Add Value type and replace dyn-nodes with call-exprs 🏗

- In addition to syntax trees there are now `Value`s, which syntax trees can be evaluated into (e.g. the tree is `5+5` and the value is `10`)
- Parsing is completely pure, function calls are not parsed into nodes, but into simple call expressions, which are resolved later
- Functions aren't dynamic nodes anymore, but simply functions which receive their arguments as a table and the layouting context
- Functions may return any `Value`
- Layouting is powered by functions which return the new `Commands` value, which informs the layouting engine what to do
- When a function returns a non-`Commands` value, the layouter simply dumps the value into the document in monospace

1 files changed, 50 insertions, 0 deletions

diff --git a/src/compute/scope.rs b/src/compute/scope.rs
new file mode 100644
index 00000000..1fd4db0b
--- /dev/null
+++ b/src/compute/scope.rs
@@ -0,0 +1,50 @@
+//! Mapping from identifiers to functions.
+
+use std::collections::HashMap;
+use std::fmt::{self, Debug, Formatter};
+
+use super::value::FuncValue;
+
+/// A map from identifiers to functions.
+pub struct Scope {
+    functions: HashMap<String, FuncValue>,
+    fallback: FuncValue,
+}
+
+impl Scope {
+    // Create a new empty scope with a fallback function that is invoked when no
+    // match is found.
+    pub fn new(fallback: FuncValue) -> Self {
+        Self {
+            functions: HashMap::new(),
+            fallback,
+        }
+    }
+
+    /// Associate the given name with the function.
+    pub fn insert(&mut self, name: impl Into<String>, function: FuncValue) {
+        self.functions.insert(name.into(), function);
+    }
+
+    /// Return the function with the given name if there is one.
+    pub fn func(&self, name: &str) -> Option<&FuncValue> {
+        self.functions.get(name)
+    }
+
+    /// Return the function with the given name or the fallback if there is no
+    /// such function.
+    pub fn func_or_fallback(&self, name: &str) -> &FuncValue {
+        self.func(name).unwrap_or_else(|| self.fallback())
+    }
+
+    /// Return the fallback function.
+    pub fn fallback(&self) -> &FuncValue {
+        &self.fallback
+    }
+}
+
+impl Debug for Scope {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.debug_set().entries(self.functions.keys()).finish()
+    }
+}