Add slice builtin

src/template/lib.zig

@@ -8,11 +8,13 @@ pub fn main() !void {
         @embedFile("./test.tmp.html"),
         .{
             .community = .{ .name = "<abcd>" },
-            .foo = [_][]const u8{ "5", "4", "3", "2", "1" },
+            .foo = &[_][]const u8{ "5", "4", "3", "2", "1" },
-            .baz = [_][]const []const u8{
+            .baz = &[_][]const []const u8{
                 &.{ "5", "4", "3", "2", "1" },
                 &.{ "5", "4", "3", "2", "1" },
             },
+            .start = 1,
+            .end = 3,
             .bar = .{ .x = "x" },
             .qux = false,
             .quxx = true,
@@ -236,7 +238,7 @@ fn deref(arg: anytype, comptime names: []const DerefDecl) DerefError!Deref(@Type
     }
 }
 
-const ExpressionError = DerefError;
+const ExpressionError = error{IndexOutOfBounds} || DerefError;
 
 fn EvaluateExpression(
     comptime expression: Expression,
@@ -251,6 +253,7 @@ fn EvaluateExpression(
         .equals => bool,
         .builtin => |call| switch (call.*) {
             .isTag => bool,
+            .slice => |sl| []const std.meta.Elem(EvaluateExpression(sl.iterable, Args, Captures, Context)),
         },
     };
 }
@@ -280,6 +283,15 @@ fn evaluateExpression(
                 const val = try evaluateExpression(hdr.expression, args, captures, context);
                 return std.meta.isTag(val, hdr.tag);
             },
+            .slice => |sl| {
+                const iterable = try evaluateExpression(sl.iterable, args, captures, context);
+                const start = try evaluateExpression(sl.start, args, captures, context);
+                const end = try evaluateExpression(sl.end, args, captures, context);
+
+                if (comptime std.meta.trait.is(.Array)(@TypeOf(iterable))) @compileError("Cannot slice an array, pass a slice or pointer to array instead");
+                if (start > iterable.len or end > iterable.len) return error.IndexOutOfBounds;
+                return iterable[start..end];
+            },
         },
     };
 }
@@ -635,7 +647,7 @@ fn parseBuiltin(comptime tokens: ControlTokenIter) ParseResult(ControlTokenIter,
         const call = switch (builtin) {
             .isTag => blk: {
                 const expr = parseExpression(iter);
-                iter = expr.new_iter;
+                iter = skipWhitespace(expr.new_iter);
                 expectToken(iter.next(), .comma);
                 iter = skipWhitespace(iter);
                 const tag = iter.next();
@@ -647,6 +659,26 @@ fn parseBuiltin(comptime tokens: ControlTokenIter) ParseResult(ControlTokenIter,
                     },
                 };
             },
+            .slice => blk: {
+                const expr = parseExpression(iter);
+                iter = skipWhitespace(expr.new_iter);
+                expectToken(iter.next(), .comma);
+                iter = skipWhitespace(iter);
+                const start = parseExpression(iter);
+                iter = skipWhitespace(start.new_iter);
+                expectToken(iter.next(), .comma);
+                iter = skipWhitespace(iter);
+                const end = parseExpression(iter);
+                iter = skipWhitespace(end.new_iter);
+
+                break :blk .{
+                    .slice = .{
+                        .iterable = expr.item,
+                        .start = start.item,
+                        .end = end.item,
+                    },
+                };
+            },
         };
         iter = skipWhitespace(iter);
         expectToken(iter.next(), .close_paren);
@@ -1165,6 +1197,7 @@ const EndKeyword = enum {
 
 const Builtin = enum {
     isTag,
+    slice,
 };
 
 const BuiltinCall = union(Builtin) {
@@ -1172,6 +1205,11 @@ const BuiltinCall = union(Builtin) {
         tag: []const u8,
         expression: Expression,
     },
+    slice: struct {
+        iterable: Expression,
+        start: Expression,
+        end: Expression,
+    },
 };
 
 const ControlToken = union(enum) {

src/template/test.tmp.html

@@ -26,6 +26,8 @@
         neither
       {=/if}
 
+      sliced: {#for @slice(.foo, .start, .end) |$s|}{$s}, {/for}
+
       format: {#format "s" .x}
 
       {#switch .snap case foo =}