Re: Curry Crash Course

From: Michael Hanus <>
Date: Thu, 24 Jan 2013 17:13:35 +0100

On 01/23/2013 02:02 PM, Sebastian Fischer wrote:
> There seems to be a tension that has not been mentioned yet in the
> discussion. Lists support the implementation of other evaluation-order
> dependent constructs such as committed choice. I could not define the
> function `once` in Evaluator.lcurry with the "pure" interface on sets
> that you describe (and is provided by the Values type in the
> SetFunctions module). Values can be turned into a list by sorting them
> but it seems counter productive to sort all values in order to commit to
> one and sorting does not terminate for infinite lists.

So maybe one could distinguish between a nondeterministic operation

choose :: Values a -> (a,Values a)

that chooses some element and returns the remaining ones from a set
of values (following Sergio's proposal) and an *indeterministic*

select :: Values a -> (a,Values a)

that commits to some element and returns the remaining ones.
Hence, if we define chooseElem by

chooseElem x = fst (choose x)

then the set function chooseElem_set of chooseElem is the identity
on value sets. The use of the indeterministic operation "select"
may destroy referential transparency if there is more than one
value in the set.

> Maybe we should aim for a pure approach (like set functions) on one hand
> and a more powerful approach that allows more control (like commited
> choice or heuristics such as findbest based on try) on the other.

In the KiCS2 implementation of set functions, there are some
possibilities for this. Set functions are parameterized over
the search strategy to be used, e.g., one can use depth-first
search or breadth-first search to construct the sets.
This is useful to test various search strategies and might
influence the result of emptiness tests (e.g., an emptiness
test might loop with dfs but could terminate with bfs)
and could be also interesting when an indeterministic select
operation is applied.

Best regards,

curry mailing list
Received on Do Jan 24 2013 - 17:16:59 CET

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