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From: Michael Hanus <hanus_at_informatik.rwth-aachen.de>

Date: Fri, 6 Nov 1998 09:45:56 +0100

Sven Panne wrote:

*> Perhaps someone on this list can enlighten me a little bit: It's not
*

*> clear to me why an entire disjunction must suspend when one disjunct
*

*> suspends.
*

The main problem is that it is not allowed to commit to the

non-suspending branch and ignore the suspended branch (as somebody

familiar with concurrent logic programming might think)

since then you would loose completeness (as you already pointed out).

Taking Sergio's remarks into account, a more appropriate example

is the following (using the new syntax):

f 0 x = success

f x 1 = success

g eval rigid

g 0 = 0

h eval flex

h 1 = 1

The goal to solve is: f (g x) (h x) & x=:=0

Now, if we ignore the suspending alternative caused by the evaluation

of (g x), we could bind x to 1 by evaluating (h x) and thus run

into a failure, so we loose the answer {x=0}.

Hence, your remark is right:

*> Of course things get incomplete when you simply commit to the
*

*> non-suspending disjunct and forget about the suspending one. Simply
*

*> keep the latter one as it is, as flexible branches similarly keep
*

*> suspending RHSs.
*

BTW, this is also done in our Java-based implementation where

or-branches are immediately executed by different threads.

The main problem w.r.t. the formal definition is a matter

of presentation. Since there is only a reduction function

on entire expressions (Eval[expr]) and not a representation

of suspended expressions with their definitional trees,

it is simpler to evaluate an or-node only if both

branches can be evaluated further. A formal description

of your proposal might require the explicit representation

of suspended expressions with definitional trees in the goals.

Any suggestions to improve it?

Another options is to describe your proposed extension informally

in the report.

Best regards,

Michael

Received on Fri Nov 06 1998 - 09:49:00 CET

Date: Fri, 6 Nov 1998 09:45:56 +0100

Sven Panne wrote:

The main problem is that it is not allowed to commit to the

non-suspending branch and ignore the suspended branch (as somebody

familiar with concurrent logic programming might think)

since then you would loose completeness (as you already pointed out).

Taking Sergio's remarks into account, a more appropriate example

is the following (using the new syntax):

f 0 x = success

f x 1 = success

g eval rigid

g 0 = 0

h eval flex

h 1 = 1

The goal to solve is: f (g x) (h x) & x=:=0

Now, if we ignore the suspending alternative caused by the evaluation

of (g x), we could bind x to 1 by evaluating (h x) and thus run

into a failure, so we loose the answer {x=0}.

Hence, your remark is right:

BTW, this is also done in our Java-based implementation where

or-branches are immediately executed by different threads.

The main problem w.r.t. the formal definition is a matter

of presentation. Since there is only a reduction function

on entire expressions (Eval[expr]) and not a representation

of suspended expressions with their definitional trees,

it is simpler to evaluate an or-node only if both

branches can be evaluated further. A formal description

of your proposal might require the explicit representation

of suspended expressions with definitional trees in the goals.

Any suggestions to improve it?

Another options is to describe your proposed extension informally

in the report.

Best regards,

Michael

Received on Fri Nov 06 1998 - 09:49:00 CET

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