Weird duplicate execution in `case`

[ngeiswei]

What is your problem?

Combining case with Empty evaluates twice the non empty branch.

How to reproduce your problem?

Run the following code with PeTTa:

!(case 42
   ((42 (trace! Here 42))
    (Empty (trace! There 0))))

What would normally expect?

Here
42

What do you get instead?

Here
Here
42

What else do you have say?

Taking the empty branch by running the following

!(case (empty)
   ((42 (trace! Here 42))
    (Empty (trace! There 0))))

works as expected.

Also, when replacing the Empty case by NotEmpty

!(case 42
   ((42 (trace! Here 42))
    (NotEmpty (trace! There 0))))

it also works as expected (i.e. (trace! Here 42) is only taken once).

[patham9]

Thank you for finding!
Tricky issue. case in PeTTa was a major step forward from H-E, because it avoids findall / collapse entirely via \+ (negation by failure), which allows case to run over space matches with millions of items and even cheaply stream a matching item via once and in nested case expressions. The double-evaluation corner case that appears with Empty is subtle. I'll think about how to resolve it without losing the streaming semantics that make case scalable. While a findall-based solution is trivial, we really need to avoid it. In the meanwhile, this issue shows one needs to be careful with side-effects with case-with-Empty-case, while without Empty the issue does not exist.

[patham9]

@rTreutlein Any idea?

[ngeiswei]

BTW, a temporary workaround seems to be to place a (cut), as in:

!(case 42
   ((42 (progn (cut) (trace! Here 42)))
    (Empty (trace! There 0))))

[patham9]

Fixed by @rTreutlein #104

[ngeiswei]

Maybe this issue needs to be re-opened because the following duplicates the call of foo:

(= (foo $x) (trace! (Call foo with argument $x) $x))
!(case (foo 42)
   ((42 (trace! Here 42))
    (Empty (trace! There 0))))

which outputs

(Call foo with argument 42)
Here
(Call foo with argument 42)
42

Let me know if I need to create a new issue instead.

[rTreutlein]

@ngeiswei Good catch it's fixed in #112