progress

gpx-rs/src/engine/command/apply.rs

@@ -0,0 +1,30 @@
+use std::fmt;
+
+use crate::State;
+
+/// The effect of a command on the engine state.
+///
+/// Each command is a small struct holding its arguments, with its effect implemented in its
+/// own file. Effects are built from the reusable patterns in `pattern/`.
+pub trait Apply {
+    fn apply(self, state: &mut State) -> Result<(), CommandError>;
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum CommandError {
+    /// The command would not change anything.
+    NothingToDo,
+    /// The input data could not be parsed.
+    InvalidData(String),
+}
+
+impl fmt::Display for CommandError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::NothingToDo => write!(f, "nothing to do"),
+            Self::InvalidData(err) => write!(f, "invalid data: {err}"),
+        }
+    }
+}
+
+impl std::error::Error for CommandError {}