The document summarizes a team meeting discussion about using the ATS programming language. Key points include: - ATS is a safer systems programming language than C due to features like dependent and linear types that can enforce invariants. - Linear types in ATS manage memory like resources to help prevent bugs. - Examples are provided showing how ATS type checking detects errors like using freed variables or accessing out of bounds indices. - An overview of linear list functions in ATS like make_pair, append, get_at, reverse, and free is given.

1. Metasepi team meeting #17:
Invariant captured by ATS's API
Kiwamu Okabe @ Metasepi Project

2. Who am I?
☆ http://www.masterq.net/
☆ Self employed software engineer
☆ Trade name := METASEPI DESIGN
☆ Founder of Metasepi Project
☆ A Debian Maintainer
☆ 10 years' experience in developing
OS using NetBSD

3. Remember Heartbleed bug?
Should we use safer language than C?
== In English ==
"Preventing heartbleed bugs with safe programming languages"
http://bluishcoder.co.nz/2014/04/11/preventing-heartbleed-bugs-with-
safe-languages.html
== In Japanease ==
"安全なプログラミング言語を使って heartbleed を防ぐには"
https://github.com/jats-ug/translate/blob/master/Web/
bluishcoder.co.nz/2014/04/11/preventing-heartbleed-bugs-with-safe-languages.
md
"A safer systems programming language
could have prevented the bug."

4. Want the safer language...
It's the ATS http://www.ats-lang.org/ !
☆ Syntax like ML
☆ Dependent types
☆ Linear types
☆ Without any runtime
☆ Optional GC

5. ATS code can run on 8-bit AVR
hhhttttttpppsss::://////gggiiittthhhuuubbb...cccooommm///fffpppiiiooottt///aaarrrddduuuiiinnnooo---mmmeeegggaaa222555666000---aaatttsss

6. ATS compile flow

7. Why ATS language is safe?
☆ Line is at between caller and callee
☆ ATS applies type to the line
☆ Type can enforce invariant in them

8. {Dependent,Linear} type
Dependent type
☆ such like Coq or Agda
☆ has universal quantification
☆ has existential quantification
Linear type
☆ based on linear logic
☆ manages resource like memory

9. Usage of Linear List
$ vi sample_list.dats
#include "share/atspre_staload.hats"
implement main0 () = {
val l1 = list_vt_make_pair<int> (1, 2)
val l2 = list_vt_make_pair<int> (3, 4)
val l3 = list_vt_append (l1, l2)
val () = println! ("l3[3] := ", l3[3])
val l4 = list_vt_reverse l3
val () = println! ("l4 := [", l4, "]")
val () = free l4
}
$ patscc -DATS_MEMALLOC_LIBC -o sample_list sample_list.dats
$ ./sample_list
l3[3] := 4
l4 := [4, 3, 2, 1]
$ size sample_list
text data bss dec hex filename
7908 772 32 8712 2208 sample_list
$ ldd sample_list | wc -l
3
$ nm sample_list| grep "U "| wc -l
10

10. Compile error: without free
$ vi sample_list.dats
#include "share/atspre_staload.hats"
implement main0 () = {
val l1 = list_vt_make_pair<int> (1, 2)
val l2 = list_vt_make_pair<int> (3, 4)
val l3 = list_vt_append (l1, l2)
val () = println! ("l3[3] := ", l3[3])
val l4 = list_vt_reverse l3
val () = println! ("l4 := [", l4, "]")
// val () = free l4 // <= Changed
}
$ patscc -DATS_MEMALLOC_LIBC -o sample_list sample_list.dats
--snip--
The 2nd translation (binding) of [sample_list.dats] is
successfully completed!
/home/kiwamu/tmp/sample_list.dats: 59(line=2, offs=22) -- 312
(line=10, offs=2): error(3): the linear dynamic variable [l4$3450
(-1)] needs to be consumed but it is preserved with the type
[S2Eapp(S2Ecst(list_vt0ype_int_vtype); S2Einvar(S2EVar(4104)),
S2EVar(4105))] instead.

11. Compile error: use freed name
$ vi sample_list.dats
#include "share/atspre_staload.hats"
implement main0 () = {
val l1 = list_vt_make_pair<int> (1, 2)
val l2 = list_vt_make_pair<int> (3, 4)
val l3 = list_vt_append (l1, l2)
val l4 = list_vt_reverse l3
val () = println! ("l3[3] := ", l3[3]) // <= Changed
val () = println! ("l4 := [", l4, "]")
val () = free l4
}
$ patscc -DATS_MEMALLOC_LIBC -o sample_list sample_list.dats
--snip--
The 2nd translation (binding) of [sample_list.dats] is
successfully completed!
/home/kiwamu/tmp/sample_list.dats: 242(line=7, offs=35) -- 245
(line=7, offs=38): error(3): the linear dynamic variable [l3$3449
(-1)] is no longer available.

12. Compile error: out of range
$ vi sample_list.dats
#include "share/atspre_staload.hats"
implement main0 () = {
val l1 = list_vt_make_pair<int> (1, 2)
val l2 = list_vt_make_pair<int> (3, 4)
val l3 = list_vt_append (l1, l2)
val () = println! ("l3[4] := ", l3[4]) // <= Changed
val l4 = list_vt_reverse l3
val () = println! ("l4 := [", l4, "]")
val () = free l4
}
$ patscc -DATS_MEMALLOC_LIBC -o sample_list sample_list.dats
--snip--
The 2nd translation (binding) of [sample_list.dats] is
successfully completed!
/home/kiwamu/tmp/sample_list.dats: 215(line=6, offs=38) -- 216
(line=6, offs=39): error(3): unsolved constraint: C3NSTRprop(main;
S2Eapp(S2Ecst(<); S2EVar(4101->S2Eintinf(4)), S2Eapp(S2Ecst
(add_int_int); S2Eintinf(2), S2Eintinf(2))))
typechecking has failed: there are some unsolved constraints:
please inspect the above reported error message(s) for information.

13. Typechecked: with assert
$ vi sample_list.dats
#include "share/atspre_staload.hats"
implement main0 () = {
val l1 = list_vt_make_pair<int> (1, 2)
val l2 = list_vt_make_pair<int> (3, 4)
val l3 = list_vt_append (l1, l2)
val () = if length l3 > 4 // <= Changed
then println! ("l3[4] := ", l3[4]) // <= Changed
val l4 = list_vt_reverse l3
val () = println! ("l4 := [", l4, "]")
val () = free l4
}
$ patscc -DATS_MEMALLOC_LIBC -o sample_list sample_list.dats
--snip--
The 1st translation (fixity) of [sample_list.dats] is successfully
completed!
The 2nd translation (binding) of [sample_list.dats] is
successfully completed!
The 3rd translation (type-checking) of [sample_list.dats] is
successfully completed!
The 4th translation (type/proof-erasing) of [sample_list.dats] is
successfully completed!

14. Type of Linear List
(* File: prelude/basics_dyn.sats *)
datavtype
list_vt0ype_int_vtype (a:vt@ype+, int) =
| {n:int | n >= 0}
list_vt_cons (a, n+1) of (a, list_vt0ype_int_vtype (a, n))
| list_vt_nil (a, 0) of ()
stadef list_vt = list_vt0ype_int_vtype

15. make_pair
(* File: prelude/SATS/list_vt.sats *)
fun{x:vt0p}
list_vt_make_pair (x1: x, x2: x):<!wrt> list_vt (x, 2)

16. append
(* File: prelude/SATS/list_vt.sats *)
fun{
a:vt0p
} list_vt_append
{n1,n2:int} (
xs1: list_vt (INV(a), n1), xs2: list_vt (a, n2)
) :<!wrt> list_vt (a, n1+n2)

17. []
(* File: prelude/SATS/list_vt.sats *)
fun{x:t0p}
list_vt_get_at{n:int}
(xs: !list_vt (INV(x), n), i: natLt n):<> x
overload [] with list_vt_get_at
(* File: prelude/basics_sta.sats *)
typedef g1intBtw
(tk:tk, lb:int, ub:int) = [i: int | lb <= i; i < ub] g1int (tk, i)
typedef intBtw (lb:int, ub:int) = g1intBtw (int_kind, lb, ub)
typedef natLt (n:int) = intBtw (0, n)

18. reverse
(* File: prelude/SATS/list_vt.sats *)
fun{x:vt0p}
list_vt_reverse{n:int}
(xs: list_vt (INV(x), n)):<!wrt> list_vt (x, n)

19. free
(* File: prelude/basics_dyn.sats *)
vtypedef
List_vt (a:vt0p) = [n:int] list_vt (a, n)
(* File: prelude/SATS/list_vt.sats *)
fun{x:t0p}
list_vt_free (xs: List_vt (INV(x))):<!wrt> void
overload free with list_vt_free

20. Japan ATS User Group
http://jats-ug.metasepi.org/
☆ In a parody of http://jaws-ug.jp/
☆ Push the Facebook like button, now!
☆ We translate ATS docs into Japanese

21. "ATSプログラミング入門"
hhhttttttppp::://////jjjaaatttsss---uuuggg...mmmeeetttaaassseeepppiii...ooorrrggg///dddoooccc///AAATTTSSS222///IIINNNTTT222PPPRRROOOGGGIIINNNAAATTTSSS///iiinnndddeeexxx...hhhtttmmmlll

22. "ATS公式Wikiの日本語訳"
hhhttttttpppsss::://////gggiiittthhhuuubbb...cccooommm///jjjaaatttsss---uuuggg///AAATTTSSS---PPPooossstttiiiaaatttsss---wwwiiikkkiii

23. "MLプログラマ向けATS言語ガイド"
https://github.com/jats-ug/translate/blob/master/Web/cs.likai.org/
ats/ml-programmers-guide-to-ats.md

24. Paper "Applied Type System"
https://github.com/jats-ug/translate/raw/master/Paper/ATS-types03/
ATS-types03-ja.pdf

25. Follow me!
☆ https://twitter.com/jats_ug
☆ https://www.facebook.com/jatsug

26. License of photos used
* Creative Commons BBB | Flickr - Photo Sharing!
https://www.flickr.com/photos/steren/2732488224
Copyright: 2008 Steren Giannini / License: CC BY 2.0
* le coq / o galo de Barcelos | Flickr - Photo Sharing!
https://www.flickr.com/photos/guymoll/311768037
Copyright: 2006 Guy MOLL / License: CC BY 2.0
* news_twitter_facebook | Flickr - Photo Sharing!
https://www.flickr.com/photos/lioman/4324501845
Copyright: 2010 lioman123 / License: CC BY-SA 2.0
* Jordan shooting Jenna with shield | Flickr - Photo Sharing!
https://www.flickr.com/photos/jasoneppink/80772834
Copyright: 2005 Jason Eppink / License: CC BY 2.0
* Marsh Plaza, Boston University | Flickr - Photo Sharing!
https://www.flickr.com/photos/hankzby/14122296866
Copyright: 2014 Henry Zbyszynski / License: CC BY 2.0
* Love That Binds | Flickr - Photo Sharing!
https://www.flickr.com/photos/enerva/12525888074
Copyright: 2014 Sonny Abesamis / License: CC BY 2.0