diff --git a/src/main.rs b/src/main.rs
index d9e79af..42c23cc 100755
--- a/src/main.rs
+++ b/src/main.rs
@@ -14,6 +14,12 @@ fn get_copyright() -> String {
     format!("Misti {}\nCopyright (c) {} Fernando Enrique Araoz Morales", VERSION, year)
 }
 
+/// # Misti
+/// 
+/// Usage:
+/// - `misti` : Compiles the current project according to the settings in the misti.json file
+/// - `misti --watch, -w` : Starts the compiler in watch mode
+/// - `misti -i FILE -o OUTPUT` : Compiles FILE and writes the result in OUTPUT
 fn main() -> io::Result<()> {
     print!("{}", get_copyright());
     repl::run()
diff --git a/src/repl/mod.rs b/src/repl/mod.rs
index 0cbf20c..5a83968 100755
--- a/src/repl/mod.rs
+++ b/src/repl/mod.rs
@@ -1,6 +1,7 @@
 use std::io::{self, Write};
 
 use super::lexic;
+use super::syntax;
 
 fn compile(input: &String) {
     let _tokens = lexic::get_tokens(input);
@@ -8,9 +9,11 @@ fn compile(input: &String) {
     match _tokens {
         Ok(tokens) => {
             for token in tokens {
-                print!("[{}] ", token.value);
+                print!("[{:?} {}] ", token.token_type, token.value);
             }
             println!("");
+
+            let _ast = syntax::construct_ast(Vec::new());
         },
         Err(error) => {
             eprintln!("Error scanning.\n{} at pos {}", error.reason, error.position)
diff --git a/src/syntax/expression.rs b/src/syntax/expression.rs
new file mode 100644
index 0000000..f3539e4
--- /dev/null
+++ b/src/syntax/expression.rs
@@ -0,0 +1,25 @@
+use crate::token::Token;
+
+use super::types::Expression;
+
+
+pub fn try_parse(tokens: &Vec<Token>, pos: usize) -> Option<Expression> {
+    None
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::lexic::get_tokens;
+
+    #[test]
+    fn should_parse_a_number() {
+        let tokens = get_tokens(&String::from("40")).unwrap();
+        let expression = try_parse(&tokens, 0).unwrap();
+
+        match expression {
+            Expression::Number(value) => assert_eq!("40", value),
+        }
+    }
+}
diff --git a/src/syntax/grammar.md b/src/syntax/grammar.md
new file mode 100644
index 0000000..3af9772
--- /dev/null
+++ b/src/syntax/grammar.md
@@ -0,0 +1,23 @@
+# Grammar
+
+## Module
+
+A module is (commonly) a single source file.
+
+- `module = variable_binding*`
+
+
+### `variable_binding`
+
+A declaration with `var` or `val`.
+
+- `var = "var"`
+- `val = "val"`
+- `variable_binding = (var | val), identifier, "=", expression`
+
+### `expression`
+
+For now just a number
+
+- `expression = number`
+
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index 2816caf..c412f75 100755
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -1,7 +1,11 @@
-
 use super::token::Token;
 
+mod expression;
+mod types;
+
 /// Constructs the Misti AST from a vector of tokens
-pub fn _construct_ast(_tokens: Vec<Token>) -> Result<(), String> {
+pub fn construct_ast(_tokens: Vec<Token>) -> Result<types::ModuleAST, String> {
     Err(String::from("NOT IMPLEMENTED"))
 }
+
+
diff --git a/src/syntax/types.rs b/src/syntax/types.rs
new file mode 100644
index 0000000..197e7bf
--- /dev/null
+++ b/src/syntax/types.rs
@@ -0,0 +1,17 @@
+
+pub struct ModuleAST {
+    bindings: Vec<Binding>,
+}
+
+pub enum Binding {
+    Val(ValBinding),
+}
+
+pub struct ValBinding {
+    identifier: String,
+    expression: Expression,
+}
+
+pub enum Expression {
+    Number(String),
+}