# Lower bound viewtype errors

**URL:** <https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236>\
**Category:** Google Group\
**Created:** [March 13, 2013, 7:01pm UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236 "2013-03-13T19:01:00Z")\
**Posts on this page:** 12\
**Page:** 1

<div class="post-metadata">

**Author:** ![Brandon\_Barker](https://sea1.discourse-cdn.com/flex019/user_avatar/discourse.ats-lang.org/brandon_barker/32/61_2.png) [@Brandon\_Barker](https://discourse.ats-lang.org/u/Brandon_Barker)\
**Post date:** [March 13, 2013, 7:01pm UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/1 "2013-03-13T19:01:00Z")

</div>

I think I’ve come across this error before, but I’m not sure or don’t  
recall what it means (error line in yellow):

/media/RAID5/share/ATS\_learning/minDisj.dats: 23248(line=817, offs=24) –  
23255(line=817, offs=31): error(3): sort mismatch: the sort of [s2V(3163)]  
is [t@ype], but the sort of its lower bound is [viewtype].  
/media/RAID5/share/ATS\_learning/minDisj.dats: 23248(line=817, offs=24) –  
23255(line=817, offs=31): error(3): mismatch of static terms (tyleq):

implement  
minConj(gr, emap): GREXP = let  
macdef d2g(gr) = let  
val rgr = case+ ,(gr) of  
| GRgenes(gs) =\> GRgenes(gs)  
| GRconj(gs) =\> GRgenes(gs)  
| GRdisj(gs) =\> let  
val dsz = int\_of\_size(genes\_size(gs))  
val rgr1 = if 1 = dsz then GRgenes(gs)  
else ($raise InvalidCase; GRdisj(gs))  
in rgr1 end  
| X =\> X  
in rgr end  
in (case+ gr of  
| ~GRconj(ex1,ex2) =\> let  
val (ex1,ex2) = (d2g(ex1),d2g(ex2))  
in (case+ (ex1,ex2) of  
| (~GRgenes(s1), ~GRgenes(s2)) =\> minConj(GRconj (s1+s2),emap)  
| (~GRconj(ex11, ex12), ex2) =\>  
minConj(GRconj(minConj(GRconj(ex11, ex12),emap),ex2),emap)  
| (ex1, ~GRconj(ex21, ex22)) =\>  
minConj(GRconj(ex1, minConj(GRconj(ex21, ex22),emap)),emap)  
| (LL,RR) =\> GRconj(LL,RR)  
):GREXP end  
| ~GRconj(gs) =\> let  
val glist = genes\_listize(gs)  
val mingene = list\_min(glist, emap)  
val \_ = genes\_free(gs)  
in GRconj(genes\_make\_sing(mingene)) end  
| X =\> X  
): GREXP  
end // end of [minConj]

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<div class="post-metadata">

**Author:** ![Brandon\_Barker](https://sea1.discourse-cdn.com/flex019/user_avatar/discourse.ats-lang.org/brandon_barker/32/61_2.png) [@Brandon\_Barker](https://discourse.ats-lang.org/u/Brandon_Barker)\
**Post date:** [March 13, 2013, 11:43pm UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/2 "2013-03-13T23:43:48Z")

</div>

First, I do see what you are saying about this getting a little out of  
hand. Sorry for being obtuse … I know you mentioned a reference-counted  
absviewtype previously; are there any relevant examples (assuming I can do  
this without a drastic rewrite of the code)?

I know I said I was basically done with this, it is just that minor changes  
and bugs keep popping up (as they always seem to).

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<div class="post-metadata">

**Author:** ![Brandon\_Barker](https://sea1.discourse-cdn.com/flex019/user_avatar/discourse.ats-lang.org/brandon_barker/32/61_2.png) [@Brandon\_Barker](https://discourse.ats-lang.org/u/Brandon_Barker)\
**Post date:** [March 13, 2013, 10:07pm UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/3 "2013-03-13T22:07:51Z")

</div>

OK, that issue is solved, I’ll try and be more careful with case/ifs.

I am wondering the best way to tackle this problem from the same code:  
extern  
fun print\_pretty\_grexp (grexp: !GREXP): void

implement  
minConj(gr, emap): GREXP = let  
fun d2g(gr:GREXP): GREXP = let  
val rgr = (case+ gr of  
| ~GRgenes(gs) =\> GRgenes(gs)  
| ~GRconj(gs) =\> GRgenes(gs)  
| ~GRdisj(gs) =\> let  
val dsz = int\_of\_size(genes\_size(gs))  
val rgr1 = (if 1 = dsz then GRgenes(gs)  
else (print\_pretty\_grexp(GRdisj(gs));  
$raise InvalidCase; GRdisj(gs))): GREXP  
in rgr1 end  
| X =\> X): GREXP  
in rgr end

Is it to just create a val binding for GRdisj(gs)?

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<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 14, 2013, 1:08am UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/4 "2013-03-14T01:08:50Z")

</div>

I have to say that I don’t see an easy way to fix your code.

Maybe turning GREXP into a datatype is the best way to go.

A very typical scenario in practice can be described as follows.  
A programmer often picks a data representation at the beginning when he  
knows very little about what he really needs. He uses the representation  
and starts to find problems with it. However, he has no easy way to change  
the data representation as doing so would require a lot of code to be  
re-written.  
So he bites the bullet and spends more and more of his time on debugging.

Properly using abstract types in ATS is a highly effective way to fight the  
above issue.

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<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 14, 2013, 12:54am UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/5 "2013-03-14T00:54:52Z")

</div>

Using datatype for GREXP makes a lot sense. But why did you use  
dataviewtype for GREXP? Because you did not want to have GC running?

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<div class="post-metadata">

**Author:** ![Brandon\_Barker](https://sea1.discourse-cdn.com/flex019/user_avatar/discourse.ats-lang.org/brandon_barker/32/61_2.png) [@Brandon\_Barker](https://discourse.ats-lang.org/u/Brandon_Barker)\
**Post date:** [March 14, 2013, 1:23am UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/6 "2013-03-14T01:23:20Z")

</div>

For the sake of completeness, the code was easy to fix using a binding as I  
guessed, but it does make things look far more complex:

```
  | ~GRdisj(gs) => let
      val dsz = int_of_size(genes_size(gs))
      val rgr1 = (if 1 = dsz then GRgenes(gs)
        else let
          val gr = GRdisj(gs)
        in (print_pretty_grexp(gr);
          $raise InvalidCase; gr)
        end): GREXP
    in rgr1 end
  | X => X): GREXP
```

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<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 13, 2013, 8:52pm UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/7 "2013-03-13T20:52:13Z")

</div>

In general, a case-expression or if-expression should be annotated with a  
type.

Unfortunately, a macdef in ATS cannot involve types (but it can in ATS2).  
So I suggest that you turn d2g into a function.

I suggest that you use macros only in cases where using functions would be  
inconvenient. As gcc (with -O2) does inlining  
pretty aggressively, using macros so as to gain efficiency is rarely needed  
if at all.

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<div class="post-metadata">

**Author:** ![Brandon\_Barker](https://sea1.discourse-cdn.com/flex019/user_avatar/discourse.ats-lang.org/brandon_barker/32/61_2.png) [@Brandon\_Barker](https://discourse.ats-lang.org/u/Brandon_Barker)\
**Post date:** [March 14, 2013, 12:58am UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/8 "2013-03-14T00:58:06Z")

</div>

Honestly, I did it because I found out about linset and linmap but did not  
know about funset and funmap, so I thought I had to have a dataviewtype.

As for exceptions, I’m just using them to track bugs right now as unhandled  
exceptions - probably not the best style (thanks for pointing out that  
memory leak complication though).

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<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 14, 2013, 1:13am UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/9 "2013-03-14T01:13:00Z")

</div>

I see. Then switching GREXP to a datatype should be the right way to go.  
I first thought that you just wanted to learn how to use dataviewtype.  
Anyway,  
you learned your lesson 🙂

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<div class="post-metadata">

**Author:** ![Brandon\_Barker](https://sea1.discourse-cdn.com/flex019/user_avatar/discourse.ats-lang.org/brandon_barker/32/61_2.png) [@Brandon\_Barker](https://discourse.ats-lang.org/u/Brandon_Barker)\
**Post date:** [March 13, 2013, 11:50pm UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/10 "2013-03-13T23:50:14Z")

</div>

I suppose one thing I could do is to change my sets/maps to be  
funsets/funmaps, but I’d still be working with a datatype (I guess this  
particular problem would go away in that case).

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<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 13, 2013, 11:34pm UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/11 "2013-03-13T23:34:32Z")

</div>

I really think that this is an example that should persuade you not to make  
GREXP a dataviewtype.  
Pattern matching is supposed to help you write clean code. If you have to  
fight the typechecker all the time,  
then I think it may be time to change GREXP into an abstract viewtype.

By the way, linear types and exceptions usually don’t mix. Unless you have  
strong justification, I would  
suggest avoiding such a programming style.

–Hongwei

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<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 14, 2013, 1:31am UTC](https://discourse.ats-lang.org/t/lower-bound-viewtype-errors/236/12 "2013-03-14T01:31:36Z")

</div>

Yes, that is the right fix.

You have many lines like

case+ x of  
| ~GRgenes (genes) =\> GRgenes (genes)

This means that you free a node and then create another node that is just  
the same as the one freed.

A more efficient way to do this would be

case+ x of  
| GRgenes \_ =\> (fold@ (…); x) // fold@ is compiled into a no-op.