Add default argument for str.first and str.last (#6554)

Co-authored-by: Laurenz <laurmaedje@gmail.com>

crates/typst-library/src/foundations/str.rs

@@ -179,24 +179,40 @@ impl Str {
     }
 
     /// Extracts the first grapheme cluster of the string.
-    /// Fails with an error if the string is empty.
+    ///
+    /// Returns the provided default value if the string is empty or fails with
+    /// an error if no default value was specified.
     #[func]
-    pub fn first(&self) -> StrResult<Str> {
+    pub fn first(
+        &self,
+        /// A default value to return if the string is empty.
+        #[named]
+        default: Option<Str>,
+    ) -> StrResult<Str> {
         self.0
             .graphemes(true)
             .next()
             .map(Into::into)
+            .or(default)
             .ok_or_else(string_is_empty)
     }
 
     /// Extracts the last grapheme cluster of the string.
-    /// Fails with an error if the string is empty.
+    ///
+    /// Returns the provided default value if the string is empty or fails with
+    /// an error if no default value was specified.
     #[func]
-    pub fn last(&self) -> StrResult<Str> {
+    pub fn last(
+        &self,
+        /// A default value to return if the string is empty.
+        #[named]
+        default: Option<Str>,
+    ) -> StrResult<Str> {
         self.0
             .graphemes(true)
             .next_back()
             .map(Into::into)
+            .or(default)
             .ok_or_else(string_is_empty)
     }
 

tests/suite/foundations/str.typ

@@ -103,6 +103,10 @@
 #test("Hello".last(), "o")
 #test("🏳️‍🌈A🏳️‍⚧️".first(), "🏳️‍🌈")
 #test("🏳️‍🌈A🏳️‍⚧️".last(), "🏳️‍⚧️")
+#test("hey".first(default: "d"), "h")
+#test("".first(default: "d"), "d")
+#test("hey".last(default: "d"), "y")
+#test("".last(default: "d"), "d")
 
 --- string-first-empty ---
 // Error: 2-12 string is empty