feat: format pipe can now accept custom formats as quoted strings.

mustache/EXTENSIONS.md

@@ -56,13 +56,16 @@ Errors (returned as `Data_Error` at render time):
 - Any element is missing the named field.
 - Any element has an empty value for the named field.
 
-#### `format` (no args yet)
+#### `format`
 
 Formats an ISO 8601 date string as a display string. Takes a string, returns a string (e.g. `"2026-03-15T08:49:54-04:00"` → `"15 Mar 2026"`). Invalid input (empty, too-short, non-string, or unparseable) returns a `Data_Error`. Templates that need to skip dateless pages should gate with a section — `{{#date}}<time datetime="{{.}}">{{. | format}}</time>{{/date}}` — so the section's truthiness check catches empty before the filter runs. Commonly used inline as `{{date | format}}` to render a display string while keeping the raw ISO available via `{{date}}` for the `datetime=` attribute.
 
 Internally: parses the invariant `YYYY-MM-DD` prefix by char offset, stringifies `time.Month(month_num)` and slices `[:3]` for the abbreviation. Accepts any of these ISO 8601 forms (the date prefix is what matters): `2023-10-15T13:18:50-07:00`, `2023-10-15T13:18:50-0700`, `2023-10-15T13:18:50Z`, `2023-10-15T13:18:50`, `2023-10-15`.
 
-Future: will accept Go reference-date format strings (e.g. `{{date | format "Mon Jan 2 2006"}}`) and pull default format/timezone from site configuration.
+Takes an optional arg for the Go reference-date layout to use:
+- A double-quoted literal, spaces allowed: `{{date | format "Mon Jan 2 2006"}}`.
+- A bare key, resolved from context like any other field: `{{date | format long}}` uses the value of `long` (e.g. a site-config field) as the layout.
+- No arg: falls back to the `date_format` context key (typically `date.format` from `thor.json`).
 
 ### Memory ownership
 

mustache/pipes.odin

@@ -25,6 +25,51 @@ Group :: struct {
 	items: [dynamic]any,
 }
 
+// is_pipe_space reports whether c is whitespace for the purposes of
+// tokenizing a filter segment.
+is_pipe_space :: proc(c: u8) -> bool {
+	return c == ' ' || c == '\t' || c == '\n' || c == '\r'
+}
+
+// tokenize_fields splits seg on whitespace like strings.fields, but a
+// double-quoted span (spaces allowed inside) becomes a single token. The
+// quote characters are kept in the token (not stripped) so callers can
+// distinguish a quoted literal from a bare key name. No escape sequences.
+tokenize_fields :: proc(seg: string, pos: int) -> (tokens: [dynamic]string, err: Error) {
+	i := 0
+	for i < len(seg) {
+		for i < len(seg) && is_pipe_space(seg[i]) {
+			i += 1
+		}
+		if i >= len(seg) {
+			break
+		}
+		if seg[i] == '"' {
+			start := i
+			j := i + 1
+			for j < len(seg) && seg[j] != '"' {
+				j += 1
+			}
+			if j >= len(seg) {
+				return tokens, Error_Body {
+					msg  = fmt.tprintf("unterminated string literal: %s", seg),
+					pos  = pos,
+					kind = .Syntax,
+				}
+			}
+			append(&tokens, seg[start:j + 1])
+			i = j + 1
+		} else {
+			start := i
+			for i < len(seg) && !is_pipe_space(seg[i]) {
+				i += 1
+			}
+			append(&tokens, seg[start:i])
+		}
+	}
+	return tokens, nil
+}
+
 // Returned strings are slices into content — no cloning, lifetime bound to
 // the caller's source.
 parse_pipeline :: proc(
@@ -70,7 +115,11 @@ parse_pipeline :: proc(
 			return "", Error_Body{msg = "empty filter", pos = pos, kind = .Syntax}
 		}
 
-		tokens := strings.fields(seg)
+		tokens, terr := tokenize_fields(seg, pos)
+		if terr != nil {
+			delete(tokens)
+			return "", terr
+		}
 		if len(tokens) == 0 {
 			return "", Error_Body{msg = "filter missing op name", pos = pos, kind = .Syntax}
 		}
@@ -119,6 +168,30 @@ apply_pipeline :: proc(
 	return
 }
 
+// resolve_format_string looks up name as a context key and returns its
+// string value. Used both for an explicit bare-key filter arg (e.g.
+// `format long`) and for the implicit "date_format" fallback when no arg
+// is given.
+resolve_format_string :: proc(name: string, ctx: []any, pos: int) -> (string, Error) {
+	raw := resolve_name(name, ctx)
+	if raw == nil {
+		return "", Error_Body {
+			msg  = fmt.tprintf("unable to resolve date format key '%s'", name),
+			pos  = pos,
+			kind = .Data,
+		}
+	}
+	str, ok := reflect.as_string(raw)
+	if !ok {
+		return "", Error_Body {
+			msg  = fmt.tprintf("date format key '%s' is not a string", name),
+			pos  = pos,
+			kind = .Data,
+		}
+	}
+	return str, nil
+}
+
 // TODO: diagnostics don't  show anything relevent
 apply_filter :: proc(value: any, filter: ^Pipe_Filter, pos: int, ctx: []any) -> (any, Error) {
 	switch filter.op {
@@ -137,26 +210,22 @@ apply_filter :: proc(value: any, filter: ^Pipe_Filter, pos: int, ctx: []any) ->
 		date_format: string
 
 		if len(filter.args) > 0 {
-			date_format = filter.args[0]
-		} else {
-			date_format_raw := resolve_name("date_format", ctx)
-			if date_format_raw == nil {
-				return value, Error_Body {
-					msg = "Unable to determine date format",
-					pos = pos,
-					kind = .Data,
-				}
+			arg := filter.args[0]
+			if len(arg) >= 2 && arg[0] == '"' && arg[len(arg) - 1] == '"' {
+				date_format = arg[1:len(arg) - 1]
 			} else {
-				valid: bool
-				date_format, valid = reflect.as_string(date_format_raw)
-				if !valid {
-					return value, Error_Body {
-						msg = "date format is not a string",
-						pos = pos,
-						kind = .Data,
-					}
+				df, ferr := resolve_format_string(arg, ctx, pos)
+				if ferr != nil {
+					return value, ferr
 				}
+				date_format = df
 			}
+		} else {
+			df, ferr := resolve_format_string("date_format", ctx, pos)
+			if ferr != nil {
+				return value, ferr
+			}
+			date_format = df
 		}
 
 		str2, err := apply_format(str, filter.args[:], pos, date_format)

mustache/pipes_test.odin

@@ -358,6 +358,72 @@ test_format_inside_section_skips_when_empty :: proc(t: ^testing.T) {
 	testing.expect_value(t, result, "[]")
 }
 
+// ---------------------------------------------------------------------------
+// format filter args: quoted literal strings and bare context keys
+// ---------------------------------------------------------------------------
+
+@(test)
+test_format_quoted_literal_arg :: proc(t: ^testing.T) {
+	data := Format_Data {
+		date        = "2026-03-15T08:49:54-04:00",
+		date_format = "2 Jan 2006",
+	}
+	tpl, _ := parse(`{{date | format "Jan 2, 2006"}}`, "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "Mar 15, 2026")
+}
+
+Key_Format_Data :: struct {
+	date: string,
+	long: string,
+}
+
+@(test)
+test_format_bare_key_arg_resolves_from_context :: proc(t: ^testing.T) {
+	data := Key_Format_Data {
+		date = "2026-03-15T08:49:54-04:00",
+		long = "2 January 2006",
+	}
+	tpl, _ := parse("{{date | format long}}", "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "15 March 2026")
+}
+
+@(test)
+test_format_bare_key_arg_missing_errors :: proc(t: ^testing.T) {
+	data := Key_Format_Data {
+		date = "2026-03-15T08:49:54-04:00",
+		long = "2 January 2006",
+	}
+	tpl, _ := parse("{{date | format missing}}", "<test>", allocator = context.temp_allocator)
+	defer delete_template(&tpl)
+	_, err := render(tpl, data, {}, context.temp_allocator)
+	testing.expect(t, err != nil, "unresolved format key should error")
+}
+
+@(test)
+test_format_bare_key_arg_non_string_errors :: proc(t: ^testing.T) {
+	Int_Key_Data :: struct {
+		date:  string,
+		count: int,
+	}
+	data := Int_Key_Data {
+		date  = "2026-03-15T08:49:54-04:00",
+		count = 42,
+	}
+	tpl, _ := parse("{{date | format count}}", "<test>", allocator = context.temp_allocator)
+	defer delete_template(&tpl)
+	_, err := render(tpl, data, {}, context.temp_allocator)
+	testing.expect(t, err != nil, "non-string format key should error")
+}
+
+@(test)
+test_format_unterminated_quote_arg_is_parse_error :: proc(t: ^testing.T) {
+	src := `{{date | format "Jan 2, 2006}}`
+	_, err := parse(src)
+	testing.expect(t, err != nil, "unterminated string literal should fail to parse")
+}
+
 @(test)
 test_format_context_date_format_weekday :: proc(t: ^testing.T) {
 	data := Format_Data {
@@ -402,3 +468,18 @@ test_format_handles_all_iso8601_variants :: proc(t: ^testing.T) {
 	}
 }
 
+
+@(test)
+test_format_bare_numeric_arg_treated_as_key_not_literal :: proc(t: ^testing.T) {
+	// "2006" happens to also be a valid Go layout token — make sure an
+	// unquoted arg is still resolved as a context key (and fails, since
+	// no field is named "2006"), not silently used as the literal layout.
+	data := Format_Data {
+		date        = "2026-03-15T08:49:54-04:00",
+		date_format = "2 Jan 2006",
+	}
+	tpl, _ := parse("{{date | format 2006}}", "<test>", allocator = context.temp_allocator)
+	defer delete_template(&tpl)
+	_, err := render(tpl, data, {}, context.temp_allocator)
+	testing.expect(t, err != nil, "bare numeric-looking arg should error as an unresolved key")
+}