[Haskell] threading mutable state through callbacks
antony at galois.com
Wed Oct 13 11:33:07 EDT 2004
Wolfgang Thaller wrote:
> 2) Evaluation order.
> That's what everyone has been fighting about.
>> The basic idea is that your entire program behaves as if in a giant
>> 'mdo' block, ordered in module dependency order.
> What is module dependency order? There's no such thing when there are
> circular dependencies, and when there aren't any, it might still
> depend on the order of import directives, which is probably not
> something I want program semantics to depend on.
> I feel that In a large program, you'll probably get more surprises
> from this.
The Modula-3 language specification gives one reasonable definition:
From http://research.compaq.com/SRC/m3defn/html/complete.html#idx.148 :
The order of execution of the modules in a program is constrained by the
If module M depends on module N and N does not depend on M, then N's
body will be executed before M's body, where:
A module M depends on a module N if M uses an interface that N exports
or if M depends on a module that depends on N.
A module M uses an interface I if M imports or exports I or if M uses an
interface that (directly or indirectly) imports I.
Except for this constraint, the order of execution is
According to the above constraint, the initialization order is
implementation-defined in the case of circular dependencies.
Note, too, that the order of import directives in a module does not
affect the partial order imposed by the above constraint.
(The above was delayed by the haskell mailing list anti-spam measures.
Anyway, looks like the above rule is entirely consistent with Simon
Marlow's description of what ghc already implements for initialization
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