# Tail Calls

**URL:** <https://discourse.ats-lang.org/t/tail-calls/260>\
**Category:** Google Group\
**Created:** [March 21, 2014, 12:55am UTC](https://discourse.ats-lang.org/t/tail-calls/260 "2014-03-21T00:55:45Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Shahab\_Tasharrofi](https://avatars.discourse-cdn.com/v4/letter/s/a88e57/32.png) [@Shahab\_Tasharrofi](https://discourse.ats-lang.org/u/Shahab_Tasharrofi)\
**Post date:** [March 21, 2014, 12:55am UTC](https://discourse.ats-lang.org/t/tail-calls/260/1 "2014-03-21T00:55:45Z")

</div>

Hi there,

Do all the possible tail calls in ATS translate to jumps? For example,  
consider the Ackerman  
function below:

fun ack {n:int | n \>= 0} {m:int | m \>= 0} (x:int(n), y:int(m)) : [k:int | k

> = 0] int(k) =  
> if (x \> 0) then  
> if (y \> 0) then  
> ack(x - 1, ack(x, y - 1))  
> else  
> ack(x - 1, 1)  
> else  
> y + 1

Ideally, I would like this to translate to a C code as below and my first  
question is if this is  
indeed the case:

int ack(int x, int y)  
{  
while (x \> 0)  
{  
y = ack(x, y - 1);  
x = x - 1;  
}  
return y + 1;  
}

Also, another somehow related question is how to read the C file generated  
by ATS. I tried  
to answer my own question above by looking at the C code that ATS generates  
but I could  
not understand anything from it. So, my second question is if there is a  
way to view  
ATS-generated code in a more human-friendly manner?

Thanks,  
Shahab

---

<div class="post-metadata">

**Author:** ![gmhwxi](https://avatars.discourse-cdn.com/v4/letter/g/67e7ee/32.png) [@gmhwxi](https://discourse.ats-lang.org/u/gmhwxi)\
**Post date:** [March 21, 2014, 1:14am UTC](https://discourse.ats-lang.org/t/tail-calls/260/2 "2014-03-21T01:14:05Z")

</div>

Yes, all the tail-calls are translated into jumps. For instance,  
the first tail-call in the body of ack translates into the following  
sequence:

ATSINSmove(tmp4, ack\_0(arg0, tmp5)) ;  
ATStailcalbeg()  
ATSINSmove\_tlcal(argx0, tmp3) ;  
ATSINSmove\_tlcal(argx1, tmp4) ;  
ATSINSargmove\_tlcal(arg0, argx0) ;  
ATSINSargmove\_tlcal(arg1, argx1) ;  
ATSgoto(\_\_patsflab\_ack\_0) ;  
ATStailcalend()

The second one translates to the following:

ATStailcalbeg()  
ATSINSmove\_tlcal(argx0, tmp6) ;  
ATSINSmove\_tlcal(argx1, ATSPMVi0nt(1)) ;  
ATSINSargmove\_tlcal(arg0, argx0) ;  
ATSINSargmove\_tlcal(arg1, argx1) ;  
ATSgoto(\_\_patsflab\_ack\_0) ;  
ATStailcalend()

Currently, there is no documentation on the C code generated from ATS  
source.  
This is something I intend to be working on in the near future. Hopefully,  
I can still  
remember what I did 🙂

The C code is hard to read largely because of the very heavy use of  
templates in  
the code.On Thursday, March 20, 2014 8:55:45 PM UTC-4, Shahab Tasharrofi wrote:

> Hi there,
> 
> Do all the possible tail calls in ATS translate to jumps? For example,  
> consider the Ackerman  
> function below:
> 
> fun ack {n:int | n \>= 0} {m:int | m \>= 0} (x:int(n), y:int(m)) : [k:int |k
> 
> > = 0] int(k) =  
> > if (x \> 0) then  
> > if (y \> 0) then  
> > ack(x - 1, ack(x, y - 1))  
> > else  
> > ack(x - 1, 1)  
> > else  
> > y + 1
> 
> Ideally, I would like this to translate to a C code as below and my first  
> question is if this is  
> indeed the case:
> 
> int ack(int x, int y)  
> {  
> while (x \> 0)  
> {  
> y = ack(x, y - 1);  
> x = x - 1;  
> }  
> return y + 1;  
> }
> 
> Also, another somehow related question is how to read the C file generated  
> by ATS. I tried  
> to answer my own question above by looking at the C code that ATS  
> generates but I could  
> not understand anything from it. So, my second question is if there is a  
> way to view  
> ATS-generated code in a more human-friendly manner?
> 
> Thanks,  
> Shahab

---

<div class="post-metadata">

**Author:** ![Shahab\_Tasharrofi](https://avatars.discourse-cdn.com/v4/letter/s/a88e57/32.png) [@Shahab\_Tasharrofi](https://discourse.ats-lang.org/u/Shahab_Tasharrofi)\
**Post date:** [March 21, 2014, 1:28am UTC](https://discourse.ats-lang.org/t/tail-calls/260/3 "2014-03-21T01:28:26Z")

</div>

Ok, great 🙂 It would be very nice to see the translation of a piece of  
code in readable C.  
It would help tremendously with writing the code.

So, I assume that tail calls would also be translated into C even if they  
are not the same  
function. That is, if the last part of computing function “f” is to call a  
different function “g”,  
then it is translated into a jump as well. Am I right?

---

<div class="post-metadata">

**Author:** ![gmhwxi](https://avatars.discourse-cdn.com/v4/letter/g/67e7ee/32.png) [@gmhwxi](https://discourse.ats-lang.org/u/gmhwxi)\
**Post date:** [March 21, 2014, 1:35am UTC](https://discourse.ats-lang.org/t/tail-calls/260/4 "2014-03-21T01:35:53Z")

</div>

No. If it is a different function, then it cannot be translated into a jump  
in general as it is not clear where is the location to jump to.

You can use the keyword ‘fnx’ to combine f and g; then the compiler is  
able to turn mutually tail-recursive calls into jumps. See:

[http://www.ats-lang.org/EXAMPLE/EFFECTIVATS/loop-as-tailrec/main.htmlOn](http://www.ats-lang.org/EXAMPLE/EFFECTIVATS/loop-as-tailrec/main.htmlOn) Thursday, March 20, 2014 9:28:26 PM UTC-4, Shahab Tasharrofi wrote:

> Ok, great 🙂 It would be very nice to see the translation of a piece of  
> code in readable C.  
> It would help tremendously with writing the code.
> 
> So, I assume that tail calls would also be translated into C even if they  
> are not the same  
> function. That is, if the last part of computing function “f” is to call a  
> different function “g”,  
> then it is translated into a jump as well. Am I right?