docs: explain some of the string parsing

src/nodes/from_str.rs

@@ -25,16 +25,30 @@ enum ParsingState {
 impl FromStr for Nodes {
     type Err = Error;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
+        /// A superset of `char::is_numeric`
+        /// that includes other characters parseable into numbers,
+        /// such as `.` (for decimal) and `e` for scientific notation.
+        fn is_number_char(c: char) -> bool {
+            match c {
+                '.' | 'e' => true,
+                c => c.is_numeric(),
+            }
+        }
+        /// A function to check if the given `char` should be used by the parser.
+        ///
+        /// We ignore whitespace since it has no semantic significance in typst aside from variables,
+        /// (which are not implemented)
+        /// and `&` because it is only used for visual alignment.
+        ///
+        /// Newlines are also ignored because they're likely accidental; like in the case of `echo 1+1 | tylc`.
+        fn is_not_ignored_char(c: &char) -> bool {
+            !matches!(c, ' ' | '&' | '\n')
+        }
+
         let mut nodes = Vec::new();
         let mut state = ParsingState::default();
 
-        let is_ignored_char = |c: &char| !matches!(c, ' ' | '&' | '\n');
-        let is_numeric_char = |c: char| match c {
-            '.' | 'e' => true,
-            c => c.is_numeric(),
-        };
-
-        let filtered_string: String = s.chars().filter(is_ignored_char).collect();
+        let filtered_string: String = s.chars().filter(is_not_ignored_char).collect();
 
         for (i, c) in filtered_string.char_indices() {
             match state {
@@ -51,7 +65,7 @@ impl FromStr for Nodes {
                     }
                 }
                 ParsingState::ReadingNumber(start_index) => {
-                    if !is_numeric_char(c) {
+                    if !is_number_char(c) {
                         nodes.push(Node::Number(
                             Float::parse(&filtered_string[start_index..i])
                                 .map_err(|e| {
@@ -69,7 +83,7 @@ impl FromStr for Nodes {
             }
 
             if state == ParsingState::Start {
-                if is_numeric_char(c) {
+                if is_number_char(c) {
                     state = ParsingState::ReadingNumber(i);
                 } else if c.is_alphabetic() {
                     state = ParsingState::ReadingFunctionName(i);