[m-rev.] Compiler bug found

[Volker Wysk]

Sorry..... I was sure it's a compiler bug. I'll try hard to do it better
from now on.  :-(

Am Dienstag, dem 25.11.2025 um 18:36 +1100 schrieb Zoltan Somogyi:
> 
> 
> On Mon, 24 Nov 2025 18:39:44 +0100, Volker Wysk <post at volker-wysk.de>
> wrote:
> > I found a compiler bug.
> 
> Once again, this is not a compiler bug. Please look at ALL the error
> messages
> the compiler gives you before claiming that a problem is in the compiler,
> instead of in your code.
> 
> > 
> > :- pred umbenennen_1(
> >     list(pair(iosupp.path, iosupp.path))::in,  
> >     pair(iosupp.path, iosupp.path)::in,        
> >     io::di, io::uo
> > ) is det.
> > 
> > umbenennen_1(Symlinks, PfadAlt - PfadNeu, !IO) :-
> > 
> >     !( iosupp.path_exists_p(no, PfadNeu), Existiert1, !IO ),
> >     (
> >         Existiert1 = yes,
> >         write_string("-1-\n", !IO),             % <-- HERE
> >         throw("Zielpfad " ++ quote(unslice_path(PfadNeu)) ++ 
> >             " gibt es schon. Kann " ++
> >             quote(unslice_path(PfadAltEcht)) ++ " nicht umbenennen.")
> >     ;
> >         Existiert1 = no
> >     ),
> 
> At the point of the throw, the value of PfadAltEcht has not been defined
> yet.
> The compiler therefore attempts to move the whole switch to a later
> position,
> and the mode error occurs because there is no later position that works.
> And as one of the possible causes of the mode error, the compiler tells
> you
> about PfadAltEcht being free at this program point.
> 
> > When the line marked with "<-- HERE" is commented out, all is fine.
> 
> That is because moving the switch to a point after the definition of
> PfadAltEcht
> also moves it after some I/O operations. State variable notation
> inherently imposes
> (in fact, it is *designed* to impose) an order on the state updates, and
> the move
> of the switch conflicts with this order. This conflict is what is reported
> as the mode error.
> 
> The compiler reposts multiple possible causes of the error because it
> cannot read
> your mind about which way you may want to resolve the conflict.
> 
> Zoltan.
> 
>