Refactored again, Compiles now.

Cargo.lock

@@ -4,13 +4,13 @@
 name = "interpreter"
 version = "0.1.0"
 dependencies = [
- "lazy_static 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
 name = "lazy_static"
-version = "1.2.0"
+version = "1.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [metadata]
-"checksum lazy_static 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "a374c89b9db55895453a74c1e38861d9deec0b01b405a82516e9d5de4820dea1"
+"checksum lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"

Cargo.toml

@@ -4,4 +4,5 @@ version = "0.1.0"
 authors = ["ishanjain28 <ishanjain28@gmail.com>"]
 
 [dependencies]
-lazy_static = "1.2.0"
+lazy_static = "1.4.0"
+

src/parser/ast/mod.rs

@@ -1,41 +1,62 @@
+use super::ParserError;
 use crate::lexer::Lexer;
+use std::error::Error;
 
 pub trait Node {
-    fn token_literal(&self) -> String;
+    fn token_literal(&self) -> &'static str;
+    fn validate(&self) -> Result<(), Box<dyn Error>>;
 }
+
 pub trait Statement: Node {
-    fn parse(&self, &mut Lexer) -> Box<dyn Statement>;
+    fn parse(&mut Lexer) -> Self
+    where
+        Self: Sized;
 }
 
 pub trait Expression: Node {}
 
-pub struct Let {
+#[derive(Debug)]
+pub struct LetStatement {
     name: Identifier,
     // value: dyn Expression,
 }
 
-impl Node for Let {
-    fn token_literal(&self) -> String {
-        "let".to_owned()
+impl Node for LetStatement {
+    fn validate(&self) -> Result<(), Box<dyn Error>> {
+        if self.token_literal() != "let" {
+            return Err(Box::new(ParserError::new(
+                "self.token_literal is not set, got=".to_owned() + self.token_literal(),
+            )));
+        }
+        Ok(())
+    }
+
+    fn token_literal(&self) -> &'static str {
+        "let"
     }
 }
 
-impl Statement for Let {
-    fn parse(&self, lexer: &mut Lexer) -> Vec<dyn Statement> {
-        vec![Let {
+impl Statement for LetStatement {
+    fn parse(lexer: &mut Lexer) -> LetStatement {
+        LetStatement {
             name: Identifier { name: "potato" },
             // value: "",
-        }]
+        }
     }
 }
 
+#[derive(Debug)]
 pub struct Identifier {
     name: &'static str,
 }
 
 impl Node for Identifier {
-    fn token_literal(&self) -> String {
-        "IDENT".to_owned()
+    fn validate(&self) -> Result<(), Box<dyn Error>> {
+        Ok(())
+    }
+    fn token_literal(&self) -> &'static str {
+        "IDENT"
     }
 }
+
 impl Expression for Identifier {}

src/parser/mod.rs

@@ -1,49 +1,41 @@
-mod ast;
+pub mod ast;
+mod program;
 
-use self::ast::{Let, Statement};
-use super::lexer::Lexer;
+pub use self::program::Program;
 
-pub struct Program {
-    statements: Vec<Box<dyn Statement>>,
-}
+use self::ast::{LetStatement, Statement};
+use crate::lexer::{Lexer, Token};
 
-impl Program {
-    pub fn token_literal(&self) -> String {
-        if self.statements.len() > 0 {
-            self.statements[0].token_literal()
-        } else {
-            "".to_owned()
-        }
-    }
+struct Parser {}
 
-    pub fn parse(lexer: Lexer) -> Program {
-        let mut statements = vec![];
-
-        Program { statements }
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use crate::lexer::Lexer;
-    use crate::parser::Program;
-
-    #[test]
-    fn let_statements() {
-        let ip = "
-        let yr = 5;
-        let qq = 10;
-        let foobar = 8388383;
-        ";
-
-        let lexer = Lexer::new(ip);
-        let ast_tree = Program::parse(lexer);
-
-        if ast_tree.statements.len() != 3 {
-            eprintln!(
-                "statements length not equal to 3. got {}",
-                ast_tree.statements
-            );
+impl Parser {
+    fn parse_statement(token: Token, lexer: &mut Lexer) -> Option<Box<dyn ast::Statement>> {
+        match token {
+            Token::Let => Some(Box::new(LetStatement::parse(lexer))),
+            _ => unimplemented!(),
         }
     }
 }
+
+#[derive(Debug)]
+struct ParserError {
+    desc: String,
+}
+
+impl std::fmt::Display for ParserError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, "ParserError: ")
+    }
+}
+
+impl std::error::Error for ParserError {
+    fn description(&self) -> &str {
+        &self.desc
+    }
+}
+
+impl ParserError {
+    fn new(desc: String) -> ParserError {
+        ParserError { desc }
+    }
+}

src/parser/program.rs

@@ -0,0 +1,61 @@
+use crate::lexer::{Lexer, Token};
+use crate::parser::ast::{LetStatement, Statement};
+use crate::parser::Parser;
+
+pub struct Program {
+    statements: Vec<Box<dyn Statement>>,
+}
+
+impl Program {
+    pub fn token_literal(&self) -> &'static str {
+        if self.statements.len() > 0 {
+            self.statements[0].token_literal()
+        } else {
+            ""
+        }
+    }
+
+    pub fn parse(mut lexer: Lexer) -> Program {
+        let mut statements = vec![];
+
+        loop {
+            if let Some(token) = lexer.next() {
+                if token == Token::EOF {
+                    break;
+                }
+
+                if let Some(stmt) = Parser::parse_statement(token, &mut lexer) {
+                    statements.push(stmt);
+                } else {
+                    continue;
+                }
+            } else {
+                break;
+            }
+        }
+
+        Program { statements }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::lexer::Lexer;
+    use crate::parser::Program;
+
+    #[test]
+    fn let_statements() {
+        let ip = "
+        let yr = 5;
+        let qq = 10;
+        let foobar = 8388383;
+        ";
+
+        let lexer = Lexer::new(ip);
+        let ast_tree = Program::parse(lexer);
+
+        if ast_tree.statements.len() != 3 {
+            assert_eq!(ast_tree.statements.len(), 3);
+        }
+    }
+}

src/repl.rs

@@ -14,7 +14,7 @@ pub fn init() {
 
 fn start<R: BufRead, W: Write>(mut ip: R, mut out: W) {
     loop {
-        out.write_fmt(format_args!("{}", PROMPT)).unwrap();
+        out.write(PROMPT.as_bytes()).unwrap();
         out.flush().unwrap();
         let mut s = String::new();
         ip.read_line(&mut s).unwrap();