let/app invariant violated by code generated with mkCoreApp

Simon Peyton Jones simonpj at microsoft.com
Tue Nov 11 14:13:03 UTC 2014

Oh bother, that is _so_ tiresome. The desugarer establishes the let/app invariant, so we get

	I# x_help

but if x_help has a compulsory unfolding to (x void), returning an Int#, that violates the let/app invariant.  Sigh.  This is a ridiculous amount of work for a tiny corner (pattern synonyms for unboxed constants).

Harump.  Let's see.  We are talking only of things like this

	pattern P = 4#

correct?  Perhaps it may be simpler to make the psWrapper in PatSyn be
	psWrapper :: Either Id Literal
and treat such patterns specially from the moment we first see them?  That would eliminate all this void stuff entirely.

Pursuing the current line, though, I suppose that the desugarer could inline compulsory unfoldings during desugaring itself.  In this line, add a case for when var has a compulsory unfolding.

dsExpr (HsVar var)            = return (varToCoreExpr var)   -- See Note [Desugaring vars]

That would, I suppose, be the quickest pathc.


|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Dr.
|  ERDI Gergo
|  Sent: 08 November 2014 14:03
|  To: GHC Devs
|  Subject: let/app invariant violated by code generated with mkCoreApp
|  Hi,
|  I'm trying to attach (f Void#) as a compulsory unfolding to an Id.
|  Here's what I tried originally:
|       let unfolding = mkCoreApp (Var worker_id) (Var voidPrimId)
|           wrapper_id' = setIdUnfolding wrapper_id $
|  mkCompulsoryUnfolding unfolding
|  However, when I try to use wrapper_id' in the desugarer, the Core
|  linter looks at me strange. This is the original Core:
|  f :: Int
|  [LclIdX, Str=DmdType]
|  f = break<1>() GHC.Types.I# Main.$WPAT
|  and this is the error message ($WPAT is the wrapper_id', PAT is the
|  worker_id in this example)
|  <no location info>: Warning:
|       In the expression: I# (PAT void#)
|       This argument does not satisfy the let/app invariant: PAT void#
|  Now, I thought I'd make sure mkCoreApp generated correct Core by
|  writing it out by hand:
|       let unfolding = Case (Var voidPrimId) voidArgId pat_ty
|  [(DEFAULT,[],App (Var worker_id) (Var voidArgId))]
|  however, bizarrely, this *still* results in *the same* error message,
|  as if something was transforming it back to a straight App.
|  Anyone have any hints what I'm doing wrong here?
|  Bye,
|   	Gergo
|  --
|     .--= ULLA! =-----------------.
|      \     http://gergo.erdi.hu   \
|       `---= gergo at erdi.hu =-------'
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