feat: Added first and last filters.

TODOS.md

@@ -18,8 +18,10 @@ Remember to polish existing features before moving on to new ones.
   - alt1: `layouts`, `partials`
   - alt2: `templates/layouts`, `templates/partials`
 - [ ] pipes
-  - [ ] first
+  - [ ] first/last
-  - [ ] last
+    - [ ] warn when using negative numbers
+    - [ ] warn when using default arg on string
+    - [ ] add the above warnings to DIAGNOSTIC_TODOS.md
   - [ ] sort_by
 
 ## Documentation

mustache/pipes.odin

@@ -4,8 +4,8 @@ import "base:runtime"
 import "core:fmt"
 import "core:log"
 import "core:reflect"
+import "core:strconv"
 import "core:strings"
-import "core:time"
 
 import diags "../diagnostics"
 import "core:time/datetime"
@@ -22,6 +22,8 @@ DEFAULT_DATE_FORMAT :: "2 Jan 2006"
 Pipe_Op :: enum {
 	Format,
 	Group_By,
+	First,
+	Last,
 }
 
 // Pipe op names are derived from the enum via reflection (lowercased).
@@ -133,11 +135,7 @@ parse_pipeline :: proc(
 	pipe_count := strings.count(content, "|")
 	if pipe_count > MAX_PIPES {
 		return "", Error_Body {
-			msg = fmt.tprintf(
+			msg = fmt.tprintf("pipe expression has %d filters, max is %d", pipe_count, MAX_PIPES),
-				"pipe expression has %d filters, max is %d",
-				pipe_count,
-				MAX_PIPES,
-			),
 			pos = pos,
 			kind = .Syntax,
 		}
@@ -267,8 +265,8 @@ apply_filter :: proc(value: any, filter: ^Pipe_Filter, pos: int, ctx: []any) ->
 			hint = fmt.tprintf("did you mean '%s'?", suggestion)
 		}
 		return nil, Error_Body {
-			msg  = fmt.tprintf("unknown pipe op '%s'", filter.op),
+			msg = fmt.tprintf("unknown pipe op '%s'", filter.op),
-			pos  = filter.op_pos,
+			pos = filter.op_pos,
 			span = len(filter.op),
 			kind = .Data,
 			hint = hint,
@@ -316,6 +314,10 @@ apply_filter :: proc(value: any, filter: ^Pipe_Filter, pos: int, ctx: []any) ->
 		} else {
 			return any{new_clone(str2, context.temp_allocator), typeid_of(string)}, nil
 		}
+	case .First:
+		return take(value, filter.args[:], pos, from_start = true)
+	case .Last:
+		return take(value, filter.args[:], pos, from_start = false)
 	}
 	return {}, nil
 }
@@ -411,3 +413,61 @@ apply_group_by :: proc(value: any, args: []string, pos: int) -> (result: any, er
 
 	return groups, nil
 }
+
+take :: proc(value: any, args: []string, pos: int, from_start: bool) -> (any, Error) {
+	elem_info, count, data := list_info(value)
+	is_list := elem_info != nil
+	from_start := from_start
+
+	str: string
+	if !is_list {
+		s, is_str := reflect.as_string(value)
+		if !is_str {
+			return nil, Error_Body{msg = "requires a list or string", pos = pos, kind = .Data}
+		}
+		str = s
+		count = len(str)
+	}
+
+	n := 1
+	if !is_list && len(args) == 0 {
+		return nil, Error_Body{msg = "requires an argument for strings", pos = pos, kind = .Data}
+	}
+	if len(args) > 0 {
+		parsed, ok := strconv.parse_int(args[0])
+		if !ok {
+			return nil, Error_Body {
+				msg = fmt.tprintf("invalid argument '%s'", args[0]),
+				pos = pos,
+				kind = .Data,
+			}
+		}
+		n = parsed
+	}
+
+	if n < 0 {
+		from_start = !from_start
+		n = 0 - n // abs(n)
+	}
+
+	start: int
+	end: int
+	if from_start {
+		start = 0
+		end = min(count, n)
+	} else {
+		start = max(0, count - n)
+		end = count
+	}
+
+	if is_list {
+		result := make([dynamic]any, 0, max(0, end - start), context.temp_allocator)
+		for j in start ..< end {
+			append(&result, extract_list_element(elem_info, data, j))
+		}
+		return result, nil
+	} else {
+		return str[start:end], nil
+	}
+}
+

mustache/pipes_test.odin

@@ -1,3 +1,4 @@
+#+feature dynamic-literals
 #+test
 package mustache
 
@@ -567,7 +568,10 @@ test_format_mst_no_offset_no_timezone :: proc(t: ^testing.T) {
 
 @(test)
 test_unknown_pipe_op_suggestion :: proc(t: ^testing.T) {
-	filter := Pipe_Filter{op = "formats", op_pos = 0}
+	filter := Pipe_Filter {
+		op     = "formats",
+		op_pos = 0,
+	}
 	_, err := apply_filter("2026-01-15", &filter, 0, nil)
 	testing.expect(t, err != nil, "should error on unknown op")
 	b := body(err)
@@ -577,9 +581,146 @@ test_unknown_pipe_op_suggestion :: proc(t: ^testing.T) {
 
 @(test)
 test_unknown_pipe_op_no_suggestion :: proc(t: ^testing.T) {
-	filter := Pipe_Filter{op = "xyz", op_pos = 0}
+	filter := Pipe_Filter {
+		op     = "xyz",
+		op_pos = 0,
+	}
 	_, err := apply_filter("2026-01-15", &filter, 0, nil)
 	testing.expect(t, err != nil, "should error on unknown op")
 	b := body(err)
 	testing.expect(t, b.hint == "", "no suggestion expected for 'xyz'")
 }
+
+// Group 1: Lists
+
+First_Last_List_Data: map[string][5]string = {
+	"items" = [5]string{"foo", "bar", "bat", "qux", "quz"},
+}
+
+@(test)
+test_first_list_no_args_equals_first_one :: proc(t: ^testing.T) {
+	tpl_a, _ := parse(
+		"{{#items | first}}<li>{{.}}</li>{{/items}}",
+		"<test>",
+		allocator = context.temp_allocator,
+	)
+	result_a, _ := render(tpl_a, First_Last_List_Data, {}, context.temp_allocator)
+
+	tpl_b, _ := parse(
+		"{{#items | first 1}}<li>{{.}}</li>{{/items}}",
+		"<test>",
+		allocator = context.temp_allocator,
+	)
+	result_b, _ := render(tpl_b, First_Last_List_Data, {}, context.temp_allocator)
+
+	testing.expect_value(t, result_a, "<li>foo</li>")
+	testing.expect_value(t, result_a, result_b)
+}
+
+@(test)
+test_first_list_n :: proc(t: ^testing.T) {
+	data := First_Last_List_Data
+	tpl, _ := parse(
+		"{{#items | first 3}}<li>{{.}}</li>{{/items}}",
+		"<test>",
+		allocator = context.temp_allocator,
+	)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "<li>foo</li><li>bar</li><li>bat</li>")
+}
+
+@(test)
+test_first_list_negative_equals_last :: proc(t: ^testing.T) {
+	data := First_Last_List_Data
+	tpl, _ := parse(
+		"{{#items | first -2}}<li>{{.}}</li>{{/items}}",
+		"<test>",
+		allocator = context.temp_allocator,
+	)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "<li>qux</li><li>quz</li>")
+}
+
+@(test)
+test_last_list_no_args_equals_last_one :: proc(t: ^testing.T) {
+	data := First_Last_List_Data
+	tpl_a, _ := parse(
+		"{{#items | last}}<li>{{.}}</li>{{/items}}",
+		"<test>",
+		allocator = context.temp_allocator,
+	)
+	result_a, _ := render(tpl_a, data, {}, context.temp_allocator)
+
+	tpl_b, _ := parse(
+		"{{#items | last 1}}<li>{{.}}</li>{{/items}}",
+		"<test>",
+		allocator = context.temp_allocator,
+	)
+	result_b, _ := render(tpl_b, data, {}, context.temp_allocator)
+
+	testing.expect_value(t, result_a, "<li>quz</li>")
+	testing.expect_value(t, result_b, "<li>quz</li>")
+}
+
+@(test)
+test_last_list_n :: proc(t: ^testing.T) {
+	data := First_Last_List_Data
+	tpl, _ := parse(
+		"{{#items | last 3}}<li>{{.}}</li>{{/items}}",
+		"<test>",
+		allocator = context.temp_allocator,
+	)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "<li>bat</li><li>qux</li><li>quz</li>")
+}
+
+@(test)
+test_last_list_negative_equals_first :: proc(t: ^testing.T) {
+	data := First_Last_List_Data
+	tpl, _ := parse(
+		"{{#items | last -2}}<li>{{.}}</li>{{/items}}",
+		"<test>",
+		allocator = context.temp_allocator,
+	)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "<li>foo</li><li>bar</li>")
+}
+
+// Group 2: Strings
+
+First_Last_String_Data := map[string]string {
+	"word" = "Hello, World!",
+}
+
+@(test)
+test_first_string_n :: proc(t: ^testing.T) {
+	data := First_Last_String_Data
+	tpl, _ := parse("{{word | first 5}}", "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "Hello")
+}
+
+@(test)
+test_last_string_n :: proc(t: ^testing.T) {
+	data := First_Last_String_Data
+	tpl, _ := parse("{{word | last 6}}", "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "World!")
+}
+
+@(test)
+test_first_string_negative_equals_last :: proc(t: ^testing.T) {
+	data := First_Last_String_Data
+	tpl, _ := parse("{{word | first -6}}", "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "World!")
+}
+
+@(test)
+test_last_string_negative_equals_first :: proc(t: ^testing.T) {
+	data := First_Last_String_Data
+	tpl, _ := parse("{{word | last -5}}", "<test>", allocator = context.temp_allocator)
+	result, _ := render(tpl, data, {}, context.temp_allocator)
+	testing.expect_value(t, result, "Hello")
+}
+