diff --git a/src/core/cmd.rs b/src/core/cmd.rs
new file mode 100644
index 0000000..4363062
--- /dev/null
+++ b/src/core/cmd.rs
@@ -0,0 +1,14 @@
+use crate::core::value::Value;
+use crate::core::expr::Expr;
+
+pub trait Command {
+    fn params(&self) -> Vec<Expr> { vec![] }
+    fn requires(&self) -> Vec<Expr> { vec![] }
+    fn provides(&self) -> Vec<Expr> { vec![] }
+    fn call(&mut self, args: Vec<Value>) -> Value;
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+}
diff --git a/src/core/eval.rs b/src/core/eval.rs
new file mode 100644
index 0000000..5a59ffb
--- /dev/null
+++ b/src/core/eval.rs
@@ -0,0 +1,183 @@
+use std::collections::HashMap;
+use std::error::Error;
+use crate::core::{
+    value::Value,
+    expr::Expr,
+    sym::Sym,
+    cmd::Command
+};
+
+pub struct Evaluator {
+    sym: Sym,
+    commands: HashMap<String, Box<dyn Command>>
+}
+
+impl Evaluator {
+    pub fn new(sym: Sym) -> Self {
+        Self {
+            sym,
+            commands: HashMap::new()
+        }
+    }
+
+    pub fn run(&mut self, expr: &Expr) -> Result<Value, Box<dyn Error>> {
+        match expr {
+            Expr::BuiltIn(value) => Ok(value.clone()),
+            Expr::Ident(value) => {
+                if let Some(val) = self.sym.get(value) {
+                    Ok(val)
+                } else {
+                    Err(format!("'{}' is not declared", value).into())
+                }
+            },
+
+            Expr::Call(value, args) => {
+                let mut val_args: Vec<Value> = vec![];
+
+                for arg in args {
+                    val_args.push(self.run(arg)?);
+                }
+
+                let mut name = String::from("");
+
+                match self.sym.get(value) {
+                    Some(Value::Command(n)) => {name = n;}
+                    None => {
+                        return Err(
+                            format!(
+                                "command '{}' is not declared",
+                                value).into()
+                        );
+                    },
+                    _ => {
+                        return Err(
+                        format!(
+                            "'{}' is not a command",
+                            value).into()
+                        );
+                    }
+                }
+
+                match self.commands.get_mut(&name) {
+                    Some(cmd) => {
+                        Ok(cmd.call(val_args))
+                    },
+
+                    None => Err(
+                        format!(
+                            "command '{}' is not declared",
+                            value).into()
+                        )
+                }
+            },
+
+            _ => Err(format!("unknown expr {:?}", expr).into())
+        }
+    }
+
+    pub fn new_command(&mut self, name: &str, cmd: Box<dyn Command>) {
+        self.commands.insert(name.to_string(), cmd);
+    }
+
+    pub fn sym<T>(&mut self, callback: T)
+    where T: Fn(&mut Sym) {
+        callback(&mut self.sym);
+    }
+}
+
+mod test {
+    use super::*;
+    type TestResult = Result<(), Box<dyn std::error::Error>>;
+
+    fn test_eval_init<T>(expr: Expr, value: Value, init: T)
+    -> TestResult
+where T: Fn(&mut Evaluator) {
+        let sym = Sym::new();
+        let mut eval = Evaluator::new(sym);
+        init(&mut eval);
+        let res = eval.run(&expr)?;
+
+        assert_eq!(res, value);
+        Ok(())
+    }
+
+    fn test_eval(expr: Expr, value: Value) -> TestResult {
+        test_eval_init(expr, value, |_| {})
+    }
+
+    #[test]
+    fn builtins() -> TestResult {
+        test_eval(
+            Expr::BuiltIn(Value::Int(42)),
+            Value::Int(42)
+        )?;
+
+        test_eval(
+            Expr::BuiltIn(Value::String("hello".to_string())),
+            Value::String("hello".to_string())
+        )?;
+
+        test_eval(
+            Expr::BuiltIn(Value::Symbol("hello".to_string())),
+            Value::Symbol("hello".to_string()),
+        )?;
+
+        test_eval(
+            Expr::BuiltIn(Value::Array(vec![
+                Value::Bool(true),
+                Value::Float(3.2),
+            ])),
+
+            Value::Array(vec![
+                Value::Bool(true),
+                Value::Float(3.2),
+            ]),
+        )?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn ident() -> TestResult {
+        test_eval_init(
+            Expr::Ident(String::from("X")),
+            Value::Symbol(String::from("hello")),
+            |eval| {
+                eval.sym(|s| {
+                    s.assign("X", Value::Symbol("hello".to_string()));
+                })
+            }
+        )?;
+
+        Ok(())
+    }
+
+    #[test]
+    fn command() -> TestResult {
+        struct MockedCmd {
+        }
+
+        impl Command for MockedCmd {
+            fn call(&mut self, args: Vec<Value>) -> Value {
+                if let Some(Value::Int(val)) = args.get(0) {
+                    Value::Int(val * 2)
+                } else {
+                    Value::Int(0)
+                }
+            }
+        }
+
+        test_eval_init(
+            Expr::Call(String::from("X"), vec![Expr::BuiltIn(Value::Int(12))]),
+            Value::Int(24),
+            |eval| {
+                eval.new_command("CMD", Box::new(MockedCmd {}));
+                eval.sym(|s| {
+                    s.assign("X", Value::Command(String::from("CMD")));
+                })
+            }
+        )?;
+
+        Ok(())
+    }
+}
diff --git a/src/core/expr.rs b/src/core/expr.rs
new file mode 100644
index 0000000..a82d925
--- /dev/null
+++ b/src/core/expr.rs
@@ -0,0 +1,10 @@
+use crate::core::value::Value;
+
+#[derive(Debug)]
+#[derive(PartialEq)]
+#[derive(Clone)]
+pub enum Expr {
+    BuiltIn(Value),
+    Ident(String),
+    Call(String, Vec<Expr>)
+}
diff --git a/src/core/instr.rs b/src/core/instr.rs
new file mode 100644
index 0000000..414bb67
--- /dev/null
+++ b/src/core/instr.rs
@@ -0,0 +1,14 @@
+use crate::core::expr::Expr;
+
+#[derive(Debug)]
+#[derive(PartialEq)]
+#[derive(Clone)]
+pub enum Instr {
+    Module(Vec<Instr>),
+    Expr(Expr),
+    Assign(String, Vec<Expr>),
+    AssignIf(String, Vec<Expr>),
+    AssignArray(String, Vec<Expr>),
+    Command(String, Vec<Expr>, Vec<Expr>, Vec<Instr>),
+    Task(String, Vec<Instr>)
+}
diff --git a/src/core/mod.rs b/src/core/mod.rs
new file mode 100644
index 0000000..915cf6b
--- /dev/null
+++ b/src/core/mod.rs
@@ -0,0 +1,6 @@
+pub mod expr;
+pub mod instr;
+pub mod value;
+pub mod eval;
+pub mod sym;
+pub mod cmd;
diff --git a/src/core/sym.rs b/src/core/sym.rs
new file mode 100644
index 0000000..f13a04e
--- /dev/null
+++ b/src/core/sym.rs
@@ -0,0 +1,91 @@
+use std::collections::HashMap;
+use crate::core::value::Value;
+
+pub struct Sym {
+    table: HashMap<String, Value>
+}
+
+impl Sym {
+    pub fn new() -> Self {
+        Self {
+            table: HashMap::new()
+        }
+    }
+
+    pub fn get(&self, identifier: &str) -> Option<Value> {
+        self.table.get(identifier).cloned()
+    }
+
+    pub fn assign(&mut self, identifier: &str, value: Value) {
+        self.table.insert(identifier.to_string(), value);
+    }
+
+    pub fn assign_array(&mut self, identifier: &str, value: Value) {
+        match self.table.get(identifier) {
+            Some(Value::Array(arr)) => {
+                let mut new_arr: Vec<Value> = vec![];
+                new_arr.extend(arr.clone());
+                new_arr.push(value.clone());
+                self.table.insert(identifier.to_string(), Value::Array(new_arr));
+            },
+
+            Some(val) => {
+                let mut new_arr: Vec<Value> = vec![val.clone()];
+                new_arr.push(value.clone());
+                self.table.insert(identifier.to_string(), Value::Array(new_arr));
+            },
+            None => {
+                self.table.insert(identifier.to_string(), Value::Array(vec![
+                    value.clone()
+                ]));
+            }
+        }
+    }
+
+    pub fn assign_if(&mut self, identifier: &str, value: Value) {
+        if self.table.get(identifier).is_none() {
+            self.table.insert(identifier.to_string(), value);
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn assign() {
+        let mut sym = Sym::new();
+        assert!(sym.get("X").is_none());
+        sym.assign("X", Value::Int(34));
+        assert_eq!(sym.get("X"), Some(Value::Int(34)));
+    }
+
+    #[test]
+    fn assign_array() {
+        let mut sym = Sym::new();
+        assert!(sym.get("X").is_none());
+        sym.assign_array("X", Value::Float(6.28));
+
+        assert_eq!(sym.get("X"), Some(Value::Array(vec![
+            Value::Float(6.28),
+        ])));
+
+        sym.assign_array("X", Value::Bool(false));
+
+        assert_eq!(sym.get("X"), Some(Value::Array(vec![
+            Value::Float(6.28),
+            Value::Bool(false)
+        ])));
+    }
+
+    #[test]
+    fn assign_if() {
+        let mut sym = Sym::new();
+        assert!(sym.get("X").is_none());
+        sym.assign_if("X", Value::Float(6.28));
+        assert_eq!(sym.get("X"), Some(Value::Float(6.28)));
+        sym.assign_if("X", Value::Int(79));
+        assert_eq!(sym.get("X"), Some(Value::Float(6.28)));
+    }
+}
diff --git a/src/core/value.rs b/src/core/value.rs
new file mode 100644
index 0000000..4261857
--- /dev/null
+++ b/src/core/value.rs
@@ -0,0 +1,14 @@
+use crate::core::cmd::Command;
+
+#[derive(Debug)]
+#[derive(PartialEq)]
+#[derive(Clone)]
+pub enum Value {
+    Int(i32),
+    Float(f64),
+    Bool(bool),
+    String(String),
+    Symbol(String),
+    Array(Vec<Value>),
+    Command(String)
+}
diff --git a/src/main.rs b/src/main.rs
index 0545a4a..7eeb05d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,4 +1,5 @@
 mod cli;
+mod core;
 
 use crate::cli::parse;