From 39439d59e8e9ddb1e2b7802f3aff092d77de7acf Mon Sep 17 00:00:00 2001
From: Vincent Ambo <mail@tazj.in>
Date: Thu, 14 Jan 2021 22:49:07 +0300
Subject: feat(tazjin/rlox): Implement early return from functions

In the book this is implemented via exceptions as control flow, and
I'm sticking somewhat closely to that by doing it via an error
variant.

Change-Id: I9c7b84d6bb28265ab94021ea681df84f16a53da2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2395
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI
---
 users/tazjin/rlox/src/errors.rs      | 12 ++++++++++++
 users/tazjin/rlox/src/interpreter.rs | 19 ++++++++++++++++++-
 users/tazjin/rlox/src/parser.rs      | 17 +++++++++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)

(limited to 'users/tazjin/rlox/src')

diff --git a/users/tazjin/rlox/src/errors.rs b/users/tazjin/rlox/src/errors.rs
index 4f173d0160..875f268ca6 100644
--- a/users/tazjin/rlox/src/errors.rs
+++ b/users/tazjin/rlox/src/errors.rs
@@ -1,3 +1,5 @@
+use crate::interpreter::Value;
+
 #[derive(Debug)]
 pub enum ErrorKind {
     UnexpectedChar(char),
@@ -12,6 +14,16 @@ pub enum ErrorKind {
     InternalError(String),
     InvalidAssignmentTarget(String),
     RuntimeError(String),
+
+    // This variant is not an error, rather it is used for
+    // short-circuiting out of a function body that hits a `return`
+    // statement.
+    //
+    // It's implemented this way because in the original book the
+    // author uses exceptions for control flow, and this is the
+    // closest equivalent that I had available without diverging too
+    // much.
+    FunctionReturn(Value),
 }
 
 #[derive(Debug)]
diff --git a/users/tazjin/rlox/src/interpreter.rs b/users/tazjin/rlox/src/interpreter.rs
index 5fb5283bc7..5fdde9adac 100644
--- a/users/tazjin/rlox/src/interpreter.rs
+++ b/users/tazjin/rlox/src/interpreter.rs
@@ -40,7 +40,18 @@ impl Callable {
                     fn_env.define(param, value)?;
                 }
 
-                lox.interpret_block(Rc::new(RwLock::new(fn_env)), &func.body)
+                let result = lox.interpret_block(Rc::new(RwLock::new(fn_env)), &func.body);
+
+                match result {
+                    // extract returned values if applicable
+                    Err(Error {
+                        kind: ErrorKind::FunctionReturn(value),
+                        ..
+                    }) => Ok(value),
+
+                    // otherwise just return the result itself
+                    _ => result,
+                }
             }
         }
     }
@@ -221,6 +232,12 @@ impl Interpreter {
             Statement::If(if_stmt) => return self.interpret_if(if_stmt),
             Statement::While(while_stmt) => return self.interpret_while(while_stmt),
             Statement::Function(func) => return self.interpret_function(func.clone()),
+            Statement::Return(ret) => {
+                return Err(Error {
+                    line: 0,
+                    kind: ErrorKind::FunctionReturn(self.eval(&ret.value)?),
+                })
+            }
         };
 
         Ok(value)
diff --git a/users/tazjin/rlox/src/parser.rs b/users/tazjin/rlox/src/parser.rs
index ed5e670ecb..495304686b 100644
--- a/users/tazjin/rlox/src/parser.rs
+++ b/users/tazjin/rlox/src/parser.rs
@@ -79,6 +79,11 @@ pub struct Var {
     pub initialiser: Option<Expr>,
 }
 
+#[derive(Debug)]
+pub struct Return {
+    pub value: Expr,
+}
+
 #[derive(Debug)]
 pub struct If {
     pub condition: Expr,
@@ -110,6 +115,7 @@ pub enum Statement {
     If(If),
     While(While),
     Function(Rc<Function>),
+    Return(Return),
 }
 
 // Parser
@@ -130,6 +136,7 @@ statement      → exprStmt
                | forStmt
                | ifStmt
                | printStmt
+               | returnStmt
                | whileStmt
                | block ;
 
@@ -137,6 +144,8 @@ forStmt        → "for" "(" ( varDecl | exprStmt | ";" )
                  expression? ";"
                  expression? ")" statement ;
 
+returnStmt     → "return" expression? ";" ;
+
 whileStmt      → "while" "(" expression ")" statement ;
 
 exprStmt       → expression ";" ;
@@ -256,6 +265,8 @@ impl Parser {
             self.while_statement()
         } else if self.match_token(&TokenKind::For) {
             self.for_statement()
+        } else if self.match_token(&TokenKind::Return) {
+            self.return_statement()
         } else {
             self.expr_statement()
         }
@@ -379,6 +390,12 @@ impl Parser {
         Ok(body)
     }
 
+    fn return_statement(&mut self) -> StmtResult {
+        let value = self.expression()?;
+        self.consume(&TokenKind::Semicolon, ErrorKind::ExpectedSemicolon)?;
+        Ok(Statement::Return(Return { value }))
+    }
+
     fn expr_statement(&mut self) -> StmtResult {
         let expr = self.expression()?;
         self.consume(&TokenKind::Semicolon, ErrorKind::ExpectedSemicolon)?;
-- 
cgit 1.4.1