PARSE: the DO keyword

The DO keyword requested by Gabriele has been added to parse in A91.

It is an advanced parse action that evaluates the input block as REBOL code then uses a rule to match the result.

For example:

>> parse [1 + 2] [do integer!]
== true

or, more specifically:

>> parse [1 + 2] [do quote 3]
== true

and:

>> parse [append "ab" "c"] [do "abc"]
== true

and it operates inline, as you can see from the literal word test used below:

>> parse [test append "ab" "c"] ['test do "abc"]
== true

The DO keyword only works with block inputs (not strings), and only specific match cases are valid: skip, none, end, quote, into, and a direct value.

More examples:

>> parse [] [do end]
== true

>> parse [none] [do none]
== true

>> parse ["abc"] [do none]
== false

>> parse [] [do none]
== false

>> parse ["abc"] [do "abc"]
== true

>> parse [append "ab" "c"] [do "abc"]
== true

>> parse [remove "abc"] [do "bc"]
== true

>> parse ['abc] [do 'abc]
== true

>> parse ['abc] [do 'a]
== false

>> parse ["abc"] [do ["a" "b" "c"]]
== false

>> parse ["abc"] [do string!]
== true

>> parse ["abc"] [do number!]
== false

>> parse ["abc"] [do series!]
== true

>> parse [1 + 2] [do quote 3]
== true

>> parse [1 + 2] [do quote (1 + 2)]
== true

>> parse [append [a] 'b] [do into ['a 'b]]
== true

>> parse [append "ab" "c"] [do into ["ab" "c"]]
== true

>> parse [num: 1 + 2] [
    set var set-word!
    do [set num integer!]
    (print [var num])
]
num: 3
== true

Note this final example. Prefix actions are not currently valid for DO. Here, the num is set within the DO match rule. That is, you must write:

do [set num integer!]

not:

set num do integer!

but, that may be added in a future release.

This feature was non-trivial to implement, and it is worth testing well to make sure that it's solid.

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