On 10/21/2016 05:50 PM, Dan Liebgold wrote:
Hi all -
In the process of putting together a somewhat complex application
using syntax-parse, I discovered that when I specified a repeated
pattern in a syntax-class (which was incorrect) AND I had a certain
usage of the syntax transformer with an error, it would lead to
degenerate performance (Racket would run out of memory or it would
need to be killed.
Please see this example and uncomment the last non-paren line to see
it fall down:
http://pasterack.org/pastes/50261
The problem is that the `remap-entry` syntax class can succeed in
multiple ways because of the repeated pattern. So when you have a macro
use like
(m remap-entry-1 remap-entry-2 ... remap-entry-n bad)
when parsing `bad` fails, syntax-parse backtracks and tries the other
way of parsing all of the previous entries, so what should be a linear
parse turns exponential.
The easiest fix is to add the #:commit option to the `remap-entry`
syntax class. Then when `remap-entry` succeeds the first time, it throws
away the choice points it created, so the repeated pattern is never
considered.
It's usually reasonable to use #:commit with regular syntax classes
(unless you're using syntax-parse to do logic programming). It's usually
a bad idea to use it with splicing syntax classes, though, since it can
lead to missed parses or bad error messages.
Two other comments:
1. A term like `(a <- blend)` will match the first pattern and treat
`blend` as a `remap:id`. If you don't want that to happen, there are two
ways to prevent it. One is to define a syntax class like `id` that
excludes all of the reserved words like `<-` and `blend` and use that
for pattern variables like `remap`. The other is to reorder the patterns
and use cut (~!):
(pattern (source:id <- blend ~! remap:id
extras:remap-entry-extra-params))
(pattern (source:id <- remap:id extras:remap-entry-extra-params))
2. `format-id` is useful for creating identifiers.
Ryan
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