Add math and random functions

Register `abs`, `min`, `max`, and `random` functions to the environment.
This also includes adding `PartialOrd` implementation for `Value` to
support comparisons for `min` and `max`.

src/ast/types.rs

@@ -117,6 +117,20 @@ impl PartialEq for Value {
     }
 }
 
+impl PartialOrd for Value {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        match (self, other) {
+            (Value::Int(a), Value::Int(b)) => a.partial_cmp(b),
+            (Value::Float(a), Value::Float(b)) => a.partial_cmp(b),
+            (Value::Int(a), Value::Float(b)) => (*a as f64).partial_cmp(b),
+            (Value::Float(a), Value::Int(b)) => a.partial_cmp(&(*b as f64)),
+            (Value::DateTime(a), Value::DateTime(b)) => a.partial_cmp(b),
+            (Value::Text(a), Value::Text(b)) => a.partial_cmp(b),
+            _ => None,
+        }
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct Signature {
     pub params: StaticType,