1. 11 Dec, 2001 1 commit
  2. 10 Dec, 2001 1 commit
  3. 07 Dec, 2001 3 commits
    • simonpj's avatar
      [project @ 2001-12-07 17:33:26 by simonpj] · 8cc5cc27
      simonpj authored
      ----------------------------
      	More jiggling in the renamer
      	----------------------------
      
      I was a little hasty before.  (Thanks Sigbjorn for finding
      this.)  This commit tidies up the handling of AvailEnvs.
      Principally:
      
        * filterImports now deals completely with hiding
          (before it handed off part of the job to mkGlobalRdrEnv)
      
        * The AvailEnv in an ExportAvails does not have class ops and
          data constructors in its domain.  This makes plusExportAvails
          more efficient, but the main thing is that it collects things
          up right.  (Previously, if we had
      	import M( C )
      	import M( op )
          then we got an AvailEnv which had C |-> AvailTC C [C]
          (no 'op').
      
        * In Rename, we do need a "filled-out" version of the overall
          AvailEnv, full_avail_env, which we construct on the spot in 'rename'.
      8cc5cc27
    • sof's avatar
      [project @ 2001-12-07 08:12:53 by sof] · c5ba8422
      sof authored
      mkExportAvails: computing the AvailEnv is rather delicate
      c5ba8422
    • sof's avatar
      [project @ 2001-12-07 07:37:43 by sof] · 24279879
      sof authored
      Tidyup - previous instance-decl commit fell a bit short:
      
       * RnEnv.lookupInstDeclBndr unceremoniously fell over when passed
         an out-of-scope class name.
      
       * the AvailEnv carried around didn't common up type/class info
         (i.e.,  AvailTCs), but rather type/class and method/label names,
         causing the renamer to (semi)randomly report instance methods as
         being out-of-scope in the presence of multiple imports for a module.
      
       * didn't support 'hiding' of class / method names (for the purposes
         of checking instance decls).
      24279879
  4. 06 Dec, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-12-06 10:45:42 by simonpj] · 61fae1d3
      simonpj authored
      --------------------------
      	Fix the instance-decl wart
      	--------------------------
      
      This commit implements the (proposed) H98 rule for
      resolving the class-method name in an instance decl.
      
      	module M( C( op1, op2 ) ) where
      		-- NB: op3 not exported
      	  class C a where
      	    op1, op2, op3 :: a -> a
      
      
      	module N where
      	  import qualified M as P( C )
      	  import qualified M as Q hiding( op2 )
      
      	  instance P.C Int where
      	    op1 x = x
      	    -- op2, op3 both illegal here
      
      The point is that
        a) only methods that can be named are legal
           in the instance decl
      	(so op2, op3 are not legal)
        b) but it doesn't matter *how* they can be named
      	(in this case Q.op1 is in scope, though
      	the class is called P.C)
      
      The AvailEnv carries the information about what's in scope,
      so we now have to carry it around in the monad, so that
      instance decl bindings can see it.  Quite simple really.
      
      Same deal for export lists. E.g.
      
      	module N( P.C( op1 ) ) where
      	  import qualified M as P( C )
      	  import qualified M as Q hiding( op2 )
      
      Actually this is what GHC has always implemented!
      61fae1d3
  5. 29 Nov, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-11-29 13:47:09 by simonpj] · 32a89583
      simonpj authored
      ------------------------------
      	Add linear implicit parameters
      	------------------------------
      
      Linear implicit parameters are an idea developed by Koen Claessen,
      Mark Shields, and Simon PJ, last week.  They address the long-standing
      problem that monads seem over-kill for certain sorts of problem, notably:
      
      	* distributing a supply of unique names
      	* distributing a suppply of random numbers
      	* distributing an oracle (as in QuickCheck)
      
      
      Linear implicit parameters are just like ordinary implicit parameters,
      except that they are "linear" -- that is, they cannot be copied, and
      must be explicitly "split" instead.  Linear implicit parameters are
      written '%x' instead of '?x'.  (The '/' in the '%' suggests the
      split!)
      
      For example:
      
          data NameSupply = ...
      
          splitNS :: NameSupply -> (NameSupply, NameSupply)
          newName :: NameSupply -> Name
      
          instance PrelSplit.Splittable NameSupply where
      	split = splitNS
      
      
          f :: (%ns :: NameSupply) => Env -> Expr -> Expr
          f env (Lam x e) = Lam x' (f env e)
      		    where
      		      x'   = newName %ns
      		      env' = extend env x x'
          ...more equations for f...
      
      Notice that the implicit parameter %ns is consumed
      	once by the call to newName
      	once by the recursive call to f
      
      So the translation done by the type checker makes
      the parameter explicit:
      
          f :: NameSupply -> Env -> Expr -> Expr
          f ns env (Lam x e) = Lam x' (f ns1 env e)
      		       where
      	 		 (ns1,ns2) = splitNS ns
      			 x' = newName ns2
      			 env = extend env x x'
      
      Notice the call to 'split' introduced by the type checker.
      How did it know to use 'splitNS'?  Because what it really did
      was to introduce a call to the overloaded function 'split',
      ndefined by
      
      	class Splittable a where
      	  split :: a -> (a,a)
      
      The instance for Splittable NameSupply tells GHC how to implement
      split for name supplies.  But we can simply write
      
      	g x = (x, %ns, %ns)
      
      and GHC will infer
      
      	g :: (Splittable a, %ns :: a) => b -> (b,a,a)
      
      The Splittable class is built into GHC.  It's defined in PrelSplit,
      and exported by GlaExts.
      
      Other points:
      
      * '?x' and '%x' are entirely distinct implicit parameters: you
        can use them together and they won't intefere with each other.
      
      * You can bind linear implicit parameters in 'with' clauses.
      
      * You cannot have implicit parameters (whether linear or not)
        in the context of a class or instance declaration.
      
      
      Warnings
      ~~~~~~~~
      The monomorphism restriction is even more important than usual.
      Consider the example above:
      
          f :: (%ns :: NameSupply) => Env -> Expr -> Expr
          f env (Lam x e) = Lam x' (f env e)
      		    where
      		      x'   = newName %ns
      		      env' = extend env x x'
      
      If we replaced the two occurrences of x' by (newName %ns), which is
      usually a harmless thing to do, we get:
      
          f :: (%ns :: NameSupply) => Env -> Expr -> Expr
          f env (Lam x e) = Lam (newName %ns) (f env e)
      		    where
      		      env' = extend env x (newName %ns)
      
      But now the name supply is consumed in *three* places
      (the two calls to newName,and the recursive call to f), so
      the result is utterly different.  Urk!  We don't even have
      the beta rule.
      
      Well, this is an experimental change.  With implicit
      parameters we have already lost beta reduction anyway, and
      (as John Launchbury puts it) we can't sensibly reason about
      Haskell programs without knowing their typing.
      
      Of course, none of this is throughly tested, either.
      32a89583
  6. 26 Nov, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-11-26 09:20:25 by simonpj] · 5e3f005d
      simonpj authored
      ----------------------
      	Implement Rank-N types
      	----------------------
      
      This commit implements the full glory of Rank-N types, using
      the Odersky/Laufer approach described in their paper
      	"Putting type annotations to work"
      
      In fact, I've had to adapt their approach to deal with the
      full glory of Haskell (including pattern matching, and the
      scoped-type-variable extension).  However, the result is:
      
      * There is no restriction to rank-2 types.  You can nest forall's
        as deep as you like in a type.  For example, you can write a type
        like
      	p :: ((forall a. Eq a => a->a) -> Int) -> Int
        This is a rank-3 type, illegal in GHC 5.02
      
      * When matching types, GHC uses the cunning Odersky/Laufer coercion
        rules.  For example, suppose we have
      	q :: (forall c. Ord c => c->c) -> Int
        Then, is this well typed?
      	x :: Int
      	x = p q
        Yes, it is, but GHC has to generate the right coercion.  Here's
        what it looks like with all the big lambdas and dictionaries put in:
      
      	x = p (\ f :: (forall a. Eq a => a->a) ->
      		 q (/\c \d::Ord c -> f c (eqFromOrd d)))
      
        where eqFromOrd selects the Eq superclass dictionary from the Ord
        dicationary:		eqFromOrd :: Ord a -> Eq a
      
      
      * You can use polymorphic types in pattern type signatures.  For
        example:
      
      	f (g :: forall a. a->a) = (g 'c', g True)
      
        (Previously, pattern type signatures had to be monotypes.)
      
      * The basic rule for using rank-N types is that you must specify
        a type signature for every binder that you want to have a type
        scheme (as opposed to a plain monotype) as its type.
      
        However, you don't need to give the type signature on the
        binder (as I did above in the defn for f).  You can give it
        in a separate type signature, thus:
      
      	f :: (forall a. a->a) -> (Char,Bool)
      	f g = (g 'c', g True)
      
        GHC will push the external type signature inwards, and use
        that information to decorate the binders as it comes across them.
        I don't have a *precise* specification of this process, but I
        think it is obvious enough in practice.
      
      * In a type synonym you can use rank-N types too.  For example,
        you can write
      
      	type IdFun = forall a. a->a
      
      	f :: IdFun -> (Char,Bool)
      	f g = (g 'c', g True)
      
        As always, type synonyms must always occur saturated; GHC
        expands them before it does anything else.  (Still, GHC goes
        to some trouble to keep them unexpanded in error message.)
      
      
      The main plan is as before.  The main typechecker for expressions,
      tcExpr, takes an "expected type" as its argument.  This greatly
      improves error messages.  The new feature is that when this
      "expected type" (going down) meets an "actual type" (coming up)
      we use the new subsumption function
      	TcUnify.tcSub
      which checks that the actual type can be coerced into the
      expected type (and produces a coercion function to demonstrate).
      
      The main new chunk of code is TcUnify.tcSub.  The unifier itself
      is unchanged, but it has moved from TcMType into TcUnify.  Also
      checkSigTyVars has moved from TcMonoType into TcUnify.
      Result: the new module, TcUnify, contains all stuff relevant
      to subsumption and unification.
      
      Unfortunately, there is now an inevitable loop between TcUnify
      and TcSimplify, but that's just too bad (a simple TcUnify.hi-boot
      file).
      
      
      All of this doesn't come entirely for free.  Here's the typechecker
      line count (INCLUDING comments)
      	Before	16,551
      	After	17,116
      5e3f005d
  7. 19 Nov, 2001 1 commit
  8. 08 Nov, 2001 1 commit
    • sof's avatar
      [project @ 2001-11-08 19:20:55 by sof] · 56883a7f
      sof authored
      rnHsForeignDecl: 'foreign import's (incl 'f.e.d's) _define_ local toplevel
      names, so better use RnEnv.lookupTopBndrRn and not RnEnv.lookupOccRn to
      resolve the name.
      
      As was, declaring ForeignImports with the same name as an imported entity
      wasn't permitted.
      56883a7f
  9. 06 Nov, 2001 1 commit
  10. 05 Nov, 2001 3 commits
  11. 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
  12. 23 Oct, 2001 1 commit
  13. 22 Oct, 2001 1 commit
    • simonmar's avatar
      [project @ 2001-10-22 16:08:10 by simonmar] · a5caedcb
      simonmar authored
      -fwarn-name-shadowing should check the global env as well as the local
      env for names that could be shadowed (the docs don't say anything
      about it applying to local names only).
      a5caedcb
  14. 18 Oct, 2001 2 commits
    • simonmar's avatar
      [project @ 2001-10-18 09:37:54 by simonmar] · 80d58cf3
      simonmar authored
      Simplify the grammar in a few places - we don't allow VARSYMs or
      CONSYMs in interface files any more (everything is z-encoded), so
      remove these productions, and inline some non-terminals that were only
      used in one place.
      80d58cf3
    • simonmar's avatar
      [project @ 2001-10-18 09:17:04 by simonmar] · 9b6eccb5
      simonmar authored
      - use var_fs instead of just VARID for type variables, so that
        specialids (as, qualified, hiding, forall etc.) don't confuse the
        interface file parser.
      
      - remove superfluous '!' in var_fs rule.  This was necessary to avoid
        conflicts caused by the previous change.
      9b6eccb5
  15. 08 Oct, 2001 1 commit
    • simonmar's avatar
      [project @ 2001-10-08 14:22:43 by simonmar] · 3881b000
      simonmar authored
      Turn the error message about package mismatch into a warning.  This is
      really harmless at the moment (only fatal with DLLs), but really gets
      in the way when you want to build a package and also use it directly
      with the appropriate -i/-l flags.
      3881b000
  16. 26 Sep, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-09-26 15:12:33 by simonpj] · e0d750be
      simonpj authored
      ------------------
      		Simon's big commit
      		------------------
      
      This commit, which I don't think I can sensibly do piecemeal, consists
      of the things I've been doing recently, mainly directed at making
      Manuel, George, and Marcin happier with RULES.
      
      
      Reogranise the simplifier
      ~~~~~~~~~~~~~~~~~~~~~~~~~
      1. The simplifier's environment is now an explicit parameter.  This
      makes it a bit easier to figure out where it is going.
      
      2. Constructor arguments can now be arbitrary expressions, except
      when the application is the RHS of a let(rec).  This makes it much
      easier to match rules like
      
      	RULES
      	    "foo"  f (h x, g y) = f' x y
      
      In the simplifier, it's Simplify.mkAtomicArgs that ANF-ises a
      constructor application where necessary.  In the occurrence analyser,
      there's a new piece of context info (OccEncl) to say whether a
      constructor app is in a place where it should be in ANF.  (Unless
      it knows this it'll give occurrence info which will inline the
      argument back into the constructor app.)
      
      3. I'm experimenting with doing the "float-past big lambda" transformation
      in the full laziness pass, rather than mixed in with the simplifier (was
      tryRhsTyLam).
      
      4.  Arrange that
      	case (coerce (S,T) (x,y)) of ...
      will simplify.  Previous it didn't.
      A local change to CoreUtils.exprIsConApp_maybe.
      
      5. Do a better job in CoreUtils.exprEtaExpandArity when there's an
      error function in one branch.
      
      
      Phase numbers, RULES, and INLINE pragmas
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      1.  Phase numbers decrease from N towards zero (instead of increasing).
      This makes it easier to add new earlier phases, which is what users want
      to do.
      
      2.  RULES get their own phase number, N, and are disabled in phases before N.
      
      e.g. 	{-# RULES "foo" [2] forall x y.  f (x,y) = f' x y #-}
      
      Note the [2], which says "only active in phase 2 and later".
      
      3.  INLINE and NOINLINE pragmas have a phase number to.  This is now treated
      in just the same way as the phase number on RULE; that is, the Id is not inlined
      in phases earlier than N.  In phase N and later the Id *may* be inlined, and
      here is where INLINE and NOINLINE differ: INLNE makes the RHS look small, so
      as soon as it *may* be inlined it probably *will* be inlined.
      
      The syntax of the phase number on an INLINE/NOINLINE pragma has changed to be
      like the RULES case (i.e. in square brackets).  This should also make sure
      you examine all such phase numbers; many will need to change now the numbering
      is reversed.
      
      Inlining Ids is no longer affected at all by whether the Id appears on the
      LHS of a rule.  Now it's up to the programmer to put a suitable INLINE/NOINLINE
      pragma to stop it being inlined too early.
      
      
      Implementation notes:
      
      *  A new data type, BasicTypes.Activation says when a rule or inline pragma
      is active.   Functions isAlwaysActive, isNeverActive, isActive, do the
      obvious thing (all in BasicTypes).
      
      * Slight change in the SimplifierSwitch data type, which led to a lot of
      simplifier-specific code moving from CmdLineOpts to SimplMonad; a Good Thing.
      
      * The InlinePragma in the IdInfo of an Id is now simply an Activation saying
      when the Id can be inlined.  (It used to be a rather bizarre pair of a
      Bool and a (Maybe Phase), so this is much much easier to understand.)
      
      * The simplifier has a "mode" environment switch, replacing the old
      black list.  Unfortunately the data type decl has to be in
      CmdLineOpts, because it's an argument to the CoreDoSimplify switch
      
          data SimplifierMode = SimplGently | SimplPhase Int
      
      Here "gently" means "no rules, no inlining".   All the crucial
      inlining decisions are now collected together in SimplMonad
      (preInlineUnconditionally, postInlineUnconditionally, activeInline,
      activeRule).
      
      
      Specialisation
      ~~~~~~~~~~~~~~
      1.  Only dictionary *functions* are made INLINE, not dictionaries that
      have no parameters.  (This inline-dictionary-function thing is Marcin's
      idea and I'm still not sure whether it's a good idea.  But it's definitely
      a Bad Idea when there are no arguments.)
      
      2.  Be prepared to specialise an INLINE function: an easy fix in
      Specialise.lhs
      
      But there is still a problem, which is that the INLINE wins
      at the call site, so we don't use the specialised version anyway.
      I'm still unsure whether it makes sense to SPECIALISE something
      you want to INLINE.
      
      
      
      
      
      Random smaller things
      ~~~~~~~~~~~~~~~~~~~~~~
      
      * builtinRules (there was only one, but may be more) in PrelRules are now
        incorporated.   They were being ignored before...
      
      * OrdList.foldOL -->  OrdList.foldrOL, OrdList.foldlOL
      
      * Some tidying up of the tidyOpenTyVar, tidyTyVar functions.  I've
        forgotten exactly what!
      e0d750be
  17. 20 Sep, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-09-20 08:47:13 by simonpj] · 2c250999
      simonpj authored
      ------------------------------
      	Fix a scoped-type-variable bug
      	------------------------------
      
      	MERGE WITH STABLE BRANCH
      
      The bug caused an incorrect failure when the same type
      variable was used more than once in a collection of patterns:
      
      	f (x :: t) (y :: t) = e
      
      On the way, I eliminated bindNakedTyVarsFVRn, which was only
      called once.
      2c250999
  18. 13 Sep, 2001 1 commit
  19. 10 Sep, 2001 1 commit
  20. 07 Sep, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-09-07 13:38:55 by simonpj] · a2415187
      simonpj authored
      Improve the error message for duplicate or invalid binders
      in a binding group.
      
      	[Consequence: rnfail034 should not be an expected failure
      		      any more]
      
      	MERGE WITH STABLE BRANCH
      a2415187
  21. 23 Aug, 2001 3 commits
    • simonpj's avatar
      [project @ 2001-08-23 15:06:59 by simonpj] · ae3231c4
      simonpj authored
      remove rnsource.hi-boot-5 (part of prev commit)
      ae3231c4
    • simonpj's avatar
      [project @ 2001-08-23 15:05:52 by simonpj] · f62fd70d
      simonpj authored
      More instance-gate fiddling.  This must be one of the most
      tiremsome bits of the entire compiler, and I appear to be
      incapable of modifying it without getting it wrong at least
      once.
      
      Still, this commit does tidy things up a bit.
      
      * The type renamers (rnHsType, etc) have moved from RnSource
        into a new module RnTypes.
      
      * This breaks a couple of loops, and lets us nuke RnSource.hi-boot.
        Hurrah!
      
      Simon
      f62fd70d
    • simonpj's avatar
      [project @ 2001-08-23 09:54:45 by simonpj] · 98bf5734
      simonpj authored
      --------------------------------------------------
      	Be a bit more liberal when slurping instance decls
      	--------------------------------------------------
      
      Functional dependencies have (as usual) made things more complicated
      
      Suppose an interface file contains
      	interface A where
      	  class C a b | a->b where op :: a->b
      	  instance C Foo Baz where ...
      
      Now we are compiling
      	module B where
      	  import A
      	  t = op (v::Foo)
      
      Should we slurp the instance decl, even though Baz is nowhere mentioned
      in module B?  YES!  Because of the fundep, the (C Foo ?) part is enough to
      select this instance decl, and the Baz part follows.
      
      Rather than take fundeps into account "properly", we just slurp
      if C is visible and *any one* of the Names in the types
      This is a slightly brutal approximation, but most instance decls
      are regular H98 ones and it's perfect for them.
      
      Changes:
      
        HscTypes:
      	generalise the types of GatedDecl a bit
      
        RnHiFiles.loadInstDecl, RnHiFiles.loadRule, RnIfaces.selectGated:
      	the meat of the solution
      
        RdrName, OccName etc:
      	some consequential wibbles
      98bf5734
  22. 22 Aug, 2001 1 commit
  23. 21 Aug, 2001 2 commits
  24. 20 Aug, 2001 2 commits
  25. 15 Aug, 2001 1 commit
  26. 14 Aug, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-08-14 06:35:56 by simonpj] · 2767767f
      simonpj authored
      1. Arrange that w/w records unfoldings
         And that the simplifier preserves them
      
      2. Greatly improve structure of checking user types in the typechecker
         Main changes:
      	TcMType.checkValidType checks for a valid type
      	TcMonoType.tcHsSigType uses checkValidType
      	Type and class decls use TcMonoType.tcHsType (which does not
      		check for validity) inside the knot in TcTyClsDecls,
      		and then runs TcTyDecls.checkValidTyCon
      		or TcClassDcl.checkValidClass to check for validity
      		once the knot is tied
      2767767f
  27. 23 Jul, 2001 1 commit
    • simonpj's avatar
      [project @ 2001-07-23 10:54:46 by simonpj] · f6cd95ff
      simonpj authored
      ---------------------------------
      	Switch to the new demand analyser
      	---------------------------------
      
      This commit makes the new demand analyser the main beast,
      with the old strictness analyser as a backup.  When
      DEBUG is on, the old strictness analyser is run too, and the
      results compared.
      
      WARNING: this isn't thorougly tested yet, so expect glitches.
      Delay updating for a few days if the HEAD is mission critical
      for you.
      
      But do try it out.  I'm away for 2.5 weeks from Thursday, so
      it would be good to shake out any glaring bugs before then.
      f6cd95ff
  28. 19 Jul, 2001 1 commit
  29. 13 Jul, 2001 2 commits
    • simonmar's avatar
      [project @ 2001-07-13 15:00:11 by simonmar] · 347ea316
      simonmar authored
      error msg wibble
      347ea316
    • simonpj's avatar
      [project @ 2001-07-13 13:29:56 by simonpj] · d4e38936
      simonpj authored
      ------------------------------------
      	Tidy up the "syntax rebinding" story
      	------------------------------------
      
      I found a bug in the code that dealt with re-binding implicit
      numerical syntax:
      	literals 	(fromInteger/fromRational)
      	negation	(negate)
      	n+k patterns	(minus)
      
      This is triggered by the -fno-implicit-prelude flag, and it
      used to be handled via the PrelNames.SyntaxMap.
      
      But I found a nicer way to do it that involves much less code,
      and doesn't have the bug.  The explanation is with
      	RnEnv.lookupSyntaxName
      d4e38936
  30. 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