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Branche: main, 11.07 Update some documentation README.Erlang: Mention the minimum required version of Erlang/OTP. README.MacOS: Mention that with XCode 4 gcc is actually a symlink for llvm-gcc.
223 lines
7.1 KiB
Plaintext
223 lines
7.1 KiB
Plaintext
-----------------------------------------------------------------------------
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INTRODUCTION
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This release of Mercury contains a port to Erlang. The Mercury
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compiler will generate Erlang source code that can be compiled into
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bytecode or native code suitable for running in the Erlang runtime
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system.
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The port is incomplete. Large parts of the Mercury standard library
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are not yet implemented. Some RTTI related features are incompletely
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or incorrectly implemented.
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PREREQUISITES
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In order to try this system you will need
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- The Erlang/OTP distribution, which is open source and can be
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downloaded from
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<http://www.erlang.org/download.html>
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- A Unix-like environment. The Erlang support has not been
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tested on Windows, but you will need a shell script
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interpreter.
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- Users of Debian or Ubuntu who are using the Erlang that is
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packaged for those systems also need have the erlang-dev
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package installed.
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WARNING
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Please note that the Erlang backend is still an experimental feature.
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-----------------------------------------------------------------------------
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INSTALLATION
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Invoke configure with the option `--enable-erlang-grade' in order to
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enable Erlang support. Make sure it was detected properly by running:
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mmake echo_libgrades
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Then run:
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mmake install
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or
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mmake install LIBGRADES=erlang
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For better performance you will need to add the following line to
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Mmake.params before installing:
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EXTRA_MCFLAGS += --erlang-native-code
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-----------------------------------------------------------------------------
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THE ERLANG GRADE
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The Mercury compiler currently supports the grade `erlang'.
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The erlang grade is enabled by using any of the options
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`--grade erlang', `--target erlang', or just `--erlang',
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in combination with `mmc --make'. Mmake does _not_ currently support
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the erlang grade.
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To run a Mercury program using the erlang grade, you need to install the
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Mercury library in the erlang grade, as described above.
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You can now build programs such as hello.m or calculator.m in the samples
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directory.
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cd samples
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mmc --make --erlang hello
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Now you can run hello
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./hello
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Note that hello is a simple shell script that starts the Erlang runtime system.
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The actual object files will be stored in the Mercury subdirectory, in `beams'.
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If you are using the Windows command-line interpreter, i.e. cmd.exe, then
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setting the value of option --target-env-type to "windows" will cause the
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compiler to generate a batch file that starts the Erlang runtime system, rather
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than a shell script.
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-----------------------------------------------------------------------------
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USING ERLANG
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The Mercury standard library has not been fully ported to Erlang yet.
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The use of unimplemented procedures will result in a run-time error,
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with a message such as "Sorry, not implemented: foreign code for this
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function", and a stack trace.
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If you find missing functionality, you can interface to Erlang using
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Mercury's foreign language interface.
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For more information about the foreign language interface, see the Mercury
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Language Reference Manual, which you can find at:
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<http://www.mercury.csse.unimelb.edu.au/information/documentation.html>
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-----------------------------------------------------------------------------
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PERFORMANCE NOTES
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The Erlang code generated by the Mercury compiler is designed to be compiled
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to native code, using the HiPE compiler. The Erlang bytecode compiler does
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not recognise static data structures so some run-time type information can
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get created repeatedly again at runtime. HiPE treats static data structures
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specially so this problem does not occur.
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You can pass the `--erlang-native-code' option to mmc to use HiPE to compile
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.erl files to .beam files. It is recommended to add this flag to
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Mmake.params before installing the standard library.
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The need to support user-defined equality and comparison predicates can
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cause significant slowdowns, even when they are unused. This problem
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usually exists when a type contains an abstract type in its definition. The
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compiler does not know if an abstract type has user-defined equality or not,
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so it makes a conservative assumption. Enabling intermodule optimisation
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gives the compiler more information to avoid this problem.
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For some programs you may want to increase the default heap size.
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You can do this by setting the ERL_FLAGS environment variable, e.g.
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export ERL_FLAGS="+h8000".
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-----------------------------------------------------------------------------
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DIFFERENCES FROM OTHER BACKENDS
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* Discriminated union values are ordered according to Erlang conventions
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(e.g. alphabetically) instead of according to the order that data
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constructors appear in the type definition.
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* Some legal Mercury code making use of partial instantiation will be
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rejected when compiling to Erlang (you will get a compiler abort).
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An example is:
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foo(Xs) :-
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( Xs = []
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; Xs = [1 | _]
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),
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( Xs = []
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; Xs = [_ | []]
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).
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-----------------------------------------------------------------------------
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RESOURCES
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You might find the following pages useful:
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<http://www.mercury.csse.unimelb.edu.au/backends.html>
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<http://www.mercury.csse.unimelb.edu.au/information/documentation.html>
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-----------------------------------------------------------------------------
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FREQUENTLY ASKED QUESTIONS (FAQS)
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Q. What are the advantages of using the Erlang back-end?
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A. The goal is to take advantage of the Erlang implementation for writing
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scalable and reliable server programs, which can be distributed over
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multiple machines and, possibly, updated at run-time.
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Q. What version of Erlang should I be using?
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A. Erlang/OTP R14B04 or later is required.
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Q. What features are not yet implemented for the Erlang back-end?
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A. The following language features are not supported and will not be:
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trailing
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tabling
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The following implementation features are not supported:
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Mercury-level debugging
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Mercury-level profiling
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The following standard library modules are completely unimplemented:
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bit_buffer
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thread
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thread.semaphore
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time
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version_array
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In addition, many modules are incompletely implemented or have placeholder
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implementations.
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Q. How do I enable Erlang-level debugging?
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A. With great difficulty. One way is to copy or symlink all the .erl and
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.hrl files used by the program, including those of the standard library,
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into a single directory. Then build the .beam files with debugging
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enabled, e.g.
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erlc +debug_info *.erl
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Start up the Erlang runtime system and the debugger:
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% erl
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...
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1> debugger:start().
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In the debugger, select Module > Interpret... and choose the modules to
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interpret (probably "All"). At the erl prompt, the program can be started
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like so:
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% hello:mercury__main_wrapper().
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To run it a second time you may need to call `main_2_p_0' instead.
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For more information, see the documentation for erlc and the Erlang
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debugger.
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-----------------------------------------------------------------------------
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