From 4d92f06c98c98c014a02c218b6c31cf81d9f0fec Mon Sep 17 00:00:00 2001
From: Nadrieril
Date: Wed, 6 Mar 2019 16:30:40 +0100
Subject: Move context to dhall_core

---
 dhall_core/src/context.rs | 56 +++++++++++++++++++++++++++++++++++++++++++++++
 dhall_core/src/lib.rs     |  1 +
 2 files changed, 57 insertions(+)
 create mode 100644 dhall_core/src/context.rs

(limited to 'dhall_core/src')

diff --git a/dhall_core/src/context.rs b/dhall_core/src/context.rs
new file mode 100644
index 0000000..b88a677
--- /dev/null
+++ b/dhall_core/src/context.rs
@@ -0,0 +1,56 @@
+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
+    }
+}
diff --git a/dhall_core/src/lib.rs b/dhall_core/src/lib.rs
index e1d0f7d..dbdd08c 100644
--- a/dhall_core/src/lib.rs
+++ b/dhall_core/src/lib.rs
@@ -8,3 +8,4 @@ lalrpop_mod!(pub grammar);
 mod grammar_util;
 pub mod lexer;
 pub mod parser;
+pub mod context;
-- 
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