Silk, a simple systems programming language

Silk

Silk is a small system programming language that I wrote for fun and clout. It is designed to be as flexible and simple as C, but more pleasant to write. To that end, Silk provides

• nicer syntax
• a sophisticated type system (including parametric types!)
• a sense of pride and accomplishment

Silk does not aim to be as safe or fast as C++, Rust, etc. The LLVM backend (you didn’t think I wrote an end-to-end compiler, did you?) does all of the optimization.

As is true of many of my projects, Silk is currently alpha -stage software. Don’t expect it to work well (or at all).

https://github.com/AjayMT/silk

Syntax

Silk is heavily inspired by Go and other modern programming languages.

// this is a comment

extern func printf(s *i8) void;

func main(argc i32, argv **i8) i32 {
  printf("hello, world\n");

  if argc > 1 {
    val arg = @(argv + 1);
    printf(arg);
  }

  return 0;
}

Notably,

• types come after names
• @ is the pointer dereference operator
• val and var are used to declare values and variables (whose types are inferred unless specified explicitly)
• func , return , if , for and while are familiar keywords
• extern declares a symbol defined in a different object file.

Silk also features parametric polymorphism:

func add:<$t> (a $t, b $t) $t {
  return a + b;
}

func main() i32 {
  val a = add:<i32>(1, 2);
  val b = add:<i8>('a', 'b');
  return a + i32(b);
}

Here is a comprehensive specification of Silk’s syntax.

Semantics and features

Silk is an imperative and statically typed language.

Though safety is not a primary design goal, Silk’s features make writing unsafe code more difficult than it would otherwise be.

Mutability

The val and var keywords declare immutable values and mutable variables respectively. The immutability of values also applies to aggregate types (i.e structures and arrays).

type my_type = struct(a i32, b i32, c i32);

func main() void {
  val foo = my_type(1, 2, 3);
  foo.a = 0; // produces a compiler error

  // bar is a zero-initialized array of 16 my_types
  val bar = [my_type; 16];
  bar[1].c = 2; // this also produces a compiler error
}

Silk has distinct mutable and immutable pointer types. Taking the address of a value produces an immutable pointer, whereas taking the address of a variable produces a mutable pointer.

type my_type = struct(a i32, b i32, c i32);

func main() void {
  val foo = my_type(1, 2, 3);
  var foo_addr *my_type = &foo;
  // foo_addr is an immutable pointer.
  // it is a var and can therefore be re-assigned
  // but cannot be write-dereferenced
  @foo_addr.a = 0; // produces a compiler error

  var bar = my_type(4, 5, 6);
  val bar_addr mut*my_type = &bar;
  // bar_addr is a mutable pointer, which cannot be re-assigned
  // but can be write-dereferenced
  @bar_addr.b = 7;
}

Mutable and immutable pointers can never be cast to/from each other. A mutable pointer can be promoted to an immutable pointer when passed as an argument to a function – the function cannot mutate the object unless it also has a mutable pointer or direct access to the object.

// p is an immutable *i32
func f(p *i32) void {
  @p = 12; // this produces a compiler error
}

func main() void {
  var a = 12;
  val a_addr = &a;
  // a_addr is a mut*i32 that is promoted to an immutable *i32
  f(a_addr);
  // but it is still a mutable pointer in this function
  @a_addr = 13;
}

Additionally, only mutable pointers can be cast to/from integers. Silk’s mutability rules guarantee that:

• mutable pointers never point to immutable objects
• immutable pointers can only point to mutable objects when the objects can be mutated in the same function
  • (or when the objects are mutated elsewhere concurrently, but that’s a big can of worms)

Aggregate types

Silk’s aggregate types behave similarly to their C counterparts, with a few key differences.

Arraysdo not decay to pointers as in C. They generally behave as a single entity of data, much like structures.

func g(arr *i32) void {}

func f(arr [3]i32) void {}

func main() void {
  val a = { 1, 2, 3 };
  g(a); // this produces a compiler error
  f(a); // this does not
  g(&a[0]); // this does not

  val b = [i32; 3];
  // b is a zero-initialized array of 3 i32s
  f(b);
}

Silk does not support variable-length arrays. The size of every array must be known at compile time and is part of its type.

Structuresare either labeled (every member has a name) or unlabeled . Unlabeled structures can be constructed with struct literals :

func main() void {
  var a = (1, 2, 3);
  // (1, 2, 3) is a struct literal
  // a is of type struct(i32, i32, i32)

  var b = a.0; // its members are accessed by 'index'
  var c = a.1;
  var d = a.2;
}

Labeled structs can be defined as newtypes or constructed ad-hoc:

type my_type = struct(num i32, chr i8, flag bool);

func main() void {
  var beans = my_type(1, 'B', true);
  var b = beans.num;
  // beans has members 'num', 'chr' and 'flag'

  var cake = struct(asdf i32, qwer u32)(12, 12u);
  var x = cake.qwer;
  // cake has members 'asdf' and 'qwer'
}

Structures are aligned by default, but can be packed .

type packed_type = packed(a i32, b i32, c i32);

func main() void {
  val a = (: 1, 2, 3 :);
  // a is a packed unlabeled struct

  val b = packed_type(4, 5, 6);
  // b is a packed labeled struct
}

User-defined types and parametric polymorphism

Types are defined with the type keyword:

type int = i32;

func main() int {
  val a int = 12;
  val b i32 = a;
  return b;
}

Notice that the int type defined in the previous example is not a newtype; it is an alias for i32 .

Unlike other types, structs are unique. To define a newtype, simply use a one-member struct:

type my_i32 = struct(i32);
type other_i32 = struct(i32);

func main() my_i32 {
  val a = 12;
  val b = my_i32(a);
  val c = other_i32(b.0);
  // a, b, c are of distinct types
}

Forward declaring a type allows it to be defined recursively. A singly linked list type could be defined as follows:

type list;
type list = struct(value i32, next mut*list);

func make_list_node(value i32) {
  return list(value, mut*list (0));
}

func main() i32 {
  var head = make_list_node(1);
  var tail = make_list_node(2);
  head.next = &tail;

  return @(head.next).value;
}

Silk’s types and functions can also accept type parameters:

// binary tree with key and value
type bintree:<$key_type, $value_type>;
type bintree:<$kt, $vt> = struct(
  key $kt, value $vt,
  left mut*bintree:<$kt, $vt>,
  right mut*bintree:<$kt, $vt>
);

func make_bintree_node:<$kt, $vt>(key $kt, value $vt) bintree:<$kt, $vt> {
  val null = mut*bintree:<$kt, $vt> (0);
  return bintree:<$kt, $vt>(key, value, null, null);
}

func main() i32 {
  var root = make_bintree_node:<*i8, i32>("hello", 1);
  var child = make_bintree_node:<*i8, i32>("world", 2);
  root.right = &child;
  return @(root.right).value;
}

(This syntax is exceptionally ugly, I’m working on it.)

Other miscellaneous things

Silk will never implicitly cast or coerce types. This is both a design choice and a product of my own laziness.

func main() i32 {
  // a is of type u64 ('u': unsigned, 'l': long i.e 64-bit)
  val a = 125ul;

  // this produces a compiler error since
  // 12 is an i32
  val b = a + 12;

  // these work as expected
  val c = a + 12ul;
  val d = a + u64(12);
}

If every member in a struct literal is an lvalue, the struct literal can be an lvalue. This enables some cool ‘group’ assignment stuff.

func main() i32 {
  var a = 1;
  var b = 2;
  var c = 3;
  val c_ptr = &c;

  // all of the following statements assign
  // a, b, c to 4, 5, 6 respectively
  (a, b, c) = (4, 5, 6);
  (a, (b, c)) = (4, (5, 6));
  (a, b, @c_ptr) = (4, 5, 6);

  // this will swap a and b
  (a, b) = (b, a);

  return a;
}

Build

In order to build Silk, you will need:

• ocaml toolchain
• dune build tool
• menhir parser generator

Then simply acquire the source code and ./build.sh .

This will produce a silk.exe executable in the _build/default/src/ directory.

Usage

In order to use Silk, you will need the LLVM toolchain.

Since I have not written a CLI yet, the silk.exe executable reads Silk from stdin and writes LLVM to stdout. LLVM can be compiled to native object files with the llc tool.

For example:

$ cat hello.silk
extern func printf(s *i8) void;

func main() i32 {
  printf("hello, world\n");
  return 0;
}
$ silk < hello.silk > hello.llvm
$ llc -filetype=obj hello.llvm -o hello.o
$ ld -o hello hello.o -lc
$ ./hello
hello, world

The LLVM output can be compiled directly without writing it to a file:

$ silk < hello.silk | llc -filetype=obj -o hello.o -

Silk does not include a preprocessor or package/module system of any kind. Your favorite C compiler can probably produce preprocessor output:

$ cc -E -x c hello.silk > hello.silk.out
$ # Compile hello.silk.out as described above

TODOs / Roadmap

• Un-spaghetti the code and produce better error messages
  • Possibly by rewriting the whole thing
• Better type parameter syntax
• Cool optimization stuff
• Make a real CLI
• More docs!

License

Silk is distributed under the terms of the MIT License.

Silk is one of mymany projects. If you find this (or any of my work) interesting, please contact me ! I am available for hire .