initial work on comparison based filters

This enables filters such as
    -time > 2020-01-01
    -time < 2020-02-03
    ...
for time and duration, and later possibly also for more things (such as
a COUNT(...) construct).

This work tries to integrate them into the existing filter system as
seamless as possible, by providing a Comparator which implements
LogFilter.

The "type checking" is done at parse time, so nonsensical comparisons
like -time > 12s flat out give a parse error. This however might be
changed to a more dynamic system with run-time type checking, in which
case we could do away with the type parameter on Producer and simply
work with a generic Value. The comparator would then return an error if
two non-identical types would be compared.

Note that the system does not support arithmetic expressions, only
simple comparisons to constant values.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/fexpr/mod.rs b/src/fexpr/mod.rs
index c6a3a39..1738e44 100644
--- a/src/fexpr/mod.rs
+++ b/src/fexpr/mod.rs
@@ -11,6 +11,9 @@ use itertools::Itertools;
 use lalrpop_util::{lalrpop_mod, lexer::Token, ParseError};
 use thiserror::Error;
 
+trait DateProducer = filters::values::Producer<Output = chrono::DateTime<chrono::Utc>>;
+trait DurationProducer = filters::values::Producer<Output = chrono::Duration>;
+
 lalrpop_mod!(#[allow(clippy::all)] pub grammar, "/fexpr/grammar.rs");
 
 #[derive(Debug)]