plan for some revamp

PLAN.md

@@ -99,4 +99,8 @@ Helpers
 * `print(rel)`
 * `print_schema(rel)`
 * `print_plan(rel)`
-* `print_optimized(rel)`
+* `print_optimized(rel)`
+
+Aggregation
+
+* Aggregation functions should implement `.step()` and `.result()`

src/eval.rs

@@ -0,0 +1,70 @@
+use std::collections::BTreeMap;
+use std::ops::Range;
+use std::sync::Arc;
+use crate::relation::value::Value;
+use crate::error::Result;
+use crate::relation::table::MegaTuple;
+use crate::relation::typing::Typing;
+
+pub struct InterpretContext;
+
+pub struct ApplyContext;
+
+pub struct ArgsIterator<'a> {
+    value: Value<'a>
+}
+
+pub struct TypingIterator;
+
+pub trait Op {
+    fn arity(&self) -> Range<usize>;
+    fn apply_raw<'a>(&self, arg: Value<'a>) -> Result<Value<'a>>;
+    fn typing(&self, arg_types: TypingIterator) -> Result<Typing>;
+    fn apply<'a>(&self, ctx: &ApplyContext, args: ArgsIterator<'a>) -> Result<Value<'a>>;
+    fn interpret<'a>(&self, ctx: &InterpretContext, arg: ArgsIterator<'a>) -> Result<(Value<'a>, bool)>;
+}
+
+pub trait AggregationOp {
+
+}
+
+// NOTE: the interpreter can hold global states for itself
+
+// Tiers of values:
+//
+// * Scalars
+// * Aggregates
+// * Typings
+// * RelPlan
+// * QueryTemplate
+
+// Global DB should be a C++ sharedptr
+// Every session gets its own temp DB
+// Sessions are reused after use
+
+// lower sectors layouts
+// [env_stack_depth; string_name, flags*] -> resolvable data
+// [env_stack_depth; tid, flags*] -> table definitions
+// table 10000 for serial numbers (auto incrementing, no transaction)
+// tables start at 10001
+
+pub struct DBInstance;
+
+impl DBInstance {
+    pub fn get_meta_by_id() {}
+    pub fn get_meta_by_name() {}
+    pub fn put_meta() {}
+}
+
+pub struct InterpreterSession {
+    global_db: DBInstance,
+    local_db: DBInstance,
+    env_depth: usize,
+    session_params: BTreeMap<String, Arc<dyn Op>>,
+}
+
+impl InterpreterSession {
+    pub fn push_env() {}
+    pub fn pop_env() {}
+    pub fn destroy() {}
+}

src/lib.rs

@@ -15,6 +15,7 @@ pub mod db;
 pub mod error;
 pub mod parser;
 pub mod relation;
+pub(crate) mod eval;
 
 #[cfg(test)]
 mod tests {