Function Template not working : segfaults.

[Chotu S]

I have not programmed in ATS for a while , so I was trying to review ATS and tried to implement Maybe monad from this  discussion thread : 

https://groups.google.com/forum/#!searchin/ats-lang-users/monad/ats-lang-users/gGOqTJwiegE/K8ieRTSodDEJ

#include "share/atspre_define.hats"

#include "share/atspre_staload.hats"

datatype Maybe (a : t@ype) =

  | Nothing (a) of ()

  | Just    (a) of (a)

  

datatype Expr = 

  | Val of Int 

  | Div of (Expr , Expr)

(* from UPENN haskell example *)

(* ok  = Div (Div (Val 1972) (Val 2)) (Val 23) *)

val ok  = Div (Div ( Val (1972) , Val (2) ) ,    Val (23))

(* val err = Div (Val 2) (Div (Val 1) (Div (Val 2) (Val 3))) *)

val err =   Div (Val (2) , Div (Val (1) , Div (Val (2) ,  Val (3))))

abst@ype M (t@ype) = ptr 

 

extern

fun {a : t@ype}

ret (v : a ) : M(a)

 

extern

fun{a,b : t@ype}

bind : M(a)  -> M(b)

 

extern

fun{a,b:t@ype}

bind$cont (x : a) : M(b)

assume M (a : t@ype) = Maybe (a)

extern

fun div (m : int , n : int) : Maybe(int) 

implement{a}

ret (x) = Just (x)

implement{a,b}

bind (ma) = 

  case ma of

    | Nothing _ => Nothing ()

    | Just (x) => bind$cont<a,b> (x)

fun eval (e : Expr) : Maybe(int) =

  case e of

    | Val n => ret (n)

    | Div (e1 , e2) => 

        bind<int,int>(eval (e1)) where {

          implement bind$cont<int,int> (m) = 

            bind<int,int>(eval (e2)) where {

              implement bind$cont<int,int> (n) = div (m,n) }}

              

implement              

div (m , n) =

  if n = 0 

     then Nothing ()

     else ret (m / n)

fun print_maybe (m : Maybe (int)) : void = 

  case m of

    | Nothing _ => println! ("Nothing")

    | Just (x)  => println! (x)

implement main0 () = () where {

  val res1 = eval (ok)

  val res2 = eval (err)

  val _ = print_maybe (res1)

  val _ = print_maybe (res2)

}

  

When run this segfaults , and I think it is because it does not see second implementation of bind$cont and keep calling the first one which leads to stack overflow .

Does not second implementation override the first one ?

[gmhwxi]

Your assessment of the situation sounds correct to me. However, I don't see an easy way to get out of the
situation.

See:
https://groups.google.com/forum/#!searchin/ats-lang-users/monad/ats-lang-users/gGOqTJwiegE/K8ieRTSodDEJ

Why not just implement 'bind' as a higher order function (as in Haskell)?

[Shea Levy]

By the way, for a better error message in a case like this you may want to try something like https://github.com/NixOS/nix/blob/master/src/libmain/stack.cc#L17-L38

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[gmhwxi]

Here is some explanation:

When the first bind<int,int> is being compiled, 'bind<int,int>' is stored on some
kind of stack. When the second bind<int,int> is countered, the one on the stack is still
there; so the second bind<int,int> and the first bind<int,int> are actually compiled into
two calls to the same function (instead of each call to a different function)

On Wednesday, April 1, 2015 at 11:39:20 PM UTC-4, Chotu S wrote: