simplified metaprogramming interface

Author: kristoff-it

src/fuzz/context.zig

@@ -62,8 +62,9 @@ const TestValue = union(Tag) {
                     gpa: std.mem.Allocator,
                     args: []const TestValue,
                 ) !TestValue {
+                    _ = gpa;
                     if (args.len != 0) return .{ .err = "'len' wants no arguments" };
-                    return TestValue.from(gpa, str.len);
+                    return TestValue.from(str.len);
                 }
             };
         };
@@ -86,7 +87,7 @@ const TestValue = union(Tag) {
         return .{ .bool = b };
     }
 
-    pub fn from(gpa: std.mem.Allocator, value: anytype) TestValue {
+    pub fn from(gpa: std.mem.Allocator, value: anytype) !TestValue {
         _ = gpa;
         const T = @TypeOf(value);
         switch (T) {

src/types.zig

@@ -44,7 +44,10 @@ pub fn defaultCall(comptime Value: type) fn (
         ) error{ OutOfMemory, Interrupt }!Value {
             switch (value) {
                 inline else => |v, tag| {
-                    const Builtin = Value.builtinsFor(tag);
+                    const Builtin = if (@hasDecl(Value, "builtinsFor"))
+                        Value.builtinsFor(tag)
+                    else
+                        defaultBuiltinsFor(Value, @TypeOf(v));
 
                     inline for (@typeInfo(Builtin).Struct.decls) |decl| {
                         if (decl.name[0] == '_') continue;
@@ -86,3 +89,24 @@ inline fn constify(comptime T: type, comptime mut: bool) type {
         false => *const T,
     };
 }
+
+pub fn defaultBuiltinsFor(comptime Value: type, comptime Field: type) type {
+    inline for (std.meta.fields(Value)) |f| {
+        if (f.type == Field) {
+            switch (@typeInfo(f.type)) {
+                .Pointer => |ptr| {
+                    if (@typeInfo(ptr.child) == .Struct) {
+                        return @field(ptr.child, "Builtins");
+                    }
+                },
+                .Struct => {
+                    return @field(f.type, "Builtins");
+                },
+                else => {},
+            }
+
+            return struct {};
+        }
+    }
+    @compileError("Value has no field of value " ++ @typeName(Field));
+}

src/vm.zig

@@ -16,10 +16,6 @@ pub fn VM(
     comptime _Context: type,
     comptime _Value: type,
 ) type {
-    if (!@hasDecl(_Value, "builtinsFor")) {
-        @compileLog("Value type must specify builtinsFor");
-    }
-
     return struct {
         parser: Parser = .{},
         stack: std.MultiArrayList(Result) = .{},
@@ -180,7 +176,7 @@ pub fn VM(
                         const path = src[start..end];
 
                         const old_value = if (global)
-                            Value.from(gpa, ctx)
+                            try Value.from(gpa, ctx)
                         else blk: {
                             if (builtin.mode == .Debug) {
                                 const stack_debug = slice.items(.debug);