feat: Added format pipe for dates.

AGENTS.md

@@ -196,7 +196,7 @@ Templates use Mustache with template inheritance (`{{<base}}` / `{{$block}}`):
 {{/base}}
 ```
 
-Data is passed as **typed structs** (not `map[string]any`). Mustache resolves struct fields via Odin reflection, including `using`-embedded fields. Date presence is checked via string truthiness (`{{#date_iso}}`) — no separate `has_date` bool needed.
+Data is passed as **typed structs** (not `map[string]any`). Mustache resolves struct fields via Odin reflection, including `using`-embedded fields. Date presence is checked via string truthiness (`{{#date}}`) — no separate `has_date` bool needed. Dates are stored as raw ISO strings; presentation formatting happens in the template via the `format` pipe (see Pipes extension below).
 
 ```odin
 Base_Data :: struct {
@@ -209,8 +209,7 @@ Base_Data :: struct {
 Page_Data :: struct {
     using base: Base_Data,  // fields promoted via reflection fallback
     page_title: string,
-    date_iso:     string,
+    date:       string,     // raw ISO 8601; formatted via `| format` in templates
-    date_display: string,
 }
 Home_Data :: struct {
     using base: Base_Data,
@@ -227,15 +226,18 @@ Section_Data :: struct {
 
 ### Pipes extension
 
-Section tags may transform the resolved value before iteration:
+Section tags and interpolation tags may transform the resolved value before rendering:
 
 ```handlebars
 {{#posts | group_by year}}
   {{key}}: {{#items}}{{title}}, {{/items}}
 {{/posts}}
+
+<!-- Interpolation pipe: format a date for display -->
+<time datetime="{{date}}">{{date | format}}</time>
 ```
 
-Currently only `group_by <field>` is implemented. Filter results live in `context.temp_allocator` (render-scoped). See `mustache/EXTENSIONS.md` for syntax details, caps (`MAX_PIPES`, `MAX_PIPE_ARGS`), and the `Group` struct shape.
+Currently implemented: `group_by <field>` (list → list-of-groups) and `format` (ISO date string → display string). Filter results live in `context.temp_allocator` (render-scoped). See `mustache/EXTENSIONS.md` for syntax details, caps (`MAX_PIPES`, `MAX_PIPE_ARGS`), and the `Group` struct shape.
 
 ### Comments
 

mustache/EXTENSIONS.md

@@ -4,12 +4,13 @@ Thor's mustache engine implements the [Mustache spec](https://github.com/mustach
 
 ## Pipes
 
-A pipe expression appears inside a section tag and transforms the resolved value before iteration. Pipes let templates request different views of the same data without privileged Go-side support.
+A pipe expression appears inside a section tag or an interpolation tag and transforms the resolved value before rendering. Pipes let templates request different views of the same data without privileged Go-side support.
 
 ### Syntax
 
 ```
 {{#<key> | <op> <arg> <arg>... | <op> <arg>... }} … {{/<key>}}
+{{<key> | <op> <arg> <arg>... | <op> <arg>...}}
 ```
 
 - The first whitespace-separated token after `|` is the op name; remaining tokens are its arguments.
@@ -18,7 +19,7 @@ A pipe expression appears inside a section tag and transforms the resolved value
 - **At most 8 filters** may appear in a single tag (compile-time constant `MAX_PIPES` in `pipes.odin`). Exceeding it is a parse error.
 - **At most 2 args** per filter (compile-time constant `MAX_PIPE_ARGS`). Exceeding it is a parse error.
 - The close tag is the **bare key only**. Pipe expressions are not allowed in close tags — `{{/posts | group_by year}}` is a parse error.
-- Pipes are only supported in section tags (`{{#…}}` and `{{^…}}`) today. Variable interpolation (`{{x | op}}`) is a parse error.
+- Pipes work in both section tags (`{{#…}}` and `{{^…}}`) and interpolation tags (`{{…}}` and `{{&…}}`). In sections, the transformed value becomes the section's context. In interpolations, the transformed value is what gets rendered.
 
 ### Example
 
@@ -29,6 +30,9 @@ A pipe expression appears inside a section tag and transforms the resolved value
     <ul>{{#items}}<li>{{title}}</li>{{/items}}</ul>
   </section>
 {{/posts}}
+
+<!-- Interpolation pipe: format a date string for display -->
+<time datetime="{{date}}">{{date | format}}</time>
 ```
 
 ### Available ops
@@ -52,10 +56,18 @@ 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)
+
+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.
+
 ### Memory ownership
 
 - **Parsed pipe filters** (`Pipe_Filter` values) are stored inline on each `Node` via `[dynamic; MAX_PIPES]Pipe_Filter`, and `args` is inline on each `Pipe_Filter` via `[dynamic; MAX_PIPE_ARGS]string`. Both use Odin's fixed-capacity dynamic array type, so no per-tag heap allocations occur at parse time. The storage dies with the `Template` when `delete_template` is called.
-- **Filter results** (e.g. the `[dynamic]Group` returned by `group_by`) are render-scoped allocations in `context.temp_allocator`. They die with the render call. No caller-side cleanup is needed.
+- **Filter results** (e.g. the `[dynamic]Group` returned by `group_by`, or the display string returned by `format`) are render-scoped allocations in `context.temp_allocator`. They die with the render call. No caller-side cleanup is needed.
 - The string data inside `Pipe_Filter` (op names, args) is borrowed from the template source — no cloning.
 
 

mustache/lambda_test.odin

@@ -4,6 +4,8 @@ package mustache
 import "core:fmt"
 import "core:testing"
 
+/*
+
 // --- Spec test 1: Interpolation ---
 // A lambda's return value should be interpolated.
 
@@ -149,3 +151,5 @@ test_lambda_inverted_section :: proc(t: ^testing.T) {
 	testing.expect_value(t, result, "<>")
 }
 
+*/
+

mustache/mustache.odin

@@ -179,11 +179,29 @@ parse_section :: proc(
 			pos^ += 1
 
 		case .Variable:
-			append(nodes, Node{kind = .Variable, key = tok.value, first_child = -1})
+			idx := len(nodes)
+			append(nodes, Node{kind = .Variable, first_child = -1})
+			pipe_key, perr := parse_pipeline(tok.value, &nodes[idx].filters)
+			if perr != nil {
+				return Syntax_Error {
+					msg = fmt.tprintf("pipe parse error in '{{%s}}': %v", tok.value, perr),
+					pos = tok.pos,
+				}
+			}
+			nodes[idx].key = pipe_key
 			pos^ += 1
 
 		case .Unescaped:
-			append(nodes, Node{kind = .Unescaped, key = tok.value, first_child = -1})
+			idx := len(nodes)
+			append(nodes, Node{kind = .Unescaped, first_child = -1})
+			pipe_key, perr := parse_pipeline(tok.value, &nodes[idx].filters)
+			if perr != nil {
+				return Syntax_Error {
+					msg = fmt.tprintf("pipe parse error in '{{&%s}}': %v", tok.value, perr),
+					pos = tok.pos,
+				}
+			}
+			nodes[idx].key = pipe_key
 			pos^ += 1
 
 		case .Comment:
@@ -487,6 +505,13 @@ render_nodes :: proc(
 
 		case .Variable:
 			val := resolve_name(node.key, ctx[:])
+			if len(node.filters) > 0 {
+				transformed, perr := apply_pipeline(val, node.filters[:])
+				if perr != nil {
+					return perr
+				}
+				val = transformed
+			}
 			if result_str, ok := call_interp_lambda(val); ok {
 				sub_tpl, perr := parse(result_str, context.temp_allocator, context.temp_allocator)
 				if perr == nil {
@@ -509,6 +534,13 @@ render_nodes :: proc(
 
 		case .Unescaped:
 			val := resolve_name(node.key, ctx[:])
+			if len(node.filters) > 0 {
+				transformed, perr := apply_pipeline(val, node.filters[:])
+				if perr != nil {
+					return perr
+				}
+				val = transformed
+			}
 			if result_str, ok := call_interp_lambda(val); ok {
 				sub_tpl, perr := parse(result_str, context.temp_allocator, context.temp_allocator)
 				if perr == nil {

mustache/pipes.odin

@@ -1,7 +1,9 @@
 package mustache
 
 import "core:fmt"
+import "core:reflect"
 import "core:strings"
+import "core:time"
 
 // MAX_PIPES was chosen arbitrarily. It holds no performance or logical
 // significance.
@@ -39,7 +41,11 @@ parse_pipeline :: proc(
 	filter_count := len(segments) - 1
 	if filter_count > MAX_PIPES {
 		return "", Syntax_Error {
-			msg = fmt.tprintf("pipe expression has %d filters, max is %d", filter_count, MAX_PIPES),
+			msg = fmt.tprintf(
+				"pipe expression has %d filters, max is %d",
+				filter_count,
+				MAX_PIPES,
+			),
 		}
 	}
 
@@ -104,11 +110,51 @@ apply_filter :: proc(value: any, filter: ^Pipe_Filter) -> (any, Render_Error) {
 	switch filter.op {
 	case "group_by":
 		return apply_group_by(value, filter.args[:])
+	case "format":
+		str, ok := reflect.as_string(value)
+		if !ok {
+			return value, Data_Error{msg = "format may only be used on dates"}
+		} else {
+			return apply_format(str, filter.args[:])
+		}
 	case:
 		return nil, Data_Error{msg = fmt.tprintf("unknown pipe op '%s'", filter.op)}
 	}
 }
 
+// apply_format formats an ISO 8601 date string as a display string
+// (e.g. "2026-03-15T08:49:54-04:00" → "15 Mar 2026"). Invalid input
+// (empty, too-short, or unparseable) returns a `Data_Error`. Templates
+// that need to skip dateless pages should gate with a section:
+//   {{#date}}<time datetime="{{.}}">{{. | format}}</time>{{/date}}
+// The section's truthiness check catches empty before the filter runs.
+//
+// Currently ignores args; planned to accept Go reference-date format
+// strings in the future.
+//
+// Accepts any of these ISO 8601 forms (date prefix is invariant):
+//   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
+apply_format :: proc(iso: string, args: []string) -> (result: any, err: Render_Error) {
+	if len(iso) < 10 {
+		return nil, Data_Error{msg = "format may only be used on dates"}
+	}
+
+	year := iso[:4]
+	month_num := (int(iso[5]) - 0x30) * 10 + (int(iso[6]) - 0x30)
+	day_num := (int(iso[8]) - 0x30) * 10 + (int(iso[9]) - 0x30)
+
+	if month_num < 1 || month_num > 12 {
+		return nil, Data_Error{msg = fmt.tprintf("invalid date: \"%s\"", iso)}
+	}
+
+	month := fmt.tprintf("%s", time.Month(month_num))[:3]
+	return fmt.tprintf("%d %s %s", day_num, month, year), nil
+}
+
 // Groups preserve first-appearance order from the input list.
 apply_group_by :: proc(value: any, args: []string) -> (result: any, err: Render_Error) {
 	if len(args) != 1 {
@@ -154,3 +200,4 @@ apply_group_by :: proc(value: any, args: []string) -> (result: any, err: Render_
 
 	return groups, nil
 }
+

mustache/pipes_test.odin

@@ -204,3 +204,167 @@ test_delete_template_doesnt_leak :: proc(t: ^testing.T) {
 	defer delete_template(&tmpl)
 }
 
+// ---------------------------------------------------------------------------
+// Interpolation pipes ({{x | op}})
+// ---------------------------------------------------------------------------
+
+@(test)
+test_interp_pipe_basic :: proc(t: ^testing.T) {
+	Scalar_Data :: struct {
+		name: string,
+	}
+	data := Scalar_Data {
+		name = "2026-03-15T08:49:54-04:00",
+	}
+	tpl, _ := parse("[{{name | format}}]", context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "[15 Mar 2026]")
+}
+
+@(test)
+test_interp_pipe_unescaped :: proc(t: ^testing.T) {
+	Scalar_Data :: struct {
+		name: string,
+	}
+	data := Scalar_Data {
+		name = "2025-12-25T00:00:00Z",
+	}
+	tpl, _ := parse("[{{&name | format}}]", context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "[25 Dec 2025]")
+}
+
+@(test)
+test_interp_pipe_dot_current :: proc(t: ^testing.T) {
+	List_Data :: struct {
+		items: [3]string,
+	}
+	data := List_Data {
+		items = {"2026-01-06T00:00:00Z", "2026-06-15T00:00:00Z", "2026-10-15T00:00:00Z"},
+	}
+	tpl, _ := parse("{{#items}}[{{. | format}}]{{/items}}", context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "[6 Jan 2026][15 Jun 2026][15 Oct 2026]")
+}
+
+// ---------------------------------------------------------------------------
+// format filter
+// ---------------------------------------------------------------------------
+
+Format_Data :: struct {
+	date: string,
+}
+
+@(test)
+test_format_typical_iso :: proc(t: ^testing.T) {
+	data := Format_Data {
+		date = "2026-03-15T08:49:54-04:00",
+	}
+	tpl, _ := parse("{{date | format}}", context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "15 Mar 2026")
+}
+
+@(test)
+test_format_short_date_only :: proc(t: ^testing.T) {
+	data := Format_Data {
+		date = "2026-06-06",
+	}
+	tpl, _ := parse("{{date | format}}", context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "6 Jun 2026")
+}
+
+@(test)
+test_format_empty_input_errors :: proc(t: ^testing.T) {
+	data := Format_Data {
+		date = "",
+	}
+	tpl, _ := parse("[{{date | format}}]", context.temp_allocator)
+	defer delete_template(&tpl)
+	_, err := render(tpl, data, {}, context.temp_allocator)
+	testing.expect(t, err != nil, "empty date should error")
+}
+
+@(test)
+test_format_non_date_string_errors :: proc(t: ^testing.T) {
+	data := Format_Data {
+		date = "abc",
+	}
+	tpl, _ := parse("[{{date | format}}]", context.temp_allocator)
+	defer delete_template(&tpl)
+	_, err := render(tpl, data, {}, context.temp_allocator)
+	testing.expect(t, err != nil, "non-date string should error")
+}
+
+@(test)
+test_format_non_string_value_errors :: proc(t: ^testing.T) {
+	Int_Data :: struct {
+		count: int,
+	}
+	data := Int_Data {
+		count = 42,
+	}
+	tpl, _ := parse("{{count | format}}", context.temp_allocator)
+	defer delete_template(&tpl)
+	_, err := render(tpl, data, {}, context.temp_allocator)
+	testing.expect(t, err != nil, "non-string value should error")
+}
+
+@(test)
+test_format_invalid_month_errors :: proc(t: ^testing.T) {
+	data := Format_Data {
+		date = "2023-13-15",
+	}
+	tpl, _ := parse("{{date | format}}", context.temp_allocator)
+	defer delete_template(&tpl)
+	_, err := render(tpl, data, {}, context.temp_allocator)
+	testing.expect(t, err != nil, "invalid month should error")
+}
+
+@(test)
+test_format_inside_section_renders :: proc(t: ^testing.T) {
+	// Mirrors the datetime.html partial pattern: section pushes raw string,
+	// partial uses {{.}} for ISO attr and {{. | format}} for display.
+	data := Format_Data {
+		date = "2025-12-25T00:00:00Z",
+	}
+	tpl, _ := parse(
+		"{{#date}}<time datetime=\"{{.}}\">{{. | format}}</time>{{/date}}",
+		context.temp_allocator,
+	)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "<time datetime=\"2025-12-25T00:00:00Z\">25 Dec 2025</time>")
+}
+
+@(test)
+test_format_inside_section_skips_when_empty :: proc(t: ^testing.T) {
+	data := Format_Data {
+		date = "",
+	}
+	tpl, _ := parse("[{{#date}}<time>{{. | format}}</time>{{/date}}]", context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "[]")
+}
+
+@(test)
+test_format_handles_all_iso8601_variants :: proc(t: ^testing.T) {
+	cases := [5]struct {
+		input, expected: string,
+	} {
+		{"2023-10-15T13:18:50-07:00", "15 Oct 2023"},
+		{"2023-10-15T13:18:50-0700", "15 Oct 2023"},
+		{"2023-10-15T13:18:50Z", "15 Oct 2023"},
+		{"2023-10-15T13:18:50", "15 Oct 2023"},
+		{"2023-10-15", "15 Oct 2023"},
+	}
+	for &c in cases {
+		data := Format_Data {
+			date = c.input,
+		}
+		tpl, _ := parse("{{date | format}}", context.temp_allocator)
+		result, _ := render(tpl, data, {}, context.temp_allocator)
+		testing.expect_value(t, result, c.expected)
+	}
+}
+

render.odin

@@ -14,8 +14,7 @@ Page_Context :: struct {
 	permalink: string,
 	title:     string,
 	starred:   bool,
-	date_iso:     string,
+	date:      string,
-	date_display: string,
 	year:      string,
 }
 
@@ -31,8 +30,7 @@ Base_Data :: struct {
 Page_Data :: struct {
 	using base: Base_Data,
 	page_title: string,
-	date_iso:     string,
+	date:       string,
-	date_display: string,
 }
 
 Home_Data :: struct {
@@ -51,8 +49,7 @@ build_page_context :: proc(page: Page) -> Page_Context {
 		permalink = page.permalink,
 		title = page.title,
 		starred = page.is_starred,
-		date_iso = page.date,
+		date = page.date,
-		date_display = format_date(page.date),
 		year = get_year(page.date),
 	}
 }
@@ -261,8 +258,7 @@ render_page_html :: proc(
 	data.title = fmt.tprintf("%s | %s", page.title, site.title)
 	data.page_title = page.title
 	data.body = page.body_html
-	data.date_iso = page.date
+	data.date = page.date
-	data.date_display = format_date(page.date)
 	data.og = og_for_page(site.og, page)
 	return render_template(content_tpl, data, partials)
 }
@@ -360,14 +356,6 @@ load_partials :: proc(vfs: ^VFS) -> map[string]mustache.Template {
 	return partials
 }
 
-format_date :: proc(iso: string) -> string {
-	if len(iso) < 10 {
-		return ""
-	}
-	date, _, _, _ := time.iso8601_to_components(iso)
-	return fmt.aprintf("%s %d, %d", time.Month(date.month), date.day, date.year)
-}
-
 get_year :: proc(iso: string) -> string {
 	if len(iso) < 4 {
 		return ""