Re: Factor

There was a discussion today on the Factor Discord server about how one might filter or reject objects in a sequence based on whether a quotation throws an error or not when applied to each item. We’re going to implement that now in Factor and hopefully learn a few things.

First, we need a way to see if a quotation throws an error or not. One way would be to call the quotation and use recover to return a different boolean if an error was thrown. That might look something like this:

: throws? ( quot -- ? )
    '[ @ f ] [ drop t ] recover ; inline

And that kinda works:

IN: scratchpad 10 20 [ + ] throws?

--- Data stack:
30
f

IN: scratchpad 10 "20" [ + ] throws?

--- Data stack:
10
"20"
t

But you can see that in the working first case, it did not throw an error and the stack has the single output of the quotation and f, but in the failed second case it has the two inputs to the quotation and a t indicating that an error was thrown.

So, ideally we could modify this so that when the quotation succeeds, we drop the outputs, and when the quotation fails, we drop the inputs, and then can consistently produce a single boolean output for any quotation this applies to.

Luckily for us, we can use the drop-outputs and drop-inputs words which infer the stack effect and handles each case correctly. This would change our implementation slightly:

: throws? ( quot -- ? )
    [ '[ _ drop-outputs f ] ]
    [ '[ drop _ drop-inputs t ] ] bi recover ; inline

And now we can see that it handles both cases nicely:

IN: scratchpad 10 20 [ + ] throws?

--- Data stack:
f

IN: scratchpad 10 "20" [ + ] throws?

--- Data stack:
t

Now, we can combine that with our sequence operations and make these useful words:

: filter-errors ( ... seq quot -- ... subseq )
    '[ _ throws? ] filter ; inline

: reject-errors ( ... seq quot -- ... subseq )
    '[ _ throws? ] reject ; inline

And then see how they work:

IN: scratchpad { t "5" 123 } [ string>number ] filter-errors .
{ t 123 }

IN: scratchpad { t "5" 123 } [ string>number ] reject-errors .
{ "5" }

These words were added recently to the development version of Factor.