Understanding strictness of ghc output

[Simon Peyton-Jones]

| That contradicts my intution for seq. I would read it as "h is forced
| before h is forced", and I would think that (h `seq` h) is equivalent
| to h.
| 
| Were I am wrong?

You're not wrong -- Malcolm is.  The function is certainly strict in h,
and GHC finds it.  

Here's what I get when I compile your program (with a suitable
definition of AVL).  The function $wpoly_addHeight is the worker for
addHeight. GHC has worked out that addHeight is strict, and has unboxed
the first argument, so that it is now of type Int#, as you can see.
The DmdType for the Int# is indeed "L" but that's irrelevant because
Int# values are always evaluated.  The demand info is always L for an
unboxed type.

Simon

Rec {
Foo.$wpoly_addHeight :: forall e.
			GHC.Prim.Int# -> Foo.AVL e -> GHC.Prim.Int#
[GlobalId]
Arity 2 NoCafRefs Str: DmdType LS
Foo.$wpoly_addHeight = \ @ e ww :: GHC.Prim.Int# w :: Foo.AVL e ->
			 case w of wild {
			   Foo.P ds ds1 r -> Foo.$wpoly_addHeight @ e
(GHC.Prim.+# ww 2) r;
			   Foo.Z l ds ds1 -> Foo.$wpoly_addHeight @ e
(GHC.Prim.+# ww 1) l;
			   Foo.N l ds ds1 -> Foo.$wpoly_addHeight @ e
(GHC.Prim.+# ww 2) l;
			   Foo.E -> ww
			 }
end Rec }

Foo.height :: forall e. Foo.AVL e -> GHC.Base.Int
[GlobalId]
Arity 1 NoCafRefs Str: DmdType Sm
Foo.height = \ @ e w :: Foo.AVL e ->
	       case Foo.$wpoly_addHeight @ e 0 w of ww { __DEFAULT ->
	       GHC.Base.I# ww
	       }

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Tomasz Zielonka
| Sent: 22 June 2004 14:18
| To: Malcolm Wallace
| Cc: glasgow-haskell-users at haskell.org
| Subject: Re: Understanding strictness of ghc output
| 
| On Tue, Jun 22, 2004 at 01:52:44PM +0100, Malcolm Wallace wrote:
| >
| > Same again.  Try
| >    addHeight h  E        = h `seq` h
| >
| > which, although it looks bizarre, actually forces the evaluation of
h,
| > whilst simply returning it does not.
| 
| That contradicts my intution for seq. I would read it as "h is forced
| before h is forced", and I would think that (h `seq` h) is equivalent
| to h.
| 
| Were I am wrong?
| 
| Best regards,
| Tom
| 
| --
| .signature: Too many levels of symbolic links
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