Static data and RULES

David Feuer david at
Fri Feb 17 14:48:34 UTC 2017

I've never used such rules myself, but when I asked Duncan Coutts about 
whether and how such rules were used in the wild, he said

> Well I've certainly tried to use that in the past.
> A previous version of the cbor lib which used a different
> representation did a lot of matching on constructors to re-arrange input to
> an interpreter, until I discovered that GHC
> actually uses constructor wrappers and that matching on constructors was
> thus not reliable .

He described such rules as "a totally legit thing to want to do". If a 
datatype represents an AST, then rewriting its terms can optimize the 
constructed programs. Of course, it's ultimately up to you. I have no dog in 
this race myself; my concern was for other people's code that could break as a 
result. Certainly such code is already fragile when strict constructors are 
involved, but if people have cleverly figured out that lazy constructors are 
more reliable in that regard, they could be using it. I don't know.


On Friday, February 17, 2017 8:06:17 AM EST Simon Peyton Jones via ghc-devs 
> {-# RULES   "L" LCon1 0 = LCon2
> Oh I missed this entirely. You want to write a rule FOR a data
> constructor????   I thought you just meant one that matches on a data
> constructor.
 That is you want (L 0) to rewrite, all by itself, to LCon2? 
> That had never occurred to me as a possibility.  Bizarre. Let’s not do
> that.
> ·         GHC does not (knowingly) support it today
> ·         It is a deeply weird thing to do
> ·         If you want to do it, write you own “smart constructor” mkLCon1,
> that inlines when you say
> mkLCon1 x = LCon1 x
> {-# INILNE [0] mkLCon1 #-}
> {-# RULES “L” mkLCon1 x = LCon2 #-}
> Problem solved.
> Simon
> From: David Feuer [mailto:david.feuer at]
> Sent: 17 February 2017 00:30
> To: Simon Peyton Jones <simonpj at>
> Cc: ghc-devs <ghc-devs at>; Reid Barton <rwbarton at>; Ben
> Gamari <bgamari at>
 Subject: RE: Static data and RULES
> Let me give an example. Suppose we have
> data L = LCon1 Int | LCon2
> data S = SCon !Int
> {-# RULES
> "L" LCon1 0 = LCon2
> "S" forall x . f (SCon x) = g x
>  #-}
> The immediate problem today is with "S". The SCon wrapper could very well
> inline before the rule has a chance to fire. We'd like to be able to phase
> that inline to give it a chance.
> The "L" rule becomes problematic when we try to identify static data the
> simplifier shouldn't have to try to optimize. If it identifies LCon 0 as
> static, the "L" rule will never fire.
> On Feb 16, 2017 7:08 PM, "David Feuer"
> <david.feuer at<mailto:david.feuer at>> wrote:
> the proposed scheme is very nearly equivalent to breaking *every* data
> constructor into a worker and a wrapper, and allowing INLINE and NOINLINE
> pragmas on the wrappers. That would allow terms built only from constructor
> workers and literals to be identified as they're constructed in any stage
> and left alone by the simplifier. It would also allow people using RULES
> that match on constructors to make those work reliably, by making sure the
> bindings they match on don't inline away or get marked static too early. Of
> course, we don't actually need to add more worker/wrapper pairs to do this;
> we can fake that. 
> On Feb 16, 2017 6:53 PM, "Simon Peyton Jones"
> <simonpj at<mailto:simonpj at>> wrote:
 I’m sorry I
> still don’t understand the problem.  Can you give an example?  It all works
> fine today; what will change in the proposed new scheme.  Indeed what IS
> the proposed new scheme? 
> I’m lost
> Simon
> From: David Feuer
> [mailto:david.feuer at<mailto:david.feuer at>]
 Sent: 16
> February 2017 23:51
> To: Simon Peyton Jones
> <simonpj at<mailto:simonpj at>>
 Cc: ghc-devs
> <ghc-devs at<mailto:ghc-devs at>>; Reid Barton
> <rwbarton at<mailto:rwbarton at>>; Ben Gamari
> <bgamari at<mailto:bgamari at>> Subject: RE: Static data and
> Sorry; guess I should have given more background on that. This goes back to
> the performance problems Ben encountered in Typeable. The goal is to avoid
> trying to optimize something over and over that's never ever going to
> change. If we know that a term is made only of static data, we can skip it
> altogether in simplification. Suppose we have
> foo = Just (Right [1])
> Then no amount of optimization will ever be useful. But what about RULES? If
> the outermost pattern in a rule matches on a data constructor, then it's
> not static anymore! We may be replacing it with something else. So we need
> a finer mechanism. We *also* need a finer mechanism for strict constructors
> in general. We need to avoid inlining those too early if they're mentioned
> in any position in RULES. Trying to make this work right automagically
> looks a bit tricky in the face of orphan rules and such.
> On Feb 16, 2017 6:35 PM, "Simon Peyton Jones"
> <simonpj at<mailto:simonpj at>> wrote:
 I don’t
> understand any of this.
> However, RULES are allowed to match on data constructors and it would be
> nice to let that keep happening.
> Why won’t it keep happening?  What is the problem you are trying to solve? 
> Why does the fast-path make it harder?
> Maybe open a ticket?
> Simon
> From: ghc-devs
> [mailto:ghc-devs-bounces at<mailto:ghc-devs-bounces at>]
> On Behalf Of David Feuer
 Sent: 16 February 2017 22:13
> To: Ben Gamari <bgamari at<mailto:bgamari at>>; Reid Barton
> <rwbarton at<mailto:rwbarton at>>
 Cc: ghc-devs
> <ghc-devs at<mailto:ghc-devs at>>
> Subject: Static data and RULES
> Ben Gamari and Reid Barton are interested in making it cheaper for static
> data to pass through simplification. The basic idea is that if a term is
> already made entirely of data constructors and literals, then there's
> nothing left to optimize.
> However, RULES are allowed to match on data constructors and it would be
> nice to let that keep happening. But on the other hand, RULES are
> apparently (according to Duncan Coutts) already broken for strict data
> constructors, because they have workers and wrappers.
> My thought: let's allow phased INLINE and NOINLINE pragmas for data
> constructors. The default would be INLINE. The ~ phase choice would not be
> available: once inline, always inline.
> Semantics
> ~~~~~~~~~~
> For all constructors:
> If a constructor is allowed by pragmas to inline in a certain phase, then in
> that phase terms built from it can be considered static. Once static,
> always static.
> If a constructor is not allowed to inline in a certain phase, terms built
> from it will be considered non-static.
> After demand analysis and worker/wrapper, all constructors are considered
> inline.
> For strict constructors:
> A strict constructor wrapper prohibited from inlining in a certain phase
> simply will not.
> Strict constructor wrappers will all be allowed to inline after demand
> analysis and worker/wrapper. This matches the way we now handle wrappers
> actually created in that phase.
> Syntax:
> For GADT syntax, this is easy:
> data Foo ... where
>   {-# INLINE [1] Bar #-}
>   Bar :: ...
> For traditional syntax, I think it's probably best to pull the pragmas to
> the top:
> {-# NOINLINE Quux #-}
> data Baz ... = Quux ... | ...

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