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|
use std::collections::HashMap;
use dhall_syntax::{Label, V};
/// Stores a pair of variables: a normal one and if relevant one
/// that corresponds to the alpha-normalized version of the first one.
/// Equality is up to alpha-equivalence.
#[derive(Clone, Eq)]
pub struct AlphaVar {
normal: V<Label>,
alpha: Option<V<()>>,
}
// Exactly like a Label, but equality returns always true.
// This is so that Value equality is exactly alpha-equivalence.
#[derive(Clone, Eq)]
pub struct AlphaLabel(Label);
pub(crate) trait Shift: Sized {
// Shift an expression to move it around binders without changing the meaning of its free
// variables. Shift by 1 to move an expression under a binder. Shift by -1 to extract an
// expression from under a binder, if the expression does not refer to that bound variable.
// Returns None if delta was negative and the variable was free in the expression.
fn shift(&self, delta: isize, var: &AlphaVar) -> Option<Self>;
fn over_binder<T>(&self, x: T) -> Option<Self>
where
T: Into<AlphaVar>,
{
self.shift(-1, &x.into())
}
fn under_binder<T>(&self, x: T) -> Self
where
T: Into<AlphaVar>,
{
// Can't fail since delta is positive
self.shift(1, &x.into()).unwrap()
}
fn under_multiple_binders(&self, shift_map: &HashMap<Label, usize>) -> Self
where
Self: Clone,
{
let mut v = self.clone();
for (x, n) in shift_map {
v = v.shift(*n as isize, &x.into()).unwrap();
}
v
}
}
pub(crate) trait Subst<T> {
fn subst_shift(&self, var: &AlphaVar, val: &T) -> Self;
}
impl AlphaVar {
pub(crate) fn to_var(&self, alpha: bool) -> V<Label> {
match (alpha, &self.alpha) {
(true, Some(x)) => V("_".into(), x.1),
_ => self.normal.clone(),
}
}
pub(crate) fn from_var(normal: V<Label>) -> Self {
AlphaVar {
normal,
alpha: None,
}
}
pub(crate) fn from_var_and_alpha(normal: V<Label>, alpha: usize) -> Self {
AlphaVar {
normal,
alpha: Some(V((), alpha)),
}
}
}
impl AlphaLabel {
pub(crate) fn to_label_maybe_alpha(&self, alpha: bool) -> Label {
if alpha {
"_".into()
} else {
self.to_label()
}
}
pub(crate) fn to_label(&self) -> Label {
self.clone().into()
}
}
impl Shift for AlphaVar {
fn shift(&self, delta: isize, var: &AlphaVar) -> Option<Self> {
Some(AlphaVar {
normal: self.normal.shift(delta, &var.normal)?,
alpha: match (&self.alpha, &var.alpha) {
(Some(x), Some(v)) => Some(x.shift(delta, v)?),
_ => None,
},
})
}
}
impl Shift for () {
fn shift(&self, _delta: isize, _var: &AlphaVar) -> Option<Self> {
Some(())
}
}
impl<T> Subst<T> for () {
fn subst_shift(&self, _var: &AlphaVar, _val: &T) -> Self {}
}
impl std::cmp::PartialEq for AlphaVar {
fn eq(&self, other: &Self) -> bool {
match (&self.alpha, &other.alpha) {
(Some(x), Some(y)) => x == y,
(None, None) => self.normal == other.normal,
_ => false,
}
}
}
impl std::cmp::PartialEq for AlphaLabel {
fn eq(&self, _other: &Self) -> bool {
true
}
}
impl std::fmt::Debug for AlphaVar {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
match &self.alpha {
Some(a) => write!(f, "AlphaVar({}, {})", self.normal, a.1),
None => write!(f, "AlphaVar({}, free)", self.normal),
}
}
}
impl std::fmt::Debug for AlphaLabel {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "AlphaLabel({})", &self.0)
}
}
impl From<Label> for AlphaVar {
fn from(x: Label) -> AlphaVar {
AlphaVar {
normal: V(x, 0),
alpha: Some(V((), 0)),
}
}
}
impl<'a> From<&'a Label> for AlphaVar {
fn from(x: &'a Label) -> AlphaVar {
x.clone().into()
}
}
impl From<AlphaLabel> for AlphaVar {
fn from(x: AlphaLabel) -> AlphaVar {
let l: Label = x.into();
l.into()
}
}
impl<'a> From<&'a AlphaLabel> for AlphaVar {
fn from(x: &'a AlphaLabel) -> AlphaVar {
x.clone().into()
}
}
impl From<Label> for AlphaLabel {
fn from(x: Label) -> AlphaLabel {
AlphaLabel(x)
}
}
impl From<AlphaLabel> for Label {
fn from(x: AlphaLabel) -> Label {
x.0
}
}
|
