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From: Michael Hanus <mh_at_informatik.uni-kiel.de>

Date: Mon, 29 Oct 2001 15:31:20 +0100 (MET)

German Vidal wrote:

*> ... For instance, if accessor functions are always rigid,
*

*> then "foo undefined" suspends with the definition
*

*>
*

*> foo eval flex
*

*> foo n = let (x,y) in [x,y]
*

*>
*

*> while it does not suspend with the definition
*

*>
*

*> foo eval flex
*

*> foo (x,y) = [x,y]
*

*>
*

*> Thus I would prefer that accessor functions have the same
*

*> evaluation annotation as the function where the local declaration
*

*> appear. Of course, I have no idea whether it is easy or difficult
*

*> to incorporate into existing compilers. Is there any special
*

*> trouble here?
*

I agree that the behavior in this case is somehow surprising

but the other solution could be equally surprising. If you use

several let's at various positions deep inside the right-hand side,

you have to consider the (far away) evaluation mode of the function

in order to determine the exact meaning of the let's. This could

be an argument in favor of a unique (local) treatment of accessor

functions (in addition to the difficulties with compilers

where it is easier to implement transformations with local conditions,

as already mentioned by Wolfgang).

It seems to me that this is again a situation where it is important

to point out that the "let" construct is just syntactic sugar

to support local definitions. It is a conservative extension

of the "let" in functional languages but becomes different

w.r.t. the functional logic extensions, e.g., the expression

"let x = f y in g x x" has the same meaning as "g (f y) (f y)"

in purely functional languages but has a different meaning

when f is a non-deterministic function.

So, I see no serious additional problems if we define the semantics

of the "let" with rigid accessor functions.

Best regards,

Michael

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Received on Mo Okt 29 2001 - 15:33:23 CET

Date: Mon, 29 Oct 2001 15:31:20 +0100 (MET)

German Vidal wrote:

I agree that the behavior in this case is somehow surprising

but the other solution could be equally surprising. If you use

several let's at various positions deep inside the right-hand side,

you have to consider the (far away) evaluation mode of the function

in order to determine the exact meaning of the let's. This could

be an argument in favor of a unique (local) treatment of accessor

functions (in addition to the difficulties with compilers

where it is easier to implement transformations with local conditions,

as already mentioned by Wolfgang).

It seems to me that this is again a situation where it is important

to point out that the "let" construct is just syntactic sugar

to support local definitions. It is a conservative extension

of the "let" in functional languages but becomes different

w.r.t. the functional logic extensions, e.g., the expression

"let x = f y in g x x" has the same meaning as "g (f y) (f y)"

in purely functional languages but has a different meaning

when f is a non-deterministic function.

So, I see no serious additional problems if we define the semantics

of the "let" with rigid accessor functions.

Best regards,

Michael

_______________________________________________

curry mailing list

curry_at_lists.RWTH-Aachen.DE

http://MailMan.RWTH-Aachen.DE/mailman/listinfo/curry

Received on Mo Okt 29 2001 - 15:33:23 CET

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