On the textual order of rules

From: Yi D <plmday_at_gmail.com>
Date: Wed, 26 Mar 2014 11:33:08 +0100


Some time ago, we had a group discussion about the paper Functional
Logic Programming<https://www.informatik.uni-kiel.de/~mh/publications/papers/CACM10.pdf>.
In the 2nd paragraph on Page 5, it goes:

To fully support non-determinism, in a functional logic program the
textual order of the rules defining an operation is irrelevant.

Curiously, one of our group members tried the following code:

data BTree = Leaf Int | Branch BTree BTree

size :: BTree -> Int
size (Branch l r) = 1 + size l + size r
size (Leaf _) = 1

in the online interpreter with the following query:

size t =:= 3 where t free

but got time out. However, after switching the order of the two rules,
he got the expected answer. Suspecting the solution needs longer time
than 5 seconds, I installed PAKCS (which is the implementation running
behind the web interface) locally and tried the same code with the same
query, it diverged! This clearly violates the statement quoted above.
I searched through the documentation but could not find any explicit
statement like the one quoted above regarding the textual order of rules.
Afterward, I installed the KiCS2 implementation of Curry, tried the same
code and query, and got the expected answer. Then we were confused, the
two implementations have different behavior of the same language. What
should we expect? Is the statement quoted above a feature or functional
logic language or not?

Later, I found out that PAKCS compile Curry code to Prolog while KiCS2
targets Haskell. So it seems that the behavior of PAKCS is due to the
inheritance of the (mis-)feature of Prolog. But the question remains:
What is the expected behavior of the Curry language we should expect?



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