hey it can calculate 4 again xP

src/language_frontend/abstract_syntax_tree/ast.rs

@@ -2,19 +2,23 @@
 
 #[derive(Debug)]
 pub enum Expression<'src> {
+    // Identifier
     Ident(&'src str),
 
+    // Types
     Integer(i64),
     Float(f64),
     String(String),
     Bool(bool),
 
+    // Operations
     Negatation(Box<Expression<'src>>),
     Add(Box<Expression<'src>>, Box<Expression<'src>>),
     Substract(Box<Expression<'src>>, Box<Expression<'src>>),
     Multiply(Box<Expression<'src>>, Box<Expression<'src>>),
     Divide(Box<Expression<'src>>, Box<Expression<'src>>),
 
+    // Keywords
     Var {
         name: &'src str,
         rhs: Box<Expression<'src>>,
@@ -37,16 +41,24 @@ pub fn eval<'src>(
     funcs: &mut Vec<(&'src str, &'src [&'src str], &'src Expression<'src>)>,
 ) -> Result<f64, String> {
     match expr {
-        Expression::Ident(_) => todo!(),
+        Expression::Ident(name) => {
+            if let Some((_, val)) = vars.iter().rev().find(|(var, _)| var == name) {
+                Ok(*val)
+            } else {
+                Err(format!("Cannot find variable `{name}` in scope"))
+            }
+        },
 
-        Expression::Integer(x) => Ok((*x) as f64),
+        // Types
+        Expression::Integer(x) => Ok((*x) as f64), // todo
 
-        Expression::Float(_) => todo!(),
+        Expression::Float(x) => Ok(*x),
 
         Expression::String(_) => todo!(),
 
         Expression::Bool(_) => todo!(),
 
+        // Operations
         Expression::Negatation(lhs) => todo!(),
 
         Expression::Add(lhs, rhs) => Ok(eval(lhs, vars, funcs)? + eval(rhs, vars, funcs)?),
@@ -57,14 +69,26 @@ pub fn eval<'src>(
 
         Expression::Divide(lhs, rhs) => Ok(eval(lhs, vars, funcs)? / eval(rhs, vars, funcs)?),
 
-        Expression::Var { name, rhs, then } => todo!(),
+        // Keywords
+        Expression::Var { name, rhs, then } => {
+            let rhs = eval(rhs, vars, funcs)?;
+            vars.push((*name, rhs));
+            let output = eval(then, vars, funcs);
+            vars.pop();
+            output
+        },
 
         Expression::Function {
             name,
             args,
             body,
             then,
-        } => todo!(),
+        } => {
+            funcs.push((name, args, body));
+            let output = eval(then, vars, funcs);
+            funcs.pop();
+            output
+        },
 
         Expression::Unit => todo!(),
     }

src/language_frontend/abstract_syntax_tree/mod.rs

@@ -1,2 +1,3 @@
 pub mod ast;
 pub mod op;
+pub mod parser;

src/language_frontend/abstract_syntax_tree/parser.rs

@@ -0,0 +1,98 @@
+use chumsky::{
+    combinator::Or, prelude::{choice, just, recursive}, recursive, select, select_ref, text::{self, ascii::ident, whitespace}, IterParser, Parser
+};
+
+use crate::{language_frontend::abstract_syntax_tree::ast::Expression, language_frontend::lexer::tokens::Token};
+
+// goal of parsing is to construct an abstract syntax tree
+
+#[allow(clippy::let_and_return)]
+pub fn parser<'src>() -> impl Parser<'src, &'src [Token<'src>], Expression<'src>> {
+    let ident = select_ref! {
+        Token::Ident(ident) => *ident
+    };
+
+    let keyword = |kw: &'static str| {
+        select! {
+            Token::Keyword(k) if k == kw => ()
+        }
+    };
+
+    let eq = just(Token::Equals);
+
+    let expr = recursive(|expr| {
+        let atom = {
+            let parenthesized = expr
+                .clone()
+                .delimited_by(just(Token::ParenBegin), just(Token::ParenEnd));
+
+            let integer = select! {
+                Token::Integer(n) => Expression::Integer(n),
+            };
+
+            parenthesized.or(integer)
+        };
+
+        let unary = just(Token::Substract)
+            .repeated()
+            .foldr(atom, |_op, rhs| Expression::Negatation(Box::new(rhs)));
+
+        // "Punktrechnung vor Strichrechnung :nerd:"
+
+        let binary_1 = unary.clone().foldl(
+            just(Token::Multiply)
+                .or(just(Token::Divide))
+                .then(unary)
+                .repeated(),
+            |lhs, (op, rhs)| match op {
+                Token::Multiply => Expression::Multiply(Box::new(lhs), Box::new(rhs)),
+                Token::Divide => Expression::Divide(Box::new(lhs), Box::new(rhs)),
+                _ => unreachable!(),
+            },
+        );
+
+        let binary_2 = binary_1.clone().foldl(
+            just(Token::Add)
+                .or(just(Token::Substract))
+                .then(binary_1)
+                .repeated(),
+            |lhs, (op, rhs)| match op {
+                Token::Add => Expression::Add(Box::new(lhs), Box::new(rhs)),
+                Token::Substract => Expression::Substract(Box::new(lhs), Box::new(rhs)),
+                _ => unreachable!(),
+            },
+        );
+
+        binary_2
+    });
+
+    let decl = recursive(|decl| {
+        let r#var = keyword("var")
+            .ignore_then(ident)
+            .then_ignore(eq.clone())
+            .then(expr.clone())
+            .then(decl.clone())
+            .map(|((name, rhs), then)| Expression::Var {
+                name,
+                rhs: Box::new(rhs),
+                then: Box::new(then),
+            });
+
+        let r#fun = keyword("fun")
+            .ignore_then(ident.clone())
+            .then(ident.repeated().collect::<Vec<_>>())
+            .then_ignore(eq.clone())
+            .then(expr.clone())
+            .then(decl)
+            .map(|(((name, args), body), then)| Expression::Function {
+                name,
+                args,
+                body: Box::new(body),
+                then: Box::new(then),
+            });
+
+        var.or(r#fun).or(expr)
+    });
+
+    decl
+}