Hi all,
A while ago Doug ran across a caveat in the macro system. He had code
like the following in the 'pack' vocab,
MACRO: pack ( str -- quot )
[ pack-table at '[ _ execute ] ] { } map-as
'[ [ [ _ spread ] input<sequence ] B{ } append-outputs-as ] ;
So the macro converts a string into a sequence of quotations by
mapping characters to words using the 'pack-table', then fries this
sequence of quotations into a nested form involving three other
macros: spread, input<sequence and append-outputs-as.
This looks all fine and dandy, but Doug noticed that the macro didn't
expand at compile time. The reason is that the fry form
'[ [ [ _ spread ] input<sequence ] B{ } append-outputs-as ]
Is parsed as
[
[ [ spread ] curry input<sequence ] curry
B{ } append-outputs-as
] curry
However, since input<sequence and append-outputs-as were macros, they
were being called on the result of a 'curry', which is not a literal,
so those macros didn't expand at compile time. At the time, I didn't
think there would be a solution, so I explained this to Doug and we
settled for the slightly less pleasing code below, which does not
suffer from the problem:
MACRO: pack ( str -- quot )
[ pack-table at '[ _ execute ] ] { } map-as
'[ _ spread ]
'[ _ input<sequence ]
'[ _ B{ } append-outputs-as ] ;
However, the above is just a mechanical transformation of the above.
Indeed, even though the macros input<sequence and append-outputs-as
are called with the result of curry, and not with a literal value,
both inputs to the curry are literals, so all the information needed
to expand them at compile time is there.
To fix this, I made macro expansion more general, allowing the first
nested curry version as well as the flat second version. This means
you can also do stuff like this,
: my-combinator ( seq quot -- result ) [ dip append ] curry
input<sequence ; inline
{ "xyz" "foo" } [ reverse ] my-combinator .
=> "zyxfoo"
So this means you can now use fry to construct macro expansions
involving additional macros without worrying as much.
Slava
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