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const std = @import("std");

fn LazyEval(val: var, tok: var) @TypeOf(val) {
    return val;
}

pub fn ErrorDispatcher(tok: var) type {
    comptime var curr_set: type = void;

return struct {
        const Self = @This();
        err: anyerror,

pub fn init(err: var) InitSet(@TypeOf(err), Self) {
            return .{ .err = err };
        }

pub fn case(self: *Self, comptime SubSet: type) RestrictSet(SubSet, SubSet!void) {
            inline for (std.meta.fields(SubSet)) |field| {
                if (@errorToInt(self.err) == field.value) return @errSetCast(SubSet, self.err);
            }
        }

pub fn rest(self: Self, uniq: var) CheckSet(uniq) {
            // use the cached type, the actual var is empty
            return @errSetCast(CheckSet(uniq), self.err);
        }

fn InitSet(comptime T: type, comptime Return: type) type {
            if (!std.meta.trait.is(.ErrorSet)(T)) @compileError("Value should be an error");
            curr_set = T;
            return Return;
        }

fn RestrictSet(comptime Removed: type, comptime Return: type) type {
            if (comptime !std.meta.trait.is(.ErrorSet)(Removed)) {
                @compileError("Sub set needs to be an error set...");
            }

if (@typeInfo(curr_set).ErrorSet) |CurrentSet| {
                var found_fields = [1]bool {false} ** CurrentSet.len;
                for (std.meta.fields(Removed)) |sub_field| {
                    var found = false;
                    for (CurrentSet) |field, idx| {
                        if (comptime std.mem.eql(u8, field.name, sub_field.name)) {
                            found_fields[idx] = true;
                            found = true;
                            break;
                        }
                    }
                    if (!found) @compileError("Subset field " ++ sub_field.name ++ " not in current set.");
                }

// Make new narrowed type
                var errors: []const std.builtin.TypeInfo.Error = &[0]std.builtin.TypeInfo.Error{};
                for (CurrentSet) |field, idx| {
                    if (!found_fields[idx]) {
                        errors = errors ++ &[1]std.builtin.TypeInfo.Error{field};
                    }
                }
                if (errors.len == 0) {
                    curr_set = void;
                } else {
                    curr_set = @Type(.{ .ErrorSet = errors });
                }
            } else {
                @compileError("Current set is empty");
            }

return Return;
        }

fn CheckSet(uniq: var) type {
            if (curr_set == void) @compileError("Error set is empty");
            return curr_set;
        }
    };
}

fn fieldStr(comptime set: type) []const u8 {
    comptime {
        var str: []const u8 = "";
        for (std.meta.fields(set)) |f, idx| {
            if (idx != 0) str = str ++ ", ";
            str = str ++ f.name;
        }
        return str;
    }
}

const FooSet = error {
    AB,
    CD,
    EF,
    GH,
};

fn foo() FooSet!void {}

pub fn main() void {
    foo() catch |err| {
        var dispatcher = ErrorDispatcher(.{}).init(err);
        dispatcher.case(error {AB, CD}) catch |sub_err| {
            // This will print AB, CD
            // @compileError(comptime fieldStr(@TypeOf(sub_err)));
        };

const narrow_err_1 = dispatcher.rest(.{});
        // This will print EF GH
        //@compileError(comptime fieldStr(@TypeOf(dispatcher.rest(.{}))));

dispatcher.case(error{EF}) catch |sub_err| {
            // This will print EF
            // @compileError(comptime fieldStr(@TypeOf(sub_err)));
        };

const narrow_err_2 = dispatcher.rest(.{});
        // This will print GH
        //@compileError(comptime fieldStr(@TypeOf(dispatcher.rest(.{}))));

dispatcher.case(error{GH}) catch |sub_err| {
            // This will print GH
            // @compileError(comptime fieldStr(@TypeOf(sub_err)));
        };
    };
}