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use crate::semantics::{AlphaVar, Value, ValueKind};
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum NzVar {
/// Reverse-debruijn index: counts number of binders from the bottom of the stack.
Bound(usize),
/// Fake fresh variable generated for expression equality checking.
Fresh(usize),
}
#[derive(Debug, Clone)]
enum NzEnvItem {
// Variable is bound with given type
Kept(Value),
// Variable has been replaced by corresponding value
Replaced(Value),
}
#[derive(Debug, Clone)]
pub(crate) struct NzEnv {
items: Vec<NzEnvItem>,
}
impl NzVar {
pub fn new(idx: usize) -> Self {
NzVar::Bound(idx)
}
pub fn fresh() -> Self {
use std::sync::atomic::{AtomicUsize, Ordering};
// Global counter to ensure uniqueness of the generated id.
static FRESH_VAR_COUNTER: AtomicUsize = AtomicUsize::new(0);
let id = FRESH_VAR_COUNTER.fetch_add(1, Ordering::SeqCst);
NzVar::Fresh(id)
}
/// Get index of bound variable.
/// Panics on a fresh variable.
pub fn idx(&self) -> usize {
match self {
NzVar::Bound(i) => *i,
NzVar::Fresh(_) => panic!(
"Trying to use a fresh variable outside of equality checking"
),
}
}
}
impl NzEnv {
pub fn new() -> Self {
NzEnv { items: Vec::new() }
}
pub fn insert_type(&self, t: Value) -> Self {
let mut env = self.clone();
env.items.push(NzEnvItem::Kept(t));
env
}
pub fn insert_value(&self, e: Value) -> Self {
let mut env = self.clone();
env.items.push(NzEnvItem::Replaced(e));
env
}
pub fn lookup_val(&self, var: &AlphaVar) -> ValueKind {
let idx = self.items.len() - 1 - var.idx();
match &self.items[idx] {
NzEnvItem::Kept(_) => ValueKind::Var(NzVar::new(idx)),
NzEnvItem::Replaced(x) => x.kind().clone(),
}
}
pub fn lookup_ty(&self, var: &AlphaVar) -> Value {
let idx = self.items.len() - 1 - var.idx();
match &self.items[idx] {
NzEnvItem::Kept(ty) => ty.clone(),
NzEnvItem::Replaced(x) => x.get_type().unwrap(),
}
}
}
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