Resurrecting the "IF" Combinator... as WHEN

Domains Parsing
1

R3-Alpha had an idea--carried forward by Red--of an arity-1 IF combinator.

red>> num: 1020

red>> parse [a a a] [if (even? num) some 'a]
== true

red>> parse [a a a] [if (odd? num) some 'a]
== false

As you see, if the expression you give it turns out to be "falsey" then it doesn't continue matching. It skips to the next alternate--if there is one.

red>> parse [a a a] [if (odd? num) some 'b | some 'a]
== true

But I always thought the arity-1 IF was a pretty alien thing that would confuse people. You might think there's a branch, but there's no "branch"... just continuing along with the variadic list of everything that follows until the next | or end of BLOCK!.

I also wondered "where does it end?" With an IF combinator, why not a CASE combinator, or SWITCH combinator?

So when I came up with GET-GROUP! doing arbitrary substitutions of the rule it evaluates to, I thought "hey, that's a lot more general!" We could just say that ~true~ and ~void~ antiforms would continue the parse, ~false~ would stop it, and ~null~ antiforms would trigger an error in case you didn't mean to do that.

What That :(GET-GROUP!) Concept Looked Like

(Note that if condition '[...] is equivalent to if condition [[...]]. This is called "soft-quoted branching")

>> num: 1020, rule: null

; generated [some 'b] rule is treated as if it had been written there
>> parse [a a a b b b] [some 'a :(if even? num '[some 'b])]
== b 

; generated ~void~ from non-taken IF gets ignored, and it kept parsing
>> parse [a a a b b b] [some 'a :(if odd? num '[some 'c]) some 'b]
== b

; generated ~true~ signal continues parse, just as ~void~ did
>> parse [a a a b b b] [some 'a :(even? num) some 'b]
== b

; generated ~false~ skips to next alternate (isn't one, so parse fails)
>> parse [a a a b b b] [some 'a :(odd? num) some 'b]
** Error: PARSE BLOCK! combinator did not match input

; treat ~null~ conservatively, use :(maybe rule) for ~void~ to keep going 
>> parse [a a a b b b] [some 'a :(rule) some 'b]
** Error: ~null~ antiform generated by GET-GROUP! in PARSE

Flexible Logic Kills [~true~ ~false~]... Breaks That Idea

In the flexible logic model, [TRUE FALSE ON OFF YES NO] are WORD!s, and hence indiscriminately trigger taking the branch in something like an IF when used directly. The ~null~ antiform is the "branch inhibitor", and it's what conditional expressions return when they don't match the condition.

>> 10 > 20
== ~null~  ; anti

I don't think it's a good idea to make substitions via GET-GROUP! (or whatever comes to replace it) silently continue on NULL. If you forgot to set a variable that was supposed to hold something (as in rule above), that should give you an error. But I don't think you should have to write :(maybe even? num) :nauseated_face:

So Having A Conditional Logic Combinator Makes Sense

I just think that IF is a rather lousy name for it.

So I'll suggest WHEN.

>> parse [a a a b b b] [some 'a, when (even? num), some 'b]
== b

It would be against the premise of flexible logic to have WHEN be biased and assume things like TRUE, YES, or NO should mean it continues or not. I like the idea that you could hold a completely arbitrary word in a variable and say when (word), that means "continue matching when word is set to a non-null value".

Hence you'd have to say when (true? flag) or when (off? toggle) etc. I'm not merely comfortable with this... I am gung-ho about it! :comet:

(Of course people can make their own combinators and build in biases of their choosing, the core just doesn't pick sides.)

BYPASS Can Be A Synonym For [when (null)]

I didn't like using FAIL for saying when to stop a rule chain and go to the next alternate, because that is used for causing "abrupt failures" in the system.

So I'd been using quasiform ~false~ the state in source (and the antiform if in a variable).

>> parse [a a a b b b] [some 'a, :(if even? num [false]), some 'b]
** Error: PARSE BLOCK! combinator did not match input

>> parse [a a a b b b] [some 'a, ~false~, some 'b]
** Error: PARSE BLOCK! combinator did not match input

But that isn't the model anymore. There is no ~false~ or ~true~ antiform. And honestly it wasn't that literate anyway. when (...) makes it clearer when you're using a variable. And the quasiform just looks confusing.

Searching for a good word that doesn't run into something serving other purposes (e.g. BREAK), I asked Claude.ai for suggestions, and one of those was BYPASS.

I like it. So for example you could write:

>> parse [a a a b b b] [some 'a [:(if even? num ['bypass]) some 'c] | some 'b]]
== b

Although that particular case is clearer as [when (odd? num) ...], but sometimes you have to throw in a bypass rule.

(Amusingly, in Rebol2 the idiom for BYPASS was [end skip], which was a rule guaranteed to mismatch at any position: either you weren't at the tail and the END wouldn't match, or you were at the tail and the END would match but then you couldn't SKIP.)

Where Does It Stop?

I also wondered "where does it end?" With an IF combinator, why not a CASE combinator, or SWITCH combinator?

So I think it's good to just say WHEN.

You don't technically need WHEN if you have BYPASS to skip to next alternate, and ~void~ to keep going (or empty block, if you like... [] will keep going too).

 when (cond) => :(if not cond ['bypass])  ; or :(if not cond 'bypass)

But that forces you to reverse the sense of your logic and write out something longer (and slower). I think if you've got logic that's complex like a case or switch, then writing it out as a splicing rule would have negligible benefit to try and shoehorn as a combinator.

2

A good name — Haskell calls it this too. It’s a very useful combinator, so good to have.

Would you consider adding it to parse dialect too? It has a natural interpretation: parse the next thing only when the condition is true. (Again, this is something Haskell has.)

1 Like
3

I really should have clocked more time using Haskell to parse stuff. :frowning:

Problem is when I go off and experiment I tend to tackle too many things at once, I tried to do Haskell on Arch Linux and got bored trying to figure it out.

Hm? This is the PARSE dialect... what do you mean?

4

Ah-ha. Do not use Haskell libraries from the Arch package repositories. They are famously broken, mostly because GHC doesn’t play well with dynamic linking. (I say this as someone using Arch myself.)

Instead, I strongly recommend using GHCup.

Hmm, maybe I’m just struggling to understand your PARSE examples. Where in that post did you show an example of WHEN in PARSE?

6

This is the example of using it in PARSE.

I didn't add the failing case, but:

>> parse [a a a b b b] [some 'a, when (odd? num), some 'b]
** Error: PARSE BLOCK! combinator did not match input

I don't know how it would be applied in the main evaluator...

7

Oh… whoops, OK, not sure how I missed that. For some reason I thought you were talking about WHEN as a top-level combinator. (Which is also a very useful thing to have!)

8

Well now I'm confused, what would a "top level" WHEN combinator be?

9

Basically, when condition block ≡ if condition block []. That is, it would run the given block if the condition is true, and do nothing otherwise.

10

Ah, so the "actual" IF combinator.

We can accomplish the most typical desire with :(if condition block)

  • When the condition is not null, the block is evaluated and its product used as a rule.

  • When the condition is null, a ~void~ antiform is returned from the IF, and the evaluator skips it and keeps going.

If we made an IF combinator, you'd have to put your conditional code in a GROUP!. Unless you meant to actually match content with a rule as your condition, like this:

>> parse [hello world 304] [if opt 'hello [word! integer!]]
== 304

So what you used as a condition was what was synthesized by [opt 'hello] ... in this case the WORD! hello that matched (had it not matched it would be null). Then since it was non-null, it matched the next block vs. skipping it.

That's something you can't do with putting the code in a GET-GROUP!. I don't know how frequent that desire is, since most of the desire has been to control a rule based on a guard flag of some kind.

>> worldnumber: 'true

>> parse [world 304] [if (true? worldnumber) [word! integer!]]
== 304

If that's the case, then your advantage can be small...as small as two characters for having the combinator.

:(if true? worldnumber '[word! integer!])
if (true? worldnumber) [word! integer!]

But...I mean, I guess an IF combinator of that sort would be all right to have. Trivial to write. Just perhaps a little leery of the slippery slope of recreating every control construct as a combinator.

Were it to exist, I'd probably find uses for it.

11

Oh, looks like there’s been more confusion, sorry… I meant, outside PARSE altogether! That is, at the ‘top level’ of the program.

12

You'll have to use an example then.

In the ordinary evaluator, IF does nothing if the condition is false...so you'll have to illustrate what you mean.

13

Having spent some time with this in practice, I'm wondering if it's doing more harm than good to not allowing NULL in GET-GROUP! substitutions to mean "rule doesn't match".

It's particularly painful with WHILE:

while :(mode = 'read) [some chunk]

; vs.

while when (mode = 'read) [some chunk]

I am uncertain that the safety advantage is really worth it. NULL variables aren't unset variables. Having the state mean "don't keep going" seems useful enough to outweigh the occasional accident of "I meant to have a rule here but forgot it" case.

So I'm changing splicing to be null tolerant (well, tolerant in the sense that it won't abruptly fail, but triggers a rule mismatch).

The bigger issue is that leading colon for this is likely not going to be what it is, given the new general semantics of "optionality" that leading colon implies. So I'm thinking that's going to be doubled groups again.

while ((mode = 'read)) [some chunk]