Implement rudimentary control flow

demo/demo.rpy

@@ -1,13 +1,14 @@
+# Control testing
+"Bob" "I will not say anything"
+if False:
+    "Bob" "W-what? Why am I saying this?"
+    if True:
+        "Testing 123"
+"..."
+"Bob" "Good."
+"Bob" "Now I will tell you my favorite food..."
+if True:
+    "Bob" "...cereal!"
+    if False:
+        "Bob" "But I actually hate it."
 "Bob sat on the bench."
-"Bob" "Good morning!"
-    "Bob" "I am in a block now!"
-    "Bob" "Isn't this cool?"
-"Bob" "And now we're back in normal indentation."
-    "Bob" "We can even go multiple levels in!"
-        "Bob" "How far does the rabbit hole go?"
-        "Bob" "rabbit hole go?"
-        "Bob" "go?"
-"Bob" "Not sure what came of me there."
-"Bob" "I suppose I should eat a potato."
-eat "potato" True
-"Bob" "Yum!"

renrs/src/lib.rs

@@ -20,6 +20,7 @@ enum Token {
 
 impl Token {
     fn print(&self) -> &str {
+        use Token::*;
         match &self {
             Keyword(keyword) => keyword,
             Str(_) => "String",
@@ -30,11 +31,10 @@ impl Token {
     }
 }
 
-use Token::*;
-
 // Indented command block
 #[derive(Debug)]
 struct CommandBlock {
+    control: Option<Control>,
     elements: Vec<BlockElement>,
     next: Option<usize>,
 }
@@ -45,6 +45,9 @@ impl CommandBlock {
             Some(next) => next,
             None => return None,
         };
+        if !self.evaluate_control() {
+            return None;
+        }
         let mut result = None;
         let count = self.elements.len();
         for element in &mut self.elements[next..] {
@@ -74,11 +77,19 @@ impl CommandBlock {
         };
         result
     }
+    
+    fn evaluate_control(&self) -> bool {
+        use Control::*;
+        self.control.as_ref().map_or(true, |control| match control {
+            If { condition } => *condition,
+        })
+    }
 }
 
 impl Default for CommandBlock {
     fn default() -> Self {
         Self {
+            control: None,
             elements: Vec::new(),
             next: Some(0),
         }
@@ -93,6 +104,12 @@ enum BlockElement {
 
 type Script = CommandBlock;
 
+#[derive(Debug)]
+#[allow(dead_code)]
+enum Control {
+    If { condition: bool },
+}
+
 // Parsed script commands
 #[derive(Debug)]
 #[allow(dead_code)]
@@ -101,10 +118,9 @@ enum Command {
     Eat { food: String, politely: bool },
 }
 
-use Command::*;
-
 impl Command {
     fn is_blocking(&self) -> bool {
+        use Command::*;
         match self {
             Say { .. } => true,
             _ => false,
@@ -139,22 +155,66 @@ fn parse(script: &str) -> CommandBlock {
 
 fn parse_block(block: Pair) -> CommandBlock {
     CommandBlock {
-        elements: block.into_inner().filter_map(|pair| match pair.as_rule() {
-            Rule::Line => Some(BlockElement::Command(parse_command(pair))),
-            Rule::Block => Some(BlockElement::Block(parse_block(pair))),
-            Rule::EOI => None, // end
-            _ => unreachable!(),
-        }).collect(),
+        elements: {
+            let mut control = None;
+            let mut elements = Vec::new();
+            for pair in block.into_inner() {
+                elements.push(match pair.as_rule() {
+                    Rule::Control => {
+                        if control.is_some() {
+                            panic!("control statement should not be empty");
+                        }
+                        control = Some(parse_control(pair));
+                        continue;
+                    },
+                    Rule::Line => BlockElement::Command(parse_command(pair)),
+                    Rule::Block => BlockElement::Block({
+                        let mut block = parse_block(pair);
+                        if control.is_none() {
+                            panic!("block should have control");
+                        }
+                        block.control = control;
+                        control = None;
+                        block
+                    }),
+                    Rule::EOI => break, // end
+                    _ => unreachable!(),
+                });
+            }
+            elements
+        },
         ..Default::default()
     }
 }
 
-fn parse_command(pair: Pair) -> Command {
-    use Token::*;
-    let line: Vec<Token> = pair
+fn parse_line(pair: Pair) -> Vec<Token> {
+    pair
         .into_inner()
         .map(|pair| parse_token(pair))
-        .collect();
+        .collect()
+}
+
+fn parse_control(pair: Pair) -> Control {
+    use Control::*;
+    use Token::*;
+    let line = parse_line(pair);
+    macro_rules! unknown {
+        () => {
+            panic!("Unknown control {}", describe_line(&line))
+        };
+    }
+    match line.as_slice() {
+        [Keyword(control), Boolean(condition)] if control.eq("if") => If {
+            condition: *condition,
+        },
+        _ => unknown!(),
+    }
+}
+
+fn parse_command(pair: Pair) -> Command {
+    use Command::*;
+    use Token::*;
+    let line = parse_line(pair);
     macro_rules! unknown {
         () => {
             panic!("Unknown command {}", describe_line(&line))
@@ -247,7 +307,7 @@ impl State {
         while let Some(command) = self.next_command() {
             if command.is_blocking() {
                 return Some(match command {
-                    Say { name, text } => Event::Say {
+                    Command::Say { name, text } => Event::Say {
                         name: name.clone(),
                         text: text.clone()
                     },

renrs/src/rpy.pest

@@ -43,12 +43,14 @@ Number = @{
 
 // comments are a # followed by
 // any Number of non-newline characters
+// TODO: Inline comments broken by no implicit whitespace
 COMMENT = _{ "#" ~ char* }
 
 Colon = { ":" }
 
 // lines are comprised of a statement
-Line = @{ (token ~ whitespace+)* ~ token ~ Colon? }
+Line = @{ (token ~ whitespace+)* ~ token }
+Control = @{ Line ~ Colon }
 
 File = { SOI ~ NEWLINE* ~ block_content* ~ NEWLINE* ~ EOI }
 
@@ -66,5 +68,5 @@ Block = {
 whitespace = _{ " " }
 
 block_content = _{
-    Line ~ (whitespace+ ~ Line)* ~ (NEWLINE | EOI) ~ Block*
+    (Control | Line) ~ (whitespace+ ~ Line)* ~ (NEWLINE | EOI) ~ Block*
 }