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use std::collections::HashMap;
/// A `(Context a)` associates `Text` labels with values of type `a`
///
/// The `Context` is used for type-checking when `(a = Expr X)`
///
/// * You create a `Context` using `empty` and `insert`
/// * You transform a `Context` using `fmap`
/// * You consume a `Context` using `lookup` and `toList`
///
/// The difference between a `Context` and a `Map` is that a `Context` lets you
/// have multiple ordered occurrences of the same key and you can query for the
/// `n`th occurrence of a given key.
///
#[derive(Debug, Clone)]
pub struct Context<'i, T>(HashMap<&'i str, Vec<T>>);
impl<'i, T> Context<'i, T> {
/// An empty context with no key-value pairs
pub fn new() -> Self {
Context(HashMap::new())
}
/// Look up a key by name and index
///
/// ```c
/// lookup _ _ empty = Nothing
/// lookup k 0 (insert k v c) = Just v
/// lookup k n (insert k v c) = lookup k (n - 1) c -- 1 <= n
/// lookup k n (insert j v c) = lookup k n c -- k /= j
/// ```
pub fn lookup<'a>(&'a self, k: &str, n: usize) -> Option<&'a T> {
self.0.get(k).and_then(|v| v.get(v.len() - 1 - n))
}
pub fn map<U, F: Fn(&T) -> U>(&self, f: F) -> Context<'i, U> {
Context(
self.0
.iter()
.map(|(k, v)| (*k, v.iter().map(&f).collect()))
.collect(),
)
}
}
impl<'i, T: Clone> Context<'i, T> {
/// Add a key-value pair to the `Context`
pub fn insert(&self, k: &'i str, v: T) -> Self {
let mut ctx = (*self).clone();
{
let m = ctx.0.entry(k).or_insert_with(Vec::new);
m.push(v);
}
ctx
}
}
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