2 files changed, 88 insertions, 0 deletions
diff --git a/src/lib/eval/arith.rs b/src/lib/eval/arith.rs
new file mode 100644
index 0000000..fae69de
--- /dev/null
+++ b/src/lib/eval/arith.rs
@@ -0,0 +1,34 @@
+use std::ops::Add;
+use super::super::types::Type;
+use super::super::types::Type::*;
+use super::super::types::Number;
+
+impl Add for Type {
+    type Output = Result<Type, String>;
+
+    fn add(self, other: Type) -> Result<Type, String> {
+        match (self, other) {
+            (Bool(_), _) | (_, Bool(_)) |
+            (Str(_), _) | (_, Str(_)) |
+            (Operator(_), _) | (_, Operator(_)) => {
+                Err("Cannot add these types".to_string())
+            },
+            (Number(Number::Int(i)), Number(Number::Float(f))) |
+            (Number(Number::Float(f)), Number(Number::Int(i))) => {
+                Ok(Number(Number::Float(f + i as f32)))
+            },
+
+            (Number(Number::Int(a)), Number(Number::Int(b))) => {
+                Ok(Number(Number::Int(a + b)))
+            },
+
+            (Number(Number::Float(a)), Number(Number::Float(b))) => {
+                Ok(Number(Number::Float(a + b)))
+            },
+
+            (Symbol(_), Number(_)) |
+            (Number(_), Symbol(_)) |
+            (Symbol(_), Symbol(_)) => Err("Not yet implemented".to_string())
+        }
+    }
+}
diff --git a/src/lib/eval/mod.rs b/src/lib/eval/mod.rs
new file mode 100644
index 0000000..b582397
--- /dev/null
+++ b/src/lib/eval/mod.rs
@@ -0,0 +1,54 @@
+use std::collections::HashMap;
+use super::types::Op;
+use super::types::Op::*;
+use super::types::SEXP;
+use super::types::SEXP::*;
+use super::types::Type::*;
+use super::types::Number;
+
+pub mod arith;
+
+pub type Env = HashMap<String, SEXP>;
+
+pub fn eval(expr: &SEXP, env: &mut Env) -> Result<SEXP, String> {
+    match expr {
+        Atom(ref x) => Ok(Atom((*x).clone())),
+        Sexpr(ref s) => seval(s, env)
+    }
+}
+
+fn seval(sexpr: &[SEXP], env: &mut Env) -> Result<SEXP, String> {
+    if sexpr.is_empty() {
+        return Err("Empty S Expression".to_string())
+    }
+
+    let op = match &sexpr[0] {
+        Atom(Operator(x)) => *x,
+        Atom(Symbol(_s)) => return Err("Not yet implemented".to_string()),
+        x => return Err(format!("{:?} is not a procedure", x))
+    };
+
+    op_eval(op, &sexpr[1..], env)
+}
+
+fn op_eval(op: Op, expr: &[SEXP], env: &mut Env) -> Result<SEXP, String> {
+    match op {
+        Add => op_add(expr, env),
+        //Define => define(expr, env),
+        _ => Err("Not yet implemented".to_string())
+    }
+}
+
+fn op_add(expr: &[SEXP], env: &mut Env) -> Result<SEXP, String> {
+    let mut acc = Number(Number::Int(0));
+    let mut i = 0;
+    while i < expr.len() {
+        let b = match eval(&expr[i], env)? {
+            Atom(x) => x,
+            Sexpr(_) => panic!("This should have evaluated to an atom")
+        };
+        acc = (acc + b)?;
+        i += 1;
+    }
+    Ok(SEXP::Atom(acc))
+}