Print dates in a nicer format

src/eval/date.rs

@@ -1,4 +1,6 @@
-use chrono::NaiveDate;
+use std::fmt;
+
+use chrono::{Duration, NaiveDate};
 
 use crate::files::commands::DoneDate;
 use crate::files::primitives::Time;
@@ -16,6 +18,25 @@ pub struct Dates {
     times: Option<Times>,
 }
 
+impl fmt::Display for Dates {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let (start, end) = self.start_end();
+        match self.start_end_time() {
+            Some((start_time, end_time)) if start == end && start_time == end_time => {
+                write!(f, "{} {}", start, start_time)
+            }
+            Some((start_time, end_time)) if start == end => {
+                write!(f, "{} {} -- {}", start, start_time, end_time)
+            }
+            Some((start_time, end_time)) => {
+                write!(f, "{} {} -- {} {}", start, start_time, end, end_time)
+            }
+            None if start == end => write!(f, "{}", start),
+            None => write!(f, "{} -- {}", start, end),
+        }
+    }
+}
+
 impl Dates {
     pub fn new(root: NaiveDate, other: NaiveDate) -> Self {
         Self {
@@ -92,6 +113,14 @@ impl Dates {
     pub fn end_time(&self) -> Option<Time> {
         self.start_end_time().map(|times| times.1)
     }
+
+    pub fn move_by(&self, delta: Duration) -> Self {
+        Self {
+            root: self.root + delta,
+            other: self.other + delta,
+            times: self.times,
+        }
+    }
 }
 
 impl From<DoneDate> for Dates {

src/eval/error.rs

@@ -4,9 +4,10 @@ use chrono::NaiveDate;
 
 use crate::files::primitives::{Span, Time};
 
-#[derive(Debug)]
+#[derive(Debug, thiserror::Error)]
 pub enum Error {
     /// A delta step resulted in an invalid date.
+    #[error("delta step resulted in invalid date")]
     DeltaInvalidStep {
         span: Span,
         start: NaiveDate,
@@ -15,6 +16,7 @@ pub enum Error {
         prev_time: Option<Time>,
     },
     /// A time-based delta step was applied to a date without time.
+    #[error("time-based delta step applied to date without time")]
     DeltaNoTime {
         span: Span,
         start: NaiveDate,
@@ -23,6 +25,7 @@ pub enum Error {
     /// A `DATE`'s repeat delta did not move the date forwards in time. Instead,
     /// it either remained at the current date (`to == from`) or moved backwards
     /// in time (`to < from`).
+    #[error("repeat delta did not move forwards")]
     RepeatDidNotMoveForwards {
         span: Span,
         from: NaiveDate,
@@ -30,12 +33,16 @@ pub enum Error {
     },
     /// A `MOVE a TO b` statement was executed, but there was no entry at the
     /// date `a`.
+    #[error("tried to move nonexisting entry")]
     MoveWithoutSource { span: Span },
     /// A division by zero has occurred.
+    #[error("tried to divide by zero")]
     DivByZero { span: Span, date: NaiveDate },
     /// A modulo operation by zero has occurred.
+    #[error("tried to modulo by zero")]
     ModByZero { span: Span, date: NaiveDate },
     /// Easter calculation failed.
+    #[error("easter calculation failed")]
     Easter {
         span: Span,
         date: NaiveDate,