Initial version of environment model

src/environment.rs

@@ -0,0 +1,31 @@
+use crate::parser::Datum;
+use crate::parser::Datum::{Procedure, Symbol};
+use crate::parser::make_symbol;
+use crate::primitives;
+
+type Mapping = (Datum, Datum);
+type Frame = Vec<Mapping>;
+pub type Env = Vec<Frame>;
+
+pub fn get_global_environment() -> Env {
+    vec![vec![(make_symbol("+"), Procedure(primitives::add)),
+              (make_symbol("*"), Procedure(primitives::mul)),
+              (make_symbol("-"), Procedure(primitives::sub))]]
+}
+
+pub fn lookup_variable_value(exp: &Datum, env: &Env) -> Datum {
+    if let Symbol(symbol_name) = exp {
+        for frame in env {
+            for mapping in frame {
+                if let (Symbol(current_name), datum) = mapping {
+                    if symbol_name == current_name {
+                        return datum.clone();
+                    }
+                }
+            }
+        }
+    } else {
+        panic!("LOOKUP-VARIABLE-VALUE -- not-a-symbol {:?}", exp)
+    }
+    panic!("LOOKUP-VARIABLE-VALUE -- unbound-variable-error {:?}", exp)
+}