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From: Michael Hanus <hanus_at_medoc.informatik.rwth-aachen.de>

Date: Mon, 16 Dec 1996 15:50:58 +0100

Dear Sergio,

Thanks for your comments on our proposal.

Concerning your comment about the simultaneous definition

of types, evals, and equations, I would like to separate them

since then we need no special rule or syntax if the type and/or

evaluation annotation is omitted. I expect that the evals

are omitted in most cases (similarly to Prolog where the

standard computation rule is also sufficient in most cases).

On the other hand, the separated pieces do not contain much redundancy

(apart from the function name) in contrast to your Algol example.

Furthermore, as Herbert already remarked, it is difficult to combine

the annotations with the single equations. However, I am open to

new suggestions. Currently, I can imagine to omit only

the repeated mentioning of the function name, e.g., to define

append (Section A.1) by

append: List t -> List t -> List t eval 1:flex

[] ys := ys

[x|xs] ys := [x|append xs ys]

However, this is a little bit against the "traditional" notation

of functional and logic languages.

*> ------------------------------------------------------------------
*

*>
*

*> The restriction on the variables in the right hand side of a
*

*> (conditional) equation makes it impossible to define a function f
*

*> such as
*

*>
*

*> f x := y if y = x
*

*>
*

*> which is certainly well-behaved and allowed in many functional
*

*> languages. There is some work by Middeldorp and others that
*

*> relaxes Curry's restrictions and yet guarantees confluence (the
*

*> function is well defined) in situations such as the above example.
*

*> We implemented it in our functional extension of Goedel and it was
*

*> not very hard. So my question is: "Has this issue been studied
*

*> enough or should we study more the issue of the extra variables in
*

*> the right hand side of conditional equations?"
*

This is an interesting point. In this proposal, I just wrote

the restrictions of Babel ensuring confluence, but I fully agree

that Curry should also provide a more general view. For instance,

it may be interesting to write functions with extra-variables

in the right-hand side where the confluence is not obvious, like in

last xs = x if append _ [x] = xs

(This example runs perfectly in our current implementation.)

This is more than the "where" clauses in current functional languages,

and such applications have not been studied enough. On the other hand,

as far as I know, the work by Middeldorp, Suzuki and Ida shows

that the common "where" clauses, where the extra-variables are

instantiated by pattern matching, ensures level-confluence and

completeness of narrowing. Since such uses of extra-variables

provides a direction in the computation, maybe it is better

to express it better by where-clauses than arbitrary conditions,

i.e., your example could be expressed by

f x := y

where y := x

I would be happy if you could extend the current proposal to

extra-variables in right-hand sides of equations.

Best regards,

Michael

Received on Mo Dez 16 1996 - 15:53:14 CET

Date: Mon, 16 Dec 1996 15:50:58 +0100

Dear Sergio,

Thanks for your comments on our proposal.

Concerning your comment about the simultaneous definition

of types, evals, and equations, I would like to separate them

since then we need no special rule or syntax if the type and/or

evaluation annotation is omitted. I expect that the evals

are omitted in most cases (similarly to Prolog where the

standard computation rule is also sufficient in most cases).

On the other hand, the separated pieces do not contain much redundancy

(apart from the function name) in contrast to your Algol example.

Furthermore, as Herbert already remarked, it is difficult to combine

the annotations with the single equations. However, I am open to

new suggestions. Currently, I can imagine to omit only

the repeated mentioning of the function name, e.g., to define

append (Section A.1) by

append: List t -> List t -> List t eval 1:flex

[] ys := ys

[x|xs] ys := [x|append xs ys]

However, this is a little bit against the "traditional" notation

of functional and logic languages.

This is an interesting point. In this proposal, I just wrote

the restrictions of Babel ensuring confluence, but I fully agree

that Curry should also provide a more general view. For instance,

it may be interesting to write functions with extra-variables

in the right-hand side where the confluence is not obvious, like in

last xs = x if append _ [x] = xs

(This example runs perfectly in our current implementation.)

This is more than the "where" clauses in current functional languages,

and such applications have not been studied enough. On the other hand,

as far as I know, the work by Middeldorp, Suzuki and Ida shows

that the common "where" clauses, where the extra-variables are

instantiated by pattern matching, ensures level-confluence and

completeness of narrowing. Since such uses of extra-variables

provides a direction in the computation, maybe it is better

to express it better by where-clauses than arbitrary conditions,

i.e., your example could be expressed by

f x := y

where y := x

I would be happy if you could extend the current proposal to

extra-variables in right-hand sides of equations.

Best regards,

Michael

Received on Mo Dez 16 1996 - 15:53:14 CET

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