reorganize compiler crates

parser/src/mode.rs

@@ -0,0 +1,40 @@
+use crate::token::Tok;
+
+#[derive(Clone, Copy)]
+pub enum Mode {
+    Module,
+    Interactive,
+    Expression,
+}
+
+impl Mode {
+    pub(crate) fn to_marker(self) -> Tok {
+        match self {
+            Self::Module => Tok::StartModule,
+            Self::Interactive => Tok::StartInteractive,
+            Self::Expression => Tok::StartExpression,
+        }
+    }
+}
+
+impl std::str::FromStr for Mode {
+    type Err = ModeParseError;
+    fn from_str(s: &str) -> Result<Self, ModeParseError> {
+        match s {
+            "exec" | "single" => Ok(Mode::Module),
+            "eval" => Ok(Mode::Expression),
+            _ => Err(ModeParseError { _priv: () }),
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct ModeParseError {
+    _priv: (),
+}
+
+impl std::fmt::Display for ModeParseError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(f, r#"mode should be "exec", "eval", or "single""#)
+    }
+}