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)
+    }
+}

src/main.rs

@@ -1,9 +1,13 @@
 mod lexer;
 mod parser;
+mod interpreter;
+
 
 fn main() {
-    let scm_code = "(+ a (* 32 b) c #t #f)";
+    // let scm_code = "(+ a (* 32 b) c #t #f)";
+    let scm_code = "+";
     let tokens = lexer::read(scm_code);
     let datum = parser::parse(tokens);
-    println!("{:?}", datum);
+    let result = interpreter::interpret(datum);
+    println!("{:?}", result);
 }

src/parser.rs

@@ -2,12 +2,14 @@ use crate::lexer::Token;
 use crate::lexer::Tokens;
 
 
+
 #[derive(Debug)]
 pub enum Datum {
     Boolean(bool),
     Number(i64),
     Symbol(String),
     List(Vec<Datum>),
+    Procedure(fn(Vec<Datum>) -> Datum),
 }
 
 pub fn parse(tokens: Tokens) -> Datum {