Release 0.9.1 - Known Problems

Release 0.9 had a couple of serious bugs in the binary distributions. One of them is that you need to type `make INSTALL_EMACS_SCRIPTS=scripts/mdb.el install' instead of `make install' to install it. The other is that installation of the binary distribution for Windows will fail. These bugs are fixed in 0.9.1.

The following is collected email of reported problems with release 0.9.x of the Mercury distribution. Included, where possible, are patches or work-arounds.

In addition to the bugs mentioned here, some bugs related to the implementation of particular languages features (existential types, nested modules, tabling) are also mentioned in the language reference manual, and some problems related to using Mercury on specific operating systems are described in the README.* files in the distribution. See also the LIMITATIONS file.

Note: please do not be alarmed by the fact that this software has some bugs. ALL useful software has bugs. During the development of the Mercury implementation we have found bugs in gcc, as, ld, the dynamic loader, and even the OS kernel. We hope that by listing the known outstanding bugs here we are doing our users a service. It would be disappointing if anyone were to infer the wrong thing from it.

Subject: bug report - Inf and NaN
Date: Wed, 4 Oct 1995

The following module causes an "undefined variable Inf" error in the generated C code, because 1E400 == Infinity, which gets printed as `Inf'.

:- module hello.
:- interface.
:- import_module io.

:- pred main(io__state::di, io__state::uo) is det.

:- implementation.

main -->

Subject: nit in error msg
Date: Thu, 16 May 1996

Here's another small error in an error message. If you comment out the [] clause for the functions car/1 or cdr/1, you get this message:

fntest.m:023: In `car(in) = out':
fntest.m:023:   Error: determinism declaration not satisfied.
fntest.m:023:   Declared `det', inferred `semidet'.
fntest.m:023:   in argument 1 of clause head:
fntest.m:023:   unification of `HeadVar__1' and `[X | V_4]' can fail.

It says Declared `det', inferred `semidet', but I never declared it at all. It's a bit misleading. Certainly not a major problem, and the later part of the message makes it quite clear what the problem is, but I thought I'd point it out to you before I forgot it.

Subject: missed mode error
Date: Tue, 28 May 1996

Another one for the bug report file:

The goal `some [X, Y] X \= Y' should be a mode error, but the current mode checker doesn't report an error. Instead, the compiler goes on to generate code which gives the wrong answer. For example, the following program prints out `no'. The same problem also occurs with `some [X, Y] (X = Y -> fail ; true)'.

:- module bug.
:- interface.
:- import_module io.

:- pred main(io__state::di, io__state::uo) is det.

:- implementation.

main --> 
	( { p } -> io__write_string("yes\n") ; io__write_string("no\n") ).

:- pred p is semidet.
p :-
	some [X, Y] X \= Y.

The bug occurs only when the variables being unified inside a negated context are not live, i.e. when it is the last occurrence of those variables.

Subject: bug with PC values on Alpha
Date: Wed, 12 Jun 1996

On the alpha, if the Mercury runtime catches a signal, it sometimes prints out the wrong value for the PC (program counter).

Subject: inter-module optimization and abstract exported equivalence types.
Date: Thu, 19 Feb 1998

In some cases the compiler reports spurious ambiguity errors when compiling with `--intermodule-optimization'. This is due to the definition of abstract exported equivalence types being made visible by inter-module optimization. In this example, with `--intermodule-optimization' the compiler sees the declaration `:- type var == int' from term.m and then cannot determine whether `Elem' has type `int' or `pair(int)'. The work-around is to add an explicit type qualification.

:- module foo.
:- interface.
:- import_module list, term.
:- pred test(list(var)::in) is det.
:- implementation.
:- import_module int, std_util.

test(Args0) :-
	MakeIndex =
		lambda([Elem0::in, Elem::out, Index0::in, Index::out] is det, (
			Elem = Elem0 - Index0,
			Index is Index0 + 1
	list__map_foldl(MakeIndex, Args0, _, 0, _).

Subject: Formatted printing of floating point numbers broken
Date: Thu, 27 May 1999

It seems as though the formatted printing of floating point numbers is broken if one uses the %g or %G format specifiers. In particular, if it decides that the %f format should be used, everything after the decimal point is discarded.

Note also that, from inspection of the code, it seems the "#" format modifier is being ignored for the "feEgG" formats, contrary to the documentation for string__format. It'd also be nice if, by default, the %g/%G format didn't print trailing zeros (as per the C standard [and implied by the Mercury docs] and the reason I was trying to use it in the first place: I don't like outputting 0.500000000000000, but I don't want to be aggressive about pruning the precision in case it's actually significant sometime).

Subject: `:- pragma does_not_terminate'
Date: Wed, 9 Jun 1999

`:- pragma does_not_terminate' declarations do not work. The compiler's termination analysis seems to ignore them.

Date: Wed, 1 Dec 1999
Subject: compiler infinite loop for cyclic type classes

According to the language reference manual:

|  Typeclass constraints on type class declarations gives rise to a
|  superclass relation.  This relation must be acyclic.  That is, it is an
|  error if a type class is its own (direct or indirect) superclass.

But if you try to compile modules containing cyclic typeclasses, the compiler goes into an infinite loop and eventually gets a stack overflow, rather than reporting a proper error message.

Date: Mon, 31 Jan 2000
Subject: installation problem on Suse 6.3 Linux with gcc

When running make, the error message:

cc1: Invalid option `-fno-gcse'
is given.

A work-around for this bug is to (after running configure, but before running make) delete the `-fno-gcse' from scripts/mmc and scripts/mgnuc.

Strangely, this bug doesn't seem to affect all versions of gcc