blog/posts/ats.md

---
title: a little ats program
date: 2022-09-16
tags: [computer, ATS, cool languages]
show-toc: true
...

[ats] is a language that is a little infamous among type system likers, for its
complexity, its weird syntax, and its thin documentation. well i won't be beat,
so here's a little implementation of simply-typed lambda calculus. right now it
doesn't actually use many ats-specific features, since i just didn't end up
needing them.

[ats]: http://www.ats-lang.org

it does leak memory without gc though, so maybe i should have.
oh well one thing at a time

# installation, et cetera

instead of trying to install ats myself, i just used [nix], since i already
have it installed as the least annoying package manager for idris. i just run
`nix shell nixpkgs#ats2` and i'm in a shell with `patscc` and `patsopt`
available. if you want to try building it yourself, that's, uh, your choice.

[nix]: https://nixos.org

to compile the program, i wrote a makefile. just a small one!

```makefile
ATSCC := patscc
ATSFLAGS := \
  -DATS_MEMALLOC_GCBDW -lgc \
  -atsccomp \
    'gcc -I$${PATSHOME} -I$${PATSHOME}/ccomp/runtime \
         -L$${PATSHOME}/ccomp/atslib/lib -std=gnu99'

%: %.dats
	$(ATSCC) $< $(ATSFLAGS) -o $@
```

(in some installs the compiler is just called `atscc` for some reason.)

the first line of `ATSFLAGS` tells it which allocator to use. in this case,
`libgc`. the rest tells it to use `-std=gnu99`, so that it can find `alloca()`;
the rest is the same as the default.

one last thing. the first two lines of the program  have to be this. i didn't
look into it too hard but they seem to define... something... and if you forget
to include them the c output by ats isn't even syntactically correct. ouch.

```ats
#include "share/atspre_define.hats"
#include "share/atspre_staload.hats"
```

# types

let's start by defining the syntax for types. it's going to be a single base
type, and functions.

$$
\newcommand\Typ{\mathit{typ}}
\newcommand\OR{\mathrel|}
\Typ ::= \bullet \OR \Typ_1 \to \Typ_2
$$

so obviously, my first attempt was to just write this:

```ats
datatype typ = Base | Arr of (typ, typ)
```

i'm going to need to check two types for equality, so let's do that next.

```ats
fun eq_typ (a : typ, b : typ) .<a>. :<> bool =
case (a, b) of
| (Base(),      Base())      => true
| (Arr(a1, a2), Arr(b1, b2)) => eq_typ(a1, b1) && eq_typ(a2, b2)
| (_,           _)           => false
```

alas: (it is talking about the `.<a>.`)

```
/home/niss/ats/lam.dats: 149(line=7, offs=33) -- 150(line=7, offs=34):
  error(2): the static identifier [a] is unrecognized.
```

surprise! as far as i can tell ats only allows termination checking to be
based on _static_ (compile time) information. so i need to index the
datatype with, uh, something. when we define functions on syntax like this,
they basically always recurse on a subterm. so the height of the syntax tree
will do. what i _actually_ have are these more messy definitions.

```ats
datatype typ(int) =
|                Base(1)
| {h1, h2 : nat} Arr(1 + max(h1, h2)) of (typ(h1), typ(h2))

typedef typ = [h : nat] typ(h)
```

the base type $\bullet$ has height 1, and an arrow type has a height one more
than the max height of its subterms. `nat` is a version of `int` constrained to
be at least zero, rather than a separate type, so that's why it says `int` one
place and `nat` the other. the typechecker has a linear arithmetic solver for
reasoning about inequalities and expressions like `1 + max(h1, h2)`.

i don't want to be talking about "types of height $h$" unless i really have to,
though, so there's a definition for just _a_ type too. the syntax `[x : t1] t2`
is an existential type that is automatically wrapped and unwrapped, so in this
case it is a `typ(h)` for some, unknown, `h`. also, names can be overloaded by
their arity so `typ(h)` and `typ` can both be defined in the same scope.

so now `eq_typ` has to have a more intricate type to talk about height. the
definition is still the same though.

```ats
fun eq_typ {ha : nat} .<ha>. (a : typ(ha), b : typ) :<> bool =
case (a, b) of
| (Base(),      Base())      => true
| (Arr(a1, a2), Arr(b1, b2)) => eq_typ(a1, b1) && eq_typ(a2, b2)
| (_,           _)           => false

overload = with eq_typ
```

since i only actually need to care about one of the arguments' height for the
termination proof, the other can still just be a `typ`. the last line lets me
write `a = b` instead of `eq_typ(a, b)`, which is nice.

next, i want to be able to print types. ideally this would be a function that
returns a string, but string stuff involves pointer weirdness i haven't worked
out yet. so it just prints. the only two precedence levels in these types are
"left of an arrow" and "not left of an arrow", so that can just be a `bool`.
unfortunately `print` has no information about its termination behaviour so
there's no point bothering with that stuff here either. :confused:

```ats
fun print_typ (parens : bool, t : typ) : void =
case t of
| Base()    => print "•"
| Arr(a, b) => begin
    if parens then print("(");
    print_typ(true, a);
    print(" → ");
    print_typ(false, b);
    if parens then print(")");
  end

fn print_typ (t : typ) : void = print_typ(false, t)
overload print with print_typ
```

# terms

types are done, now let's move on to terms. we just want names, lambdas, and
application.

$$
\newcommand\Term{\mathit{term}}
\newcommand\Var{\mathit{var}}
\Term ::= \Var \OR \lambda \Var : \Typ. \Term \OR \Term_1 \; \Term_2
$$

for the same reason as before, terms are indexed by their height, but _also_ by
the number of variables in scope, since i want de bruijn indices to be
intrinsically well-scoped. why even have a type system otherwise. so a
`term(n, h)` has $n$ variables and height $h$.

```ats
datatype term(int, int) =
| {n, i : nat | i < n} Var(n, 1) of int(i)
| {n, h : nat}         Lam(n, 1 + h) of (typ, term(1 + n, h))
| {n, h1, h2 : nat}    App(n, 1 + max(h1, h2)) of (term(n, h1), term(n, h2))

typedef term(n : int) = [h : nat] term(n, h)
```

a variable is [a de bruijn index][db], which is a natural less than `n`. to
express this we need to bridge the static world, where `n` lives, and the
dynamic, where `Var`'s field lives. so i'm using a singleton type `int(i)`,
whose only value is `i` itself. with this type, you can constrain the static
`i`, and in turn the value of the field.

[db]: https://en.wikipedia.org/wiki/De_Bruijn_index


# contexts

```ats
typedef ctx(n: int) = list(typ, n)
```

a typing context is just a list of types of the appropriate length. since
there's no binding in types here, that's all we need.

looking up into a context is just list lookup, with some constraints to make it
work.

```ats
fun lookup {n, i : nat | i < n} .<n>. (g : ctx(n), i : nat(i)) :<> typ =
let val+ list_cons(x, g) = g in
  if i = 0 then x else lookup(g, i - 1)
end
```

from the constraints $n \ge 0, i \ge 0, i < n$, the compiler can see that, as
$n \ge 1$, the list is nonempty and `list_cons` _is_ the only case. it can also
propagate the $i \ne 0$ condition from the `else` to see the recursive call is
ok too, and this function is total.

:::aside
while writing this post i discovered that this is actually a restatement of
the type of `xs[i]` in the standard library and i could have just used that.
oops!
:::


# typechecker

i'm going to write the type checker using exceptions for error handling, because
using an error monad without `do` notation was just too annoying. but from the
outside it is still going to use a `result` type.

```ats
datatype result(t@ype, t@ype) =
| {a, e : t@ype} Ok(a, e)  of a
| {a, e : t@ype} Err(a, e) of e
```

the funky looking `t@ype` refers to a type of unknown size, as opposed to `type`
which can only be instantiated with pointer-sized types. that will not come up
again here but "`t@ype`" is definitely the funniest part of ats's syntax.

the errors we need here are:

- a function takes an argument of type $A$, but is given one of type $B$;
- a term of a non-arrow type is being applied.

```ats
datatype err =
| Clash of (typ, typ)
| NotArr of typ

fn print_err (e : err) : void = … // incredibly uninteresting
overload print with print_err


typedef typ_res = result(typ, err)

fn print_res (r : typ_res) : void = …
overload print with print_res
```

so onto the typechecker. like i said, it's going to use a local exception for
errors, but then catch it on the outside so that it has a pure type.

```ats
fn typ_of {n : nat} (g : ctx n, t : term(n)) :<> typ_res =
let
  exception ERR of err
  fun go {n, h : nat} .<h>. (g : ctx(n), t : term(n, h)) :<!exn> typ =
  case t of
  | Var(i)    => lookup(g, i)
  | Lam(a, t) => Arr(a, go(list_cons(a, g), t))
  | App(s, t) => begin
      case go(g, s) of
      | Arr(a1, b) =>
          let val a2 = go(g, t) in
            if a1 = a2 then b else $raise ERR(Clash(a1, a2))
          end
      | b => $raise ERR(NotArr(b))
    end
in
  $effmask_exn(try Ok(go(g, t)) with ~ERR(e) => Err(e))
end

fn typ_of (t : term(0)) :<> typ_res = typ_of(list_nil, t)
```

most of this is fine. a bit noisy i guess. but there are a couple of new things:

- in the type of `go`, the `:<!exn>` part means it can throw exceptions, but has
  no other effects.
- `$effmask_exn(e)` hides the `exn` effect in `e`, because ats doesn't
  distinguish different exceptions in effects so it can't tell that `ERR` is the
  only one that is ever raised.
- exceptions are linear, and the pattern `~ERR(e)` is consuming the outermost
  constructor.
- exceptions are the only reason we needed `alloca` in this program
  :woozy_face:


# the end

ok, a quick `main` function, and that's it!

```ats
implement main0() = begin
  print("type of [λx:•. x] is: ");
  print(typ_of(Lam(Base, Var(0))));
  print("\ntype of [λx:•. x x] is: ");
  print(typ_of(Lam(Base, App(Var(0), Var(0)))));
  print("\n")
end
```

and it works, at least on these two examples.

```
type of [λx:•. x] is: • → •
type of [λx:•. x x] is: expected an arrow type, but got [•]
```

uh. thank's for reading. maybe i'll write another one if i think of another
nice little program that actually uses more of ats's weirdness.

--------------------------------------------------------------------------------

<details>
<summary>full code</summary>
```ats
#include "share/atspre_define.hats"
#include "share/atspre_staload.hats"

datatype typ(int) =
|                Base(1)
| {h1, h2 : nat} Arr(1 + max(h1, h2)) of (typ(h1), typ(h2))

typedef typ = [h : nat] typ(h)

fun eq_typ {ha : nat} .<ha>. (a : typ(ha), b : typ) :<> bool =
case (a, b) of
| (Base(),      Base())      => true
| (Arr(a1, a2), Arr(b1, b2)) => eq_typ(a1, b1) && eq_typ(a2, b2)
| (_,           _)           => false
overload = with eq_typ

fun print_typ (parens : bool, t : typ) : void =
case t of
| Base()    => print "•"
| Arr(a, b) => begin
    if parens then print("(");
    print_typ(true, a);
    print(" → ");
    print_typ(false, b);
    if parens then print(")");
  end

fn print_typ (t : typ) : void = print_typ(false, t)
overload print with print_typ


datatype term(int, int) =
| {n, i : nat | i < n} Var(n, 1) of int(i)
| {n, h : nat}         Lam(n, 1 + h) of (typ, term(1 + n, h))
| {n, h1, h2 : nat}    App(n, 1 + max(h1, h2)) of (term(n, h1), term(n, h2))

typedef term(n : int) = [h : nat] term(n, h)


typedef ctx(n: int) = list(typ, n)

fun lookup {n, i : nat | i < n} .<n>. (g : ctx(n), i : int(i)) :<> typ =
let val+ list_cons(x, g) = g in
  if i = 0 then x else lookup(g, i - 1)
end


datatype result(t@ype, t@ype) =
| {a, e : t@ype} Ok(a, e)  of a
| {a, e : t@ype} Err(a, e) of e


datatype err =
| Clash of (typ, typ)
| NotArr of typ

fn print_err (e : err) : void =
case e of
| Clash(a, b) => begin
    print("clash between types [");
    print(a);
    print("] and [");
    print(b);
    print("]");
  end
| NotArr(t) => begin
    print("expected an arrow type, but got [");
    print(t);
    print("]");
  end
overload print with print_err


typedef typ_res = result(typ, err)

fn print_res (r : typ_res) : void =
case r of Ok(t) => print(t) | Err(e) => print(e)
overload print with print_res


fn typ_of {n : nat} (g : ctx n, t : term(n)) :<> typ_res =
let
  exception ERR of err
  fun go {n, h : nat} .<h>. (g : ctx(n), t : term(n, h)) :<!exn> typ =
  case t of
  | Var(i)    => lookup(g, i)
  | Lam(a, t) => Arr(a, go(list_cons(a, g), t))
  | App(s, t) => begin
      case go(g, s) of
      | Arr(a1, b) =>
          let val a2 = go(g, t) in
            if a1 = a2 then b else $raise ERR(Clash(a1, a2))
          end
      | b => $raise ERR(NotArr(b))
    end
in
  $effmask_exn(try Ok(go(g, t)) with ~ERR(e) => Err(e))
end

fn typ_of (t : term(0)) :<> typ_res = typ_of(list_nil, t)


implement main0() = begin
  print("type of [λx:•. x] is: ");
  print(typ_of(Lam(Base, Var(0))));
  print("\ntype of [λx:•. x x] is: ");
  print(typ_of(Lam(Base, App(Var(0), Var(0)))));
  print("\n")
end
```
</details>