Work on interpreter

src/interpreter.rs

@@ -0,0 +1,34 @@
+use crate::parser::Datum;
+use crate::parser::Datum::*;
+
+fn add(args: Vec<Datum>) -> Datum {
+    Number(42)
+}
+
+fn lookup_variable_value(exp: Datum) -> Datum {
+    if let Symbol(string) = exp {
+        match string.as_str() {
+            "+" => Procedure(add),
+            _ => panic!("unbound-variable {:?}", string)
+        }
+    } else {
+        panic!("LOOKUP-VARIABLE-VALUE -- not-a-symbol {:?}", exp)
+    }
+}
+
+fn application(exp: Datum) -> Datum {
+    if let List(vector) = exp {
+        vector[0]
+    } else {
+        panic!("APPLICATION -- cannot-apply {:?}", exp)
+    }
+}
+
+pub fn interpret(exp: Datum) -> Datum {
+    match exp {
+        Boolean(_) | Number(_) => exp,
+        Symbol(_) => lookup_variable_value(exp),
+        List(_) => application(exp),
+        _ => panic!("unknown-expression {:?}", exp)
+    }
+}