feat(formula): support pipe-quoted identifiers |...|

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

context/design-principles.md

@@ -115,17 +115,28 @@ Formulas are parsed into a typed AST (`Expr` enum) at entry time. If the syntax
 is invalid, the user gets an error immediately. The evaluator only sees
 well-formed trees — it does not need to handle malformed input.
 
-### Formula Tokenizer: Multi-Word Identifiers and Keywords
+### Formula Tokenizer: Identifiers and Quoting
 
-The formula tokenizer supports multi-word identifiers (e.g., `Total Revenue`)
+**Bare identifiers** support multi-word names (e.g., `Total Revenue`) by
-by allowing spaces within identifier tokens when followed by non-operator
+allowing spaces when followed by non-operator, non-keyword characters. Keywords
-characters. However, keywords (`WHERE`, `SUM`, `AVG`, `MIN`, `MAX`, `COUNT`,
+(`WHERE`, `SUM`, `AVG`, `MIN`, `MAX`, `COUNT`, `IF`) act as token boundaries.
-`IF`) act as token boundaries — the tokenizer breaks an identifier when:
-1. The identifier collected **so far** is a keyword (e.g., `WHERE ` stops at `WHERE`).
-2. The **next word** after a space is a keyword (e.g., `Revenue WHERE` stops at `Revenue`).
 
-This ensures `SUM(Revenue WHERE Region = "East")` tokenizes correctly as
+**Pipe-quoted identifiers** (`|...|`) allow any characters — including spaces,
-separate tokens while `Total Revenue` remains a single identifier.
+keywords, and operators — inside the delimiters. Use pipes when a category or
+item name collides with a keyword or contains special characters:
+
+```
+|WHERE|             — category named "WHERE"
+|Revenue (USD)|     — name with parens
+|Cost + Tax|        — name with operator chars
+SUM(|Net Revenue| WHERE |Region Name| = |East Coast|)
+```
+
+Pipes produce `Token::Ident` (same as bare identifiers), so they work
+everywhere an identifier is expected: expressions, aggregate arguments, WHERE
+clause category names and filter values. Double-quoted strings (`"..."`)
+remain `Token::Str` and are used only for WHERE filter values in the
+`split_where` pre-parse step.
 
 ---
 

src/formula/parser.rs

@@ -38,6 +38,12 @@ fn split_where(s: &str) -> (&str, Option<&str>) {
                     i += 1;
                 }
             }
+            b'|' => {
+                i += 1;
+                while i < bytes.len() && bytes[i] != b'|' {
+                    i += 1;
+                }
+            }
             _ if depth == 0 => {
                 if s[i..].to_ascii_uppercase().starts_with("WHERE") {
                     let before = &s[..i];
@@ -54,14 +60,23 @@ fn split_where(s: &str) -> (&str, Option<&str>) {
     (s, None)
 }
 
+/// Strip pipe or double-quote delimiters from a value.
+fn unquote(s: &str) -> String {
+    let s = s.trim();
+    if (s.starts_with('"') && s.ends_with('"')) || (s.starts_with('|') && s.ends_with('|')) {
+        s[1..s.len() - 1].to_string()
+    } else {
+        s.to_string()
+    }
+}
+
 fn parse_where(s: &str) -> Result<Filter> {
-    // Format: Category = "Item" or Category = Item
+    // Format: Category = "Item" or Category = |Item| or Category = Item
     let eq_pos = s
         .find('=')
         .ok_or_else(|| anyhow!("WHERE clause must contain '=': {s}"))?;
-    let category = s[..eq_pos].trim().to_string();
+    let category = unquote(&s[..eq_pos]);
-    let item_raw = s[eq_pos + 1..].trim();
+    let item = unquote(&s[eq_pos + 1..]);
-    let item = item_raw.trim_matches('"').to_string();
     Ok(Filter { category, item })
 }
 
@@ -176,6 +191,18 @@ fn tokenize(s: &str) -> Result<Vec<Token>> {
                 }
                 tokens.push(Token::Str(s));
             }
+            '|' => {
+                i += 1;
+                let mut s = String::new();
+                while i < chars.len() && chars[i] != '|' {
+                    s.push(chars[i]);
+                    i += 1;
+                }
+                if i < chars.len() {
+                    i += 1;
+                }
+                tokens.push(Token::Ident(s));
+            }
             c if c.is_ascii_digit() || c == '.' => {
                 let mut num = String::new();
                 while i < chars.len() && (chars[i].is_ascii_digit() || chars[i] == '.') {
@@ -695,4 +722,78 @@ mod tests {
         let filter = f.filter.as_ref().unwrap();
         assert_eq!(filter.item, "WHERE");
     }
+
+    // ── Pipe-quoted identifiers ─────────────────────────────────────────
+
+    #[test]
+    fn pipe_quoted_identifier_in_expression() {
+        let f = parse_formula("|Total Revenue| = |Base Revenue| + Bonus", "Measure").unwrap();
+        assert_eq!(f.target, "|Total Revenue|");
+        if let Expr::BinOp(BinOp::Add, lhs, rhs) = &f.expr {
+            assert!(matches!(**lhs, Expr::Ref(ref s) if s == "Base Revenue"));
+            assert!(matches!(**rhs, Expr::Ref(ref s) if s == "Bonus"));
+        } else {
+            panic!("Expected Add, got: {:?}", f.expr);
+        }
+    }
+
+    #[test]
+    fn pipe_quoted_keyword_as_identifier() {
+        // A category named "WHERE" can be referenced with pipes
+        let f = parse_formula("X = |WHERE| + |SUM|", "Cat").unwrap();
+        if let Expr::BinOp(BinOp::Add, lhs, rhs) = &f.expr {
+            assert!(matches!(**lhs, Expr::Ref(ref s) if s == "WHERE"));
+            assert!(matches!(**rhs, Expr::Ref(ref s) if s == "SUM"));
+        } else {
+            panic!("Expected Add, got: {:?}", f.expr);
+        }
+    }
+
+    #[test]
+    fn pipe_quoted_identifier_with_special_chars() {
+        // Pipes allow characters that would normally break tokenization
+        let f = parse_formula("X = |Revenue (USD)| + |Cost + Tax|", "Cat").unwrap();
+        if let Expr::BinOp(BinOp::Add, lhs, rhs) = &f.expr {
+            assert!(matches!(**lhs, Expr::Ref(ref s) if s == "Revenue (USD)"));
+            assert!(matches!(**rhs, Expr::Ref(ref s) if s == "Cost + Tax"));
+        } else {
+            panic!("Expected Add, got: {:?}", f.expr);
+        }
+    }
+
+    #[test]
+    fn pipe_quoted_in_aggregate() {
+        let f = parse_formula("X = SUM(|Net Revenue|)", "Cat").unwrap();
+        if let Expr::Agg(AggFunc::Sum, inner, None) = &f.expr {
+            assert!(matches!(**inner, Expr::Ref(ref s) if s == "Net Revenue"));
+        } else {
+            panic!("Expected SUM aggregate, got: {:?}", f.expr);
+        }
+    }
+
+    #[test]
+    fn pipe_quoted_in_where_filter_value() {
+        let f = parse_formula(
+            "X = Revenue WHERE Region = |East Coast|",
+            "Measure",
+        )
+        .unwrap();
+        let filter = f.filter.as_ref().unwrap();
+        assert_eq!(filter.item, "East Coast");
+    }
+
+    #[test]
+    fn pipe_quoted_in_inline_where() {
+        let f = parse_formula(
+            "X = SUM(Revenue WHERE |Region Name| = |East Coast|)",
+            "Measure",
+        )
+        .unwrap();
+        if let Expr::Agg(AggFunc::Sum, _, Some(filter)) = &f.expr {
+            assert_eq!(filter.category, "Region Name");
+            assert_eq!(filter.item, "East Coast");
+        } else {
+            panic!("Expected SUM with WHERE filter, got: {:?}", f.expr);
+        }
+    }
 }