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INFO-DIR-SECTION The Mercury Programming Language
START-INFO-DIR-ENTRY
* Mercury FAQ: (mercury_faq). The Mercury Frequently Asked Questions list.
END-INFO-DIR-ENTRY

   This file is the Mercury Frequently Asked Questions List, version
14.01.1.  It contains a list of frequently asked questions about
Mercury, with answers.

   Copyright (C) 1995-1997,1999,2001-2014 The University of Melbourne.

   Permission is granted to make and distribute verbatim copies of this
FAQ list provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this FAQ list under the conditions for verbatim copying, provided also
that the entire resulting derived work is distributed under the terms
of a permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
FAQ list into another language, under the above conditions for modified
versions.


File: mercury_faq.info,  Node: Top,  Up: (mercury)

The Mercury Frequently Asked Questions List, version 14.01.1
************************************************************

* Menu:

* Installing::      Problems during compilation and installation.
* Programming::     Common programming errors.
* Unimplemented::   Problems caused by unimplemented Mercury features.
* Last resort::     What to do when all else fails.


File: mercury_faq.info,  Node: Installing,  Next: Programming,  Prev: Top,  Up: Top

1 Problems during compilation and installation.
***********************************************

  1. _The compiler crashes during the "make install" phase_

     Mercury's use of GNU C extensions can cause problems with some
     versions of GCC, notably versions 4.2 and 4.3.  Either use a
     different version of GCC, or configure Mercury so that it does not
     require any GNU C extensions.  The following describes how to do
     the latter.

     For versions of Mercury after 0.13.1, invoke the configure script
     with the option `--with-llds-base-grade=none'.  This will cause
     the compiler to be built in the `none.gc' grade, which does not
     use any GNU C extensions.

     The configure script for Mercury version 0.13.1 (and before) does
     not support the above option.  Instead create a file in the
     top-level directory of the unpacked source tree named
     `Mmake.params' containing the line `GRADE=none.gc'.

     After running the configure script, do "make" and "make install"
     as normal.


  2. _The "make install" phase takes ages!  What's going on?_

     Mercury supports many grades.  A grade is a combination the target
     language and feature options.  These features include garbage
     collection, debugging and profiling support.  See "Grades and
     grade components" in the User's Guide.

     During the "make install" phase the newly built compiler is used
     to compile the standard library in each of the selected grades.
     The runtime system is also rebuilt for each grade.

     To install fewer grades and reduce the time "make install" takes,
     see the configure script's options, in particular
     `--disable-most-grades'.  Alternatively, run `make install
     LIBGRADES=<grades>' instead, where "<grades>" is a space-separated
     list of grades.

     Users of Mac OS X should note that there appear to be performance
     problems with the linker on some versions of that operating system
     that can lead to excessive linking times with code generated by
     the Mercury compiler.



File: mercury_faq.info,  Node: Programming,  Next: Unimplemented,  Prev: Installing,  Up: Top

2 Common programming errors
***************************

  1. _What does the error message "undefined symbol `'.'/2'" mean?_

     You need to explicitly import the `list' module

          :- import_module list.

     if your code uses lists. Similarly, if your code uses arithmetic
     operations, you will need to import the `int' and possibly `float'
     modules.


  2. _Why doesn't `X <= 3' work?_

     In Mercury, less-than-or-equal-to is written as `=<' not as `<=',
     which is used for reverse implication.


  3. _I defined a type like this: `:- type number ---> int ; float.'
     Why doesn't this work?_

     You tried to define a type that is an undiscriminated union of two
     types, which is not allowed by the Mercury type system.  The
     declaration above defines an enumerated type with two constants,
     "int" and "float".  This is not what you want, but it is legal
     Mercury, which is why you don't get an error message on the type
     declaration itself.


  4. _I get a "scope error" in an if-then-else.  I checked and both
     branches bind the same variables. What is the error?_

     This error generally happens if you attempt bind non-local
     variables in the _condition_ of the if-then-else.  For example,
     the following code attempts to bind `Value' in the call to
     `map.search', but `Value' occurs outside of the if-then-else -- in
     particular, it occurs in the head of the clause.

          :- pred map.search(map(K, V), K, V).
          :- mode map.search(in, in, out) is semidet.

          :- pred lookup(map(string, int), string, int).
          :- mode lookup(in, in, out) is det.

          lookup(Map, Key, Value) :-
                  (if map.search(Map, Key, Value) then
                          true
                  else
                          Value = -1
                  ).

     Binding non-local variables in the condition of an if-then-else is
     forbidden since it would be unsound; it would lead to inconsistent
     results.  For example, `(X = 1 -> Y = 1 ; Y = 2), X = 2' would
     fail, but `X = 2, (X = 1 -> Y = 1 ; Y = 2)' would succeed --
     breaking one of the fundamental laws of logic, `(P, Q) <=> (Q, P)'.

     Mode analysis therefore rejects such programs.  (In certain rare
     circumstances, the compiler may report this as a "mode error"
     rather than a "scope error".)  The way to fix such errors is to
     avoid binding non-local variables in the _condition_, and instead
     bind them in the _then_ part of the if-then-else.  So in the above
     example, you should introduce a new local variable, which we will
     call `Value1':

          lookup(Map, Key, Value) :-
                  ( map.search(Map, Key, Value1) ->
                          Value = Value1
                  ;
                          Value = -1
                  ).

  5. _"I keep getting a link error `undefined symbol init_gc'.  Why?"_

     If you are using `mmake' to recompile your program, and you are
     overriding the default grade (e.g. by setting `GRADE=asm_fast' in
     your `Mmake' file), you must make sure that you do `mmake clean'
     every time you change grades.



File: mercury_faq.info,  Node: Unimplemented,  Next: Last resort,  Prev: Programming,  Up: Top

3 Problems caused by unimplemented Mercury features
***************************************************

  1. _How can I avoid getting a compile-time error when I try to fill
     in a partially instantiated data structure?_

     At the moment, you can create a partially instantiated data
     structure, but you can't fill in the holes.  The reason is that
     the code that does the filling in must temporarily alias two
     variables together, and the current mode checker does not allow
     this.  This limitation will go away in the future.


  2. _I'm getting an error from the C compiler:_
          foo.c:45: initializer is not computable at load time


     You're using an old version of `gcc'.  Check that the version of
     `gcc' in your PATH is version 2.6.3 or later.  Mercury does not
     (at the current time) support versions of gcc earlier than 2.6.3.
     (Using the `--no-static-ground-terms' option may also solve this
     problem, but results in less efficient code.)



File: mercury_faq.info,  Node: Last resort,  Prev: Unimplemented,  Up: Top

4 What to do when all else fails
********************************

  1. _I'm getting an error message that I don't understand.  What can I
     do?_

     Try compiling with the `-E' (`--verbose-error-messages') option.
     This option causes the compiler to give even more verbose
     descriptions than usual.


  2. _I followed the instructions in the user's guide, but it still
     didn't work. What do I do next?_

     Send email to `bugs@mercurylang.org', and we'll try to solve your
     problem.




Tag Table:
Node: Top1051
Node: Installing1474
Node: Programming3630
Node: Unimplemented6907
Node: Last resort8005

End Tag Table
