3 files changed, 103 insertions, 0 deletions
diff --git a/src/sexp/mod.rs b/src/sexp/mod.rs
new file mode 100644
index 0000000..c920e41
--- /dev/null
+++ b/src/sexp/mod.rs
@@ -0,0 +1,27 @@
+
+pub mod step;
+pub mod util;
+
+/// A leaf node for S-Expressions.
+///
+/// May represent built-in operators,
+/// variables (to be added), functions
+/// (to be added) or values.
+#[derive(Debug)]
+#[derive(PartialEq)]
+pub enum SLeaf {
+    Add,
+    Sub,
+    Mul,
+    Div,
+    Int(i32),
+}
+
+/// An S-Expression; the defining structure of the language.
+#[derive(Debug)]
+#[derive(PartialEq)]
+pub enum SExp {
+    SCons(Box<SExp>, Box<SExp>),
+    Atom(SLeaf),
+}
+
diff --git a/src/sexp/step.rs b/src/sexp/step.rs
new file mode 100644
index 0000000..86ad9ea
--- /dev/null
+++ b/src/sexp/step.rs
@@ -0,0 +1,49 @@
+
+use crate::sexp::SExp;
+
+impl SExp {
+
+    /// Evaluates the s-expression one step.
+    ///
+    /// **Integer operations**
+    ///
+    /// Addition, subtraction, multiplication and division
+    /// are supported.
+    /// ```rust
+    /// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
+    ///
+    /// assert_eq!(
+    ///     sexp(Add, sexp(1, 1)).step().unwrap(),
+    ///     Atom(Int(2))
+    /// );
+    ///
+    /// assert_eq!(
+    ///     sexp(Sub, sexp(1, 2).step().unwrap()),
+    ///     Atom(Int(-1))
+    /// );
+    ///
+    /// assert_eq!(
+    ///     sexp(Mul, sexp(2, 3).step().unwrap()),
+    ///     Atom(Int(6))
+    /// );
+    ///
+    /// assert_eq!(
+    ///     sexp(Div, sexp(6, 2).step().unwrap()),
+    ///     Atom(Int(3))
+    /// );
+    /// ```
+    ///
+    /// Division truncates the decimals.
+    /// ```rust
+    /// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
+    ///
+    /// assert_eq!(
+    ///     sexp(Div, sexp(5, 2).step().unwrap()),
+    ///     Atom(Int(2))
+    /// );
+    ///
+    /// ```
+    pub fn step(self) -> Result<Self, String> {
+        todo!();
+    }
+}
diff --git a/src/sexp/util.rs b/src/sexp/util.rs
new file mode 100644
index 0000000..a4e3ab4
--- /dev/null
+++ b/src/sexp/util.rs
@@ -0,0 +1,27 @@
+
+use crate::sexp::SExp;
+use crate::sexp::SLeaf;
+use crate::sexp::SExp::*;
+use crate::sexp::SLeaf::*;
+
+impl From<i32> for Box<SExp> {
+    fn from(int: i32) -> Self {
+        Box::new(Atom(Int(int)))
+    }
+}
+
+impl From<SLeaf> for Box<SExp> {
+    fn from(leaf: SLeaf) -> Self {
+        Box::new(Atom(leaf))
+    }
+}
+
+impl From<i32> for SLeaf {
+    fn from(int: i32) -> Self {
+        Int(int)
+    }
+}
+
+pub fn sexp(x: impl Into<Box<SExp>>, y: impl Into<Box<SExp>>) -> SExp {
+    SCons(x.into(), y.into())
+}