Re: [Caml-list] Ocamlc stack overflow (Probably a typechecking bug)

[Alex Baretta <alex@baretta.com>]

Richard Jones wrote:
> It's only 140 lines of code? Try chopping lines off the end of the
> file until the thing compiles. Then you should be able to isolate
> which statement causes the stack overflow. From there it should be a
> simple enough job to either understand the problem and work around it,
> or else come up with a minimal example which exhibits the bug.
> 
> Rich.
> 

I don't need to chop off lines of code. I know exactly what line of code 
causes the problem. Let me quote it to you:

 > let cont f init res execute =
 >   Local_rules.Rewrite.rewrite_continuation rule_base (cont f)

Let me try to explain the dependency relations in my libraries. You will 
see that it is impossible to come up with a "minimal example". I have a 
rule-based abstract processor (à la CLIPS) written as functor whose 
parameter module defines types "fact" and "fact_base" as well the 
operations which can be performed on them. I also have an SQL library 
which gives provides a statically typechecked abstract syntax for SQL 
and facilities to connect to databases through a doubly generic 
interface: the first parameter module provides a database connection 
layer, the second parameter (we call it the Access module) provides the 
type information to the ocaml typechecking system to achieve static 
typechecking of SQL in ocaml. This same generic library has a query 
rewriting rule engine à la PostgreSQL which defines a fact base module 
for the rule-based processor. This rewrite-rule functor takes an Access 
parameter, which must match the Access parameter passed to the DB 
functor. The actual rewrite rules are also defined in a functor which 
takes an Access module as a parameter.

Finally, I have an embedded-sql syntax extension which provides the 
syntactic sugar to make everything nice and easy in an ocaml source 
file. This syntax extension transforms SQL queries in concrete syntax 
into an ocaml module which instantiates the DB functor and the 
rewrite-rule engine with the same Access module. Since no module has an 
explicit signature hiding the type representations, I would expect the 
compiler to be able to figure things out correctly. Instead, I used to 
get error messages of the following kind.

 > File "inserimento_enti.xcaml", line 95, characters 56-65:
 > This expression has type
 >   Local_rules.Rewrite.Rules.rule_base =
 >     Local_rules.Rewrite.Rules.rule list
 >     Map.Make(Local_rules.Rewrite.Rules.Fact_class_order).t
 > but is here used with type
 >   RW.Rules.rule list RW.Rules.FCM.t =
 >     RW.Rules.rule list Map.Make(RW.Rules.Fact_class_order).t


Which points to the following line:

 > let cont f init res execute =
 >   RW.rewrite_continuation rule_base (cont f)

It is worth noting that the following module definitions imply that the 
error message is actually wrong.
 > (* in Inserimento_enti *)
 > module Local_rules = Generic_rules (Anagrafiche_logical)
 > module RW = Local_rules.Rewrite


Intuitively, I'd say the type checker is having some real trouble with 
the complex module operations I use. But, then again, there might be a 
problem in my code. So, I removed the RW definition and changed the 
troublesome line to the one originally mentioned in this post:
 > let cont f init res execute =
 >   Local_rules.Rewrite.rewrite_continuation rule_base (cont f)

Now, this really turns the typechecker nuts. It simply cannot state that 
there is a type error for the inferred type is identical to the actual 
type. Here comes the stack overflow problem.

So, dear caml riders and caml breeders, how do we get out of this 
impasse? I'm willing to submit my entire source tree. (I'd release it 
GPLed if only I had time to write a minimum of documentation and got it 
to compile properly...).

Alex

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