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

Date: Fri, 13 Nov 1998 16:35:25 +0100

Herbert Kuchen wrote:

*> I think, we have (at least) four options:
*

*> ...
*

*> 2) transform out nullary functions (such that the
*

*> existing theory can used)
*

*> 3) adopt Michael's original proposal
*

*> (no sharing of values for locally defined nullary functions)
*

*> 4) Michael's last proposal
*

*> locally defined nullary functions are handled like variables
*

*> (i.e. their values are shared); locally defined recursive
*

*> nullary functions are forbidden
*

*>
*

*> I will comment below on possibilities 2) and 4). Moreover, I would
*

*> like to remember that 3) leads to some surprising behaviour, e.g. in
*

*>
*

*> psort xs | sorted ys = ys where ys = permut xs
*

*>
*

*> Honestly, who was able to spot immediately that this
*

*> definition does not work as expected?!
*

Agreed, since one usually considers local declarations

of the form "where x = ..." as denoting some local variable

rather than a function. On the other hand, one considers

all global definitions as functions (rather than variables,

since a functional logic program is a set of function definitions)

and thus the "nullary function = variable" approach might

also lead to surprising results. Maybe this is an argument for

possibility 4).

*> Thus, the "top level" definitions would be handled just
*

*> like local definitions as pointed out in my last email.
*

*>
*

*> > coin = 0
*

*> > coin = 1
*

*> > ...
*

*> > f xs = ...coin...
*

*> > g ys = ...coin...
*

*> > ...
*

*>
*

*> For the example, this means:
*

*> (assuming the goal is f Xs)
*

*>
*

*> dummy_lhs = h xs
*

*> h xs = f1 xs
*

*> h xs = f2 xs
*

*> f1 xs = ... coin1 ...
*

*> f2 xs= ... coin2...
*

*> g1 xs= ... coin1...
*

*> g2 xs= ... coin2 ...
*

*> coin1 = 0
*

*> coin2 = 1
*

But this must be done for each 0-ary function which means that

the code must be duplicated for each 0-ary function which

leads to a dramatic code explosion in larger programs.

*> Note that this just serves to transform the program to the
*

*> kernel language in order to explain its meaning based on the
*

*> existing theory (i.e. mainly the ESOP'96 paper). It does
*

*> NOT imply that Curry programs have to implemented
*

*> like this. A possible implementation could preserve
*

*> the nesting and avoid the code explosion.
*

Right, but I am thinking also of other tools than compilers

like program transformation systems, partial evaluators,

abstract interpreters and so on. All such tools have then

two options:

- Consider the special treatment of 0-ary functions: such

special treatments makes these tools often much more complicated.

- Consider the transformation: this makes the tools often much

more inefficient due to the code explosion.

Therefore, I think that the sharing of 0-ary functions is

ok for local declarations (since the code explosion caused by

the transformation is usually rather limited) but not for

global functions. Thus, I am very much in favor to your

recent proposal:

*> to 4) this proposal avoids the surprising behaviour in the
*

*> permutation sort example, but restricts the language.
*

*> Unfortunately, it also destroys the compatibility with
*

*> Haskell. In order to recover it, one could allow
*

*> deterministic locally defined recursive
*

*> nullary functions (and handle them by lambda
*

*> lifting). Thus, only non-deterministic locally defined
*

*> recursive nullary functions remain excluded.
*

Yes, maybe one could specify this behavior by distinguishing

the recursive and non-recursive cases (like in the Haskell report).

If the locally introduced variables are not recursively used,

one could specify the meaning by a simple transformation

(adding arguments), and the general recursive case is

specified by your more complicated transformation

(which will be seldom used). Then there is no distinction

between deterministic and non-deterministic local functions,

which is quite reasonable.

Best regards,

Michael

Received on Fr Nov 13 1998 - 16:39:00 CET

Date: Fri, 13 Nov 1998 16:35:25 +0100

Herbert Kuchen wrote:

Agreed, since one usually considers local declarations

of the form "where x = ..." as denoting some local variable

rather than a function. On the other hand, one considers

all global definitions as functions (rather than variables,

since a functional logic program is a set of function definitions)

and thus the "nullary function = variable" approach might

also lead to surprising results. Maybe this is an argument for

possibility 4).

But this must be done for each 0-ary function which means that

the code must be duplicated for each 0-ary function which

leads to a dramatic code explosion in larger programs.

Right, but I am thinking also of other tools than compilers

like program transformation systems, partial evaluators,

abstract interpreters and so on. All such tools have then

two options:

- Consider the special treatment of 0-ary functions: such

special treatments makes these tools often much more complicated.

- Consider the transformation: this makes the tools often much

more inefficient due to the code explosion.

Therefore, I think that the sharing of 0-ary functions is

ok for local declarations (since the code explosion caused by

the transformation is usually rather limited) but not for

global functions. Thus, I am very much in favor to your

recent proposal:

Yes, maybe one could specify this behavior by distinguishing

the recursive and non-recursive cases (like in the Haskell report).

If the locally introduced variables are not recursively used,

one could specify the meaning by a simple transformation

(adding arguments), and the general recursive case is

specified by your more complicated transformation

(which will be seldom used). Then there is no distinction

between deterministic and non-deterministic local functions,

which is quite reasonable.

Best regards,

Michael

Received on Fr Nov 13 1998 - 16:39:00 CET

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