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How I built zig-sqlite

2022 May 26

Introduction

I wrote and maintain zig-sqlite, a Zig package that wraps the SQLite C API.

Its goals are to use Zig’s features to augment SQLite’s capabilities and provide a idiomatic Zig API.

Why build a wrapper

Zig has excellent C interoperability which means using SQLite is just one @cInclude away, however using the “raw” SQLite API has some drawbacks in the context of a Zig project:

All of those issues can be fixed by introducing a Zig wrapper API.

In addition, using Zig’s comptime feature we can augment SQLite’s capabilities with:

This blog post will focus on how I used comptime to implement these features.

What is comptime

Let’s start by seing what comptime actually is.

comptime is Zig’s way of defining code that must evaluate to something known at compile-time.

You can do a lot with this:

We use all of these capabilities in zig-sqlite.

A brief demo of zig-sqlite

With all that in mind, this is how you use zig-sqlite today:

const std = @import("std");

const sqlite = @import("sqlite");

pub fn main() anyerror!void {
    var db = try sqlite.Db.init(.{
        .mode = sqlite.Db.Mode{ .Memory = {} },
        .open_flags = .{ .write = true },
    });
    defer db.deinit();

    try db.exec("CREATE TABLE user(id integer primary key, age integer, name text)", .{}, .{});

    const user_name: []const u8 = "Vincent";

    // Insert some data
    try db.exec(
        "INSERT INTO user(id, age, name) VALUES($id{usize}, $age{u32}, $name{[]const u8})",
        .{},
        .{ @as(usize, 10), @as(u32, 34), user_name },
    );
    try db.exec(
        "INSERT INTO user(id, age, name) VALUES($id{usize}, $age{u32}, $name{[]const u8})",
        .{},
        .{ @as(usize, 20), @as(u32, 84), @as([]const u8, "José") },
    );

    // Read one row into a struct
    const User = struct {
        id: usize,
        age: u32,
        name: []const u8,
    };
    const user_opt = try db.oneAlloc(
        User,
        std.testing.allocator,
        "SELECT id, age, name FROM user WHERE name = $name{[]const u8}",
        .{},
        .{
            .name = user_name,
        },
    );

    // Read single integers; reuse the same prepared statement
    var stmt = try db.prepare("SELECT id FROM user WHERE age = $age{u32}");
    defer stmt.deinit();

    const id1 = try stmt.one(usize, .{}, .{@as(u32, 34)});
    stmt.reset();
    const id2 = try stmt.one(usize, .{}, .{@as(u32, 84)});
}

The code is not complete, full demo is available here.

This showcases the features I want to talk about. I encourage you to play with the code (try to remove or change the type of a bind parameter).

comptime-checked bind parameters

When you prepare a statement zig-sqlite creates a brand new type only for this prepared statement. This new type contains the parsed query which will be used to perform checks when you call either iterator, exec, one or all on a statement.

This implies the query is known at compile-time; in my experience and my projects this is almost always the case.

This is the trimmed code from zig-sqlite which creates this custom type (actual code here):

pub fn StatementType(comptime query: []const u8) type {
    return Statement(ParsedQuery(query));
}

pub fn Statement(comptime query: anytype) type {
    return struct {
        // NOTE: lots of code here, omitted for clarity
    };
}

const query = "SELECT name FROM user WHERE age = $age{u32}";
var stmt: StatementType(query) = try db.prepare(query);

(If you’re wondering what anytype is, look here).

You can see how the statement type will be different for different queries.

When you call exec, one or all zig-sqlite will use this data on the statement to do two things:

Number of bind parameters

The first check is straightforward: executing a SQL query which has for example 4 bind markers with any other number of bind parameters is never correct.

With the query metadata in our statement type we can enforce this at compile-time.

Let’s see what happens if we provide the wrong number of bind parameters.

Given the following statement:

var stmt = try db.prepare("SELECT id FROM user WHERE age = $age{u32}");

Here’s what happens if we don’t respect the contract:

./third_party/zig-sqlite/sqlite.zig:2057:17: error: expected 1 bind parameters but got 2
                @compileError(comptime std.fmt.comptimePrint("expected {d} bind parameters but got {d}", .{
                ^
./third_party/zig-sqlite/sqlite.zig:2148:26: note: called from here
            try self.bind(.{}, values);
                         ^
./third_party/zig-sqlite/sqlite.zig:2194:41: note: called from here
            var iter = try self.iterator(Type, values);
                                        ^
./src/main.zig:43:29: note: called from here
    const id1 = try stmt.one(usize, .{}, .{ @as(u32, 34), @as(usize, 2000) });
                            ^
./src/main.zig:5:29: note: called from here
pub fn main() anyerror!void {

The error tells us that we expect exactly 1 bind parameter (because there is 1 bind marker) but we provided 2. We can also see directly in the error that we call stmt.one with a tuple of 2 values, which is the source of the compile error.

Type of parameters

The second check is more involved and depends on a type annotation.

You might have already guessed with the demo code, zig-sqlite supports an “extended” version of SQL which allows a user to annotate a bind marker, instructing zig-sqlite to check that all values bound to this marker have this exact Zig type.

Let’s go back to the example above:

var stmt = try db.prepare("SELECT id FROM user WHERE age = $age{u32}");

This annotates the bind marker $age with the Zig type u32. Now anytime this statement is executed the $age bind parameter must have been bound to a u32; it’s a compilation error if that’s not the case.

Let’s see how it fails to compile when we try to pass a u16 when a u32 is expected:

_ = try stmt.one(usize, .{}, .{@as(u16, 34)});

Here is the error from the Zig compiler:

./third_party/zig-sqlite/sqlite.zig:2085:17: error: value type u16 is not the bind marker type u32
                @compileError("value type " ++ @typeName(Actual) ++ " is not the bind marker type " ++ @typeName(Expected));
                ^
./third_party/zig-sqlite/sqlite.zig:2072:58: note: called from here
                        else => comptime assertMarkerType(struct_field.field_type, typ),
                                                         ^
./third_party/zig-sqlite/sqlite.zig:2148:26: note: called from here
            try self.bind(.{}, values);
                         ^
./third_party/zig-sqlite/sqlite.zig:2194:41: note: called from here
            var iter = try self.iterator(Type, values);
                                        ^
./src/main.zig:43:29: note: called from here
    const id1 = try stmt.one(usize, .{}, .{@as(u16, 34)});
                            ^
./src/main.zig:5:29: note: called from here
pub fn main() anyerror!void {

The first line tells us exactly what’s wrong: we try to bind a value of type u16 where a value of type u32 is expected.

Read a row of data into a type

Thanks to Zig’s type reflection we can read a row of data into a user-provided type without needing to write any “mapping” function: we know the type we want to read (here the User struct) and can analyse it at compile-time. With this data we can call the appropriate internal “read” functions.

How it works

Let’s go back to the example in the demo:

const User = struct {
    id: usize,
    age: u32,
    name: []const u8,
};

const user_opt = try db.oneAlloc(
    User,
    std.testing.allocator,
    "SELECT id, age, name FROM user WHERE name = $name{[]const u8}",
    .{},
    .{
        .name = user_name,
    },
);

Notice we pass the type as first parameter. This type is ultimately used to create an Iterator which is responsible for reading data when next or nextAlloc is called.

So the meat of the code is in the iterator and it starts in nextAlloc. The first step is to get the type info of the user type with @typeInfo: this returns a value of type std.builtin.Type which is a tagged union we can analyse using a simple switch statement, for example:

switch (@typeInfo(UserType)) {
    .Int => processInt(),
    .Optional => processOptional(),
    .Array => processArray(),
    .Struct => processStruct(),
    else => @compileError("invalid type " ++ @typeName(UserType)),
}

Of course not all types make sense in the context of our nextAlloc function (it makes no sense to read into a ComptimeInt (which is the type of a comptime integer) or Fn (which is the type of a function) for example).

In our example we end up taking the .Struct prong which calls readStruct. The std.builtin.Type.Struct type contains the struct fields which we iterate over at compile-time using a inline for.

Next we call readField for each struct field, giving it the field type and its position in the struct fields slice. readField also uses @typeInfo to do its thing and ultimately it will end up calling a specific read function depending on the field type.

In our example this would be:

Finally the result of the readField call is assigned to the field in the user type using @field.

It’s important to remember that all these steps are done at compile-time; the code in the final binary would do something like this (not real code):

const user = User{
    .id = try stmt.readInt(usize, 1),
    .age = try stmt.readInt(u32, 2),
    .name = try stmt.readBytes([]const u8, std.testing.allocator, 3, .Text),
};

Examples

I already demoed this before but here’s reading into a struct:

const User = struct {
    id: usize,
    age: u32,
    name: []const u8,
};

const user_opt = try db.oneAlloc(
    User,
    std.testing.allocator,
    "SELECT id, age, name FROM user WHERE name = $name{[]const u8}",
    .{},
    .{
        .name = user_name,
    },
);

You can also read a single type:

const age_opt = try db.one(usize, "SELECT age FROM user WHERE name = $name{[]const u8}", .{}, .{
    .name = user_name,
});

Note we’re not using the oneAlloc method here since we don’t need to allocate memory when reading a simple integer.

We also support reading into enums but this is a little more involved:

const Foo = enum(u7) {
    pub const BaseType = u16;

    low = 34,
    high = 84,
};

const foo_opt = try db.one(Foo, "SELECT age FROM user WHERE name = $name{[]const u8}", .{}, .{
    .name = user_name,
});

The BaseType type is mandatory and tells zig-sqlite how to read the underlying column from SQLite. Here we first read a u16 value.

Next we convert the u16 value to a Foo enum value.

We can also use the BaseType to store the enum value as a string:

const Foo = enum {
    pub const BaseType = []const u8;

    low,
    high,
};

const user_opt = try db.oneAlloc(
    Foo,
    std.testing.allocator,
    "SELECT name FROM user WHERE name = $name{[]const u8}",
    .{},
    .{
        .name = user_name,
    },
);

Now the stored value will be the enum tag name (here low or high). When the BaseType is a string we must use the oneAlloc function because reading a string requires allocation.

Conclusion

Zig is my first real experience using powerful meta-programming and the experience has been mostly positive. Despite the general lack of documentation around Zig I didn’t have much trouble learning about comptime by looking at the existing source code in the standard library (specifically the std.fmt.format function and the JSON parser). If you’re interested in comptime I encourage you to look at the standard library too.

I started this project in October 2020, it wasn’t always fun (I stumbled on quite a few compiler bugs) but I’m really happy with what I ended up with.