add better error messages for syntax issues

src/main.rs

@@ -62,9 +62,21 @@ impl<T: Read> Iterator for Tape<T> {
     }
 }
 
+#[derive(Debug)]
+struct SyntaxError {
+    position: usize,
+    message: String,
+}
+
+impl SyntaxError {
+    pub fn new(position: usize, message: String) -> Self {
+        Self { position, message }
+    }
+}
+
 #[derive(Debug)]
 enum ProgramErrorKind {
-    InvalidSyntax,
+    SyntaxError(SyntaxError),
     IllegalMemoryAccess(isize),
     ConversionError,
     EndOfFile,
@@ -73,7 +85,7 @@ enum ProgramErrorKind {
 impl fmt::Display for ProgramErrorKind {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            Self::InvalidSyntax => write!(f, "invalid syntax"),
+            Self::SyntaxError(e) => write!(f, "invalid syntax at {}: {}", e.position, e.message),
             Self::EndOfFile => write!(f, "end of file"),
             Self::IllegalMemoryAccess(loc) => write!(f, "illegal memory access @ {loc}"),
             Self::ConversionError => todo!(),
@@ -102,6 +114,15 @@ impl From<ProgramErrorKind> for ProgramError {
     }
 }
 
+impl From<SyntaxError> for ProgramError {
+    fn from(value: SyntaxError) -> Self {
+        Self {
+            kind: ProgramErrorKind::SyntaxError(value),
+            source: None,
+        }
+    }
+}
+
 impl From<TryFromIntError> for ProgramError {
     fn from(value: TryFromIntError) -> Self {
         Self::new(ProgramErrorKind::ConversionError, value)
@@ -131,31 +152,37 @@ struct Program {
 }
 
 impl Program {
-    fn word_length(mut word: impl Iterator<Item = u8>) -> Result<isize> {
-        let result = word.try_fold((0isize, true, true), |(count, was_w, pos), byte| {
+    fn word_length(mut word: impl Iterator<Item = (usize, u8)>) -> Result<isize> {
+        let result = word.try_fold((0isize, true, true, 0usize), |(count, was_w, pos, _), (i, byte)| {
             match (byte, was_w, pos) {
-                (b'a', true, true) => Some((count, false, true)),
-                (b'w', false, true) => Some((count + 1, true, true)),
-                (b'!', false, true) => Some((count, false, false)),
-                _ => None,
+                (b'a', true, true) => Ok((count, false, true, i)),
+                (b'w', false, true) => Ok((count + 1, true, true, i)),
+                (b'!', false, true) => Ok((count, false, false, i)),
+                // errors
+                (b'a', false, true) => Err(SyntaxError::new(i, r#"unexpected "a". expected "w", "!" if continuing a word, or anything else to end it"#.to_string())),
+                (b'w', true, true) => Err(SyntaxError::new(i, r#"unexpected "w". expected "a""#.to_string())),
+                (b'!', true, _) | (b'!', _, false)=> Err(SyntaxError::new(i, r#"unexpected "!": may only appear at the end of a word"#.to_string())),
+                (v, _, false) => Err(SyntaxError::new(i, format!("unexpected {v}: whitespace or any non-word character should follow"))),
+                (v, _, _) => Err(SyntaxError::new(i, format!("unexpected {v}"))),
             }
         });
 
         let sign = match result {
-            Some((_, _, false)) => -1,
+            Ok((_, _, false, _)) => -1,
             _ => 1,
         };
 
         match result {
-            Some((count, false, _)) => Ok(count * sign),
-            _ => Err(ProgramError::from(ProgramErrorKind::InvalidSyntax)),
+            Ok((count, false, _, _)) => Ok(count * sign),
+            Ok((_, true, _, i)) => Err(SyntaxError::new(i, r#""#.to_string()).into()),
+            Err(e) => Err(e.into()),
         }
     }
 
-    fn next_word(program: impl Iterator<Item = u8>) -> Result<isize> {
+    fn next_word(program: impl Iterator<Item = (usize, u8)>) -> Result<isize> {
         let mut word = program
-            .skip_while(|ch| *ch != b'a' && *ch != b'w' && *ch != b'!')
-            .take_while(|ch| *ch == b'a' || *ch == b'w' || *ch == b'!')
+            .skip_while(|(_, ch)| *ch != b'a' && *ch != b'w' && *ch != b'!')
+            .take_while(|(_, ch)| *ch == b'a' || *ch == b'w' || *ch == b'!')
             .peekable();
 
         if word.peek().is_none() {
@@ -165,7 +192,7 @@ impl Program {
         }
     }
 
-    fn parse(mut program: impl Iterator<Item = u8>) -> Result<Vec<isize>> {
+    fn parse(mut program: impl Iterator<Item = (usize, u8)>) -> Result<Vec<isize>> {
         let mut memory = Vec::new();
 
         loop {
@@ -184,7 +211,7 @@ impl Program {
 
     pub fn from_program(program: impl Iterator<Item = u8>) -> Result<Self> {
         Ok(Self {
-            memory: Self::parse(program)?,
+            memory: Self::parse(program.enumerate())?,
             index: 0,
         })
     }
@@ -217,26 +244,10 @@ impl Program {
         }
     }
 
-    fn set_index(&mut self, value: isize) -> Result<()> {
-        if value < 0 {
-            Err(ProgramErrorKind::IllegalMemoryAccess(value).into())
-        } else {
-            match usize::try_from(value) {
-                Ok(value) => {
-                    self.index = value;
-                    Ok(())
-                }
-                Err(e) => Err(ProgramError::new(ProgramErrorKind::ConversionError, e)),
-            }
-        }
-    }
-
     fn get(&self, n: usize) -> Result<&isize> {
         match self.memory.get(n) {
             Some(n) => Ok(n),
-            None => Err(ProgramError::from(ProgramErrorKind::IllegalMemoryAccess(
-                n as isize,
-            ))),
+            None => Err(ProgramErrorKind::IllegalMemoryAccess(n as isize).into()),
         }
     }