatchLit.partitionEqnsByLit became possible.
      
      * Desugaring of HsLits is now much tidier (DsExpr.dsLit)
      
      * Some stuff to do with RdrNames is moved from ParseUtil.lhs to RdrHsSyn.lhs,
        which is where it really belongs.
      
      * I also removed 
      	many unnecessary imports from modules 
      	quite a bit of dead code
        in divers places
      1bba522f
  33. 14 Jul, 2000 2 commits
    • lewie's avatar
      [project @ 2000-07-14 23:54:06 by lewie] · f6d9b940
      lewie authored
      Functional Dependencies were not getting simplified away when the dictionary
      that generated them was simplified by instance resolution.  Fixed.
      f6d9b940
    • simonpj's avatar
      [project @ 2000-07-14 08:17:36 by simonpj] · 77a8c0db
      simonpj authored
      This commit completely re-does the kind-inference mechanism.
      Previously it was inter-wound with type inference, but that was
      always hard to understand, and it finally broke when we started
      checking for ambiguity when type-checking a type signature (details
      irrelevant).
      
      So now kind inference is more clearly separated, so that it never
      takes place at the same time as type inference.  The biggest change
      is in TcTyClsDecls, which does the kind inference for a group of
      type and class declarations.  It now contains comments to explain
      how it all works.
      
      There are also comments in TypeRep which describes the slightly
      tricky way in which we deal with the fact that kind 'type' (written
      '*') actually has 'boxed type' and 'unboxed type' as sub-kinds.
      The whole thing is a bit of a hack, because we don't really have 
      sub-kinding, but it's less of a hack than before.
      
      A lot of general tidying up happened at the same time.
      In particular, I removed some dead code here and there
      77a8c0db
  34. 11 Jul, 2000 1 commit
  35. 07 Jul, 2000 1 commit
    • simonpj's avatar
      [project @ 2000-07-07 12:13:43 by simonpj] · 7bb06950
      simonpj authored
      This commit moves the instance environment out of the Class data
      structure, where it was immutable, to part of the type-checker
      environment.  This change is absolutely essential as part of
      our move to GHCi, and I think it's also going to be necessary
      for Andrei's work on generic functions.
      
      As part of this change, we can remove
      
        a) types/InstEnv.*	(thereby also removing a hi-boot loop)
        b) a tc-fixpoint-loop in TcModule
      
      Both of these are worthwhile simplifications.
      7bb06950
  36. 22 Jun, 2000 1 commit
    • simonpj's avatar
      [project @ 2000-06-22 14:45:41 by simonpj] · 5f352824
      simonpj authored
      *** NO NEED TO MERGE WITH 4.07 ***
          (but it would do no harm)
      
      * Improve an error message when overlapping instance
        declarations are present.  Carl Witty reported this
        infelicitous message.  The problem arises for this code:
      
      	class Foo a
      	class (Foo a) => Bar a
      	
      	data Dat a = Dat
      	
      	instance Foo (Dat a)
      	instance Foo (Dat Integer)
      	
      	instance Bar (Dat a)
      
        The instance decl for Bar should say
      
      	instance Foo (Dat a) => Bar (Dat a) 
      
        because the overlapping instance decls for Foo can't
        be resolved (or at least might vary depending on how
        a is instantiated).
      5f352824