[Haskell-cafe] What causes <<loop>>?

Janis Voigtlaender voigt at tcs.inf.tu-dresden.de
Wed Dec 3 04:30:17 EST 2008

Martin Hofmann wrote:
> I've already posted this mail on haskell-cafe, but apparently the
> subject suggested a too simple question, so I try it here again. I am
> picking up a discussion with the same topic from haskell-users on
> 8th November. 

Note that you have been sending to haskell-cafe again. Your recipient
name says haskell-beginners, but the address is haskell-cafe.

Anyway, <<loop>> really means a loop in evalutation order, not some
statebased deadlock (see below).

> Thunks with reference on themselves was mentioned as main reason for
> <<loop>>.
>>A safe recursive definition would be
>>  let x = Foo (x+1)
>>However, if you leave out the constructor,
>>  let x = x + 1
>>you get a <<loop>> (or a deadlock).
> Are there any other reasons? 
> I am trying to debug monadic code which stores state information in a
> record maintaining several Data.Maps, but in vain so far. A state is
> modified/changed in several steps by a compound function i.e.
> 	changeA $ changeB $ changeC state
> Could this also lead to a deadlock? 

I don't think so. At least not in a way that leads to <<loop>>. If you
get <<loop>> then you really have some infinite recursion in your
program. Maybe you can track it down with Debug.Trace.trace.
If so, can I prevent this using CPS?

Dr. Janis Voigtlaender
mailto:voigt at tcs.inf.tu-dresden.de

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