[Haskell-cafe] Bad recursion? When can something be on both sides of an equation?

[Jeffrey Brown]

That makes sense! I did not realize the shadowing a where clauses causes
happens within the where clause itself; I thought it only applied outside.
Thanks, Tom!

On Thu, Mar 12, 2015 at 2:46 AM, Tom Ellis <
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:

> On Thu, Mar 12, 2015 at 12:49:57AM -0700, Jeffrey Brown wrote:
> >     totalDescendents :: S.Set Glab -> Graph -> S.Set Glab
> >     totalDescendents roots graph = b
> >       where (a,b,c,graph) = totalDescendentsCtrlr  -- tricky
> >                             (roots, S.empty, M.empty, graph)
> >
> >     totalDescendentsCtrlr :: TotalDescendentsData -> TotalDescendentsData
> >     totalDescendentsCtrlr (a,b,c,gr) =
> >       if S.null a' -- fails, as does a == a'
> >         -- a == a' is one iteration slower but should have same effect
> >         then                       (a',b',c',gr')
> >         else totalDescendentsCtrlr (a',b',c',gr')
> >       where (a',b',c',gr') = visitUndetPrds $ visitTdSnvSucs (a,b,c,gr)
> >
> > Notice that in the equation marked "tricky", the object "graph" appears
> on
> > both sides. I thought that was the problem, so I rewrote that line as the
> > following:
> >       where (_,b,_,_) = totalDescendentsCtrlr
> > and sure enough, the problem vanished.
>
> It's somewhat bizarre to write
>
>        where (a,b,c,graph) = totalDescendentsCtrlr  -- tricky
>                              (roots, S.empty, M.empty, graph)
>
> if
>
>        where (_,b,_,_) = totalDescendentsCtrlr
>
> works, but it's a fair question from a desire to understand what's going
> on.
>
> When you write
>
>       where (a,b,c,graph) = totalDescendentsCtrlr  -- tricky
>                              (roots, S.empty, M.empty, graph)
>
> you are introducing a new name `graph` which shadows the old one.  The
> result `graph` is "fed back in" as the input.  You are saying that the
> value
> of `graph` depends on itself, which in general can lead to looping.  You
> probably meant
>
>       where (a,b,c,graph') = totalDescendentsCtrlr (roots, S.empty,
> M.empty, graph)
>
> or, since many of those variables are unused, as you said
>
>       where (_,b,_,_) = totalDescendentsCtrlr (roots, S.empty, M.empty,
> graph)
>
> > So I tried to distill that problem to something tiny, and came up with
> this:
> >
> >   f (a) = a'
> >     where (a',b) = fBackend (a,b) -- tricky
> >
> >   fBackend (a,b) =
> >     if a' < 1
> >       then          (a',b)
> >       else fBackend (a',b)
> >     where a' = a - 1
> >
> > I see no substantive difference between the problem with the first body
> of
> > code and the problem I expected f and fBackend to have -- but f and
> > fBackend work fine!
>
> `fBackend` never does anything with the value of `b` whereas presumably
> `visitUndetPrds` and `visitTdSnvSucs` do do something with the value of
> `graph`.
>
> Tom
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