Prepared for supporting multiple function arguments. The types now support several arguments, and the only thing left is to parse this.

src/interpreter.rs

@@ -75,11 +75,16 @@ impl Visitor<f64, f64> for Interpreter {
                 self.symbol_table.insert(identifier.clone(), stmt.clone());
                 0f64
             }
-            Stmt::FnDecl(identifier, argument, _) => {
-                self.visit_stmt(&Stmt::VarDecl(
-                    argument.clone(),
-                    Box::new(Expr::Literal(String::from("0"))),
-                ));
+            Stmt::FnDecl(identifier, arguments, _) => {
+                // Initialise each of the arguments as their own variable.
+                for argument in arguments {
+                    self.visit_stmt(&Stmt::VarDecl(
+                        argument.clone(),
+                        Box::new(Expr::Literal(String::from("0"))),
+                    ));
+                }
+
+                // Add the function to the symbol table.
                 self.symbol_table
                     .insert(format!("{}()", identifier.clone()), stmt.clone());
                 0f64
@@ -134,9 +139,16 @@ impl Visitor<f64, f64> for Interpreter {
             }
             Expr::Literal(value) => value.parse().unwrap(),
             Expr::Group(expr) => self.visit_expr(&expr),
-            Expr::FnCall(identifier, expr) => {
-                let x = self.visit_expr(&expr);
-                if let Some(result) = self.prelude.call_unary_func(identifier, x) {
+            Expr::FnCall(identifier, expressions) => {
+                let prelude_func = match expressions.len() {
+                    1 => {
+                        let x = self.visit_expr(&expressions[0]);
+                        self.prelude.call_unary_func(identifier, x)
+                    }
+                    _ => None,
+                };
+
+                if let Some(result) = prelude_func {
                     result
                 } else {
                     let stmt = self
@@ -144,13 +156,24 @@ impl Visitor<f64, f64> for Interpreter {
                         .get(&format!("{}()", identifier))
                         .expect("Undefined function")
                         .clone();
-                    match stmt {
-                        Stmt::FnDecl(_, argument, fn_body) => {
-                            self.visit_stmt(&Stmt::VarDecl(argument.clone(), expr.clone()));
-                            self.visit_expr(&*fn_body)
+
+                    if let Stmt::FnDecl(_, arguments, fn_body) = stmt {
+                        if arguments.len() != expressions.len() {
+                            panic!("Incorrect amount of arguments.");
                         }
-                        _ => panic!(),
+
+                        // Initialise the arguments as their own variables.
+                        for (i, argument) in arguments.iter().enumerate() {
+                            self.visit_stmt(&Stmt::VarDecl(
+                                argument.clone(),
+                                Box::new(expressions[i].clone()),
+                            ));
+                        }
+
+                        return self.visit_expr(&*fn_body);
                     }
+
+                    panic!("Unexpected error.");
                 }
             }
         }

src/lexer.rs

@@ -5,17 +5,22 @@ pub enum TokenKind {
     Unknown,
     Literal,
     Identifier,
+
     Plus,
     Minus,
     Star,
     Slash,
     Power,
     Equals,
+
     Deg,
     Rad,
+
     Pipe,
     OpenParenthesis,
     ClosedParenthesis,
+    Comma,
+
     EOF,
 }
 
@@ -101,6 +106,10 @@ impl<'a> Lexer<'a> {
                 self.advance();
                 self.build(TokenKind::Equals, "")
             }
+            ',' => {
+                self.advance();
+                self.build(TokenKind::Comma, "")
+            }
             _ => {
                 if c.is_digit(10) {
                     self.next_number_literal()

src/parser.rs

@@ -5,7 +5,7 @@ use crate::lexer::{Token, TokenKind};
 #[derive(Debug, Clone)]
 pub enum Stmt {
     VarDecl(String, Box<Expr>),
-    FnDecl(String, String, Box<Expr>),
+    FnDecl(String, Vec<String>, Box<Expr>),
     Expr(Box<Expr>),
 }
 
@@ -16,7 +16,7 @@ pub enum Expr {
     Unit(Box<Expr>, TokenKind),
     Var(String),
     Group(Box<Expr>),
-    FnCall(String, Box<Expr>),
+    FnCall(String, Vec<Expr>),
     Literal(String),
 }
 
@@ -85,15 +85,23 @@ impl Parser {
             self.advance();
             let expr = self.parse_expr();
 
-            match primary {
-                Expr::FnCall(identifier, argument) => match *argument {
-                    Expr::Var(argument_identifier) => {
-                        Stmt::FnDecl(identifier, argument_identifier, Box::new(expr))
+            // Use the "function call" expression that was parsed, and put its values into a function declaration statement instead.
+            if let Expr::FnCall(identifier, parameters) = primary {
+                let mut parameter_identifiers = Vec::new();
+
+                // All the "arguments" are expected to be parsed as variables,
+                // since parameter definitions look the same as variable references.
+                // Extract these.
+                for parameter in parameters {
+                    if let Expr::Var(parameter_identifier) = parameter {
+                        parameter_identifiers.push(parameter_identifier);
                     }
-                    _ => panic!("Unexpected error."),
-                },
-                _ => panic!("Unexpected error."),
+                }
+
+                return Stmt::FnDecl(identifier, parameter_identifiers, Box::new(expr));
             }
+
+            panic!("Unexpected error.");
         } else {
             // It is a function call, not a function declaration.
             // Redo the parsing for this specific part.
@@ -199,7 +207,7 @@ impl Parser {
         let group_expr = Expr::Group(Box::new(self.parse_expr()));
         self.consume(TokenKind::Pipe);
 
-        Expr::FnCall(String::from("abs"), Box::new(group_expr))
+        Expr::FnCall(String::from("abs"), vec![group_expr])
     }
 
     fn parse_identifier(&mut self) -> Expr {
@@ -210,7 +218,7 @@ impl Parser {
             let parameter = self.parse_expr();
             self.consume(TokenKind::ClosedParenthesis);
 
-            Expr::FnCall(identifier.value, Box::new(parameter))
+            Expr::FnCall(identifier.value, vec![parameter])
         } else {
             Expr::Var(identifier.value)
         }