So TRY is a way to convert definitional errors into soft failure, e.g. a NULL
>> first null
** Error: FIRST on NULL requires use of TRY
>> try first null
== ~null~ ; anti
I had a thought that instead of just always returning NULL, there might be a way to have constructs raise errors that carried a payload... which would give a "fallback" value that represented something you might want vs. NULL in the case of acknowledging the error. (The only rule would be that a function emitting such an error should not have any side effects if it emits one of these errors.)
So various constructs could offer meanings for what the TRY actually does:
>> unquote first ['abc]
== abc
>> unquote first [def]
** Error: Can't Unquote Plain WORD!, use TRY if Intentional
>> try unquote first [def]
== def
Another application: at one point I suggested it would be nice to be able to have a COLLECT that didn't actually KEEP anything return NULL, so it could be ELSE-reactive. That meant if you really wanted an empty block, you'd have to do something like any [collect [...], copy []] which was ugly. But the error could hold a freshly-copied block to be the result of a TRY:
>> collect [keep if false [<not kept>]]
** Error: COLLECT didn't KEEP anything, use TRY if intended
>> try collect [keep if false [<not kept>]]
== []
There are various technical reasons why ELSE can't be reactive to TRY-style errors. One fairly important one would be that if you used ELSE with a branching construct, it wouldn't be able to tell the difference between a branch that had a TRY-error and no branch taken:
>> case [true [unquote first [def]]] else [print "Took branch, runs due to UNQUOTE"]
Took branch, runs due to UNQUOTE ; this would be bad
But... maybe the trick could be that if you pass ELSE a function that takes an argument, it assumes that means you handle TRY-style errors? (Because otherwise it could only be void/null... why take an arg?)
>> (unquote first [def]) else arg -> [print ["TRY error" arg]]
TRY error def
It's a weird thought. Anyway, just wanted to write up this parallel-universe TRY concept...