=================================== Nim Compiler User Guide =================================== :Author: Andreas Rumpf :Version: |nimversion| .. contents:: "Look at you, hacker. A pathetic creature of meat and bone, panting and sweating as you run through my corridors. How can you challenge a perfect, immortal machine?" Introduction ============ This document describes the usage of the *Nim compiler* on the different supported platforms. It is not a definition of the Nim programming language (therefore is the `manual `_). Nim is free software; it is licensed under the `MIT License `_. Compiler Usage ============== Command line switches --------------------- Basic command line switches are: .. include:: basicopt.txt Advanced command line switches are: .. include:: advopt.txt List of warnings ---------------- Each warning can be activated individually with ``--warning[NAME]:on|off`` or in a ``push`` pragma. ========================== ============================================ Name Description ========================== ============================================ CannotOpenFile Some file not essential for the compiler's working could not be opened. OctalEscape The code contains an unsupported octal sequence. Deprecated The code uses a deprecated symbol. ConfigDeprecated The project makes use of a deprecated config file. SmallLshouldNotBeUsed The letter 'l' should not be used as an identifier. AnalysisLoophole The thread analysis was incomplete due to an indirect call. DifferentHeaps The code mixes different local heaps in a very dangerous way. WriteToForeignHeap The code contains a threading error. EachIdentIsTuple The code contains a confusing ``var`` declaration. ShadowIdent A local variable shadows another local variable of an outer scope. User Some user defined warning. ========================== ============================================ Verbosity levels ---------------- ===== ============================================ Level Description ===== ============================================ 0 Minimal output level for the compiler. 1 Displays compilation of all the compiled files, including those imported by other modules or through the `compile pragma<#compile-pragma>`_. This is the default level. 2 Displays compilation statistics, enumerates the dynamic libraries that will be loaded by the final binary and dumps to standard output the result of applying `a filter to the source code `_ if any filter was used during compilation. 3 In addition to the previous levels dumps a debug stack trace for compiler developers. ===== ============================================ Compile time symbols -------------------- Through the ``-d:x`` or ``--define:x`` switch you can define compile time symbols for conditional compilation. The defined switches can be checked in source code with the `when statement `_ and `defined proc `_. The typical use of this switch is to enable builds in release mode (``-d:release``) where certain safety checks are omitted for better performance. Another common use is the ``-d:ssl`` switch to activate `SSL sockets `_. Configuration files ------------------- **Note:** The *project file name* is the name of the ``.nim`` file that is passed as a command line argument to the compiler. The ``nim`` executable processes configuration files in the following directories (in this order; later files overwrite previous settings): 1) ``$nim/config/nim.cfg``, ``/etc/nim.cfg`` (UNIX) or ``%NIMROD%/config/nim.cfg`` (Windows). This file can be skipped with the ``--skipCfg`` command line option. 2) ``/home/$user/.config/nim.cfg`` (UNIX) or ``%APPDATA%/nim.cfg`` (Windows). This file can be skipped with the ``--skipUserCfg`` command line option. 3) ``$parentDir/nim.cfg`` where ``$parentDir`` stands for any parent directory of the project file's path. These files can be skipped with the ``--skipParentCfg`` command line option. 4) ``$projectDir/nim.cfg`` where ``$projectDir`` stands for the project file's path. This file can be skipped with the ``--skipProjCfg`` command line option. 5) A project can also have a project specific configuration file named ``$project.nim.cfg`` that resides in the same directory as ``$project.nim``. This file can be skipped with the ``--skipProjCfg`` command line option. Command line settings have priority over configuration file settings. The default build of a project is a `debug build`:idx:. To compile a `release build`:idx: define the ``release`` symbol:: nim c -d:release myproject.nim Search path handling -------------------- Nim has the concept of a global search path (PATH) that is queried to determine where to find imported modules or include files. If multiple files are found an ambiguity error is produced. ``nim dump`` shows the contents of the PATH. However before the PATH is used the current directory is checked for the file's existance. So if PATH contains ``$lib`` and ``$lib/bar`` and the directory structure looks like this:: $lib/x.nim $lib/bar/x.nim foo/x.nim foo/main.nim other.nim And ``main`` imports ``x``, ``foo/x`` is imported. If ``other`` imports ``x`` then both ``$lib/x.nim`` and ``$lib/bar/x.nim`` match and so the compiler should reject it. Currently however this check is not implemented and instead the first matching file is used. Generated C code directory -------------------------- The generated files that Nim produces all go into a subdirectory called ``nimcache`` in your project directory. This makes it easy to delete all generated files. Files generated in this directory follow a naming logic which you can read about in the `Nim Backend Integration document `_. However, the generated C code is not platform independent. C code generated for Linux does not compile on Windows, for instance. The comment on top of the C file lists the OS, CPU and CC the file has been compiled for. Compilation cache ================= **Warning**: The compilation cache is still highly experimental! The ``nimcache`` directory may also contain so called `rod`:idx: or `symbol files`:idx:. These files are pre-compiled modules that are used by the compiler to perform `incremental compilation`:idx:. This means that only modules that have changed since the last compilation (or the modules depending on them etc.) are re-compiled. However, per default no symbol files are generated; use the ``--symbolFiles:on`` command line switch to activate them. Unfortunately due to technical reasons the ``--symbolFiles:on`` needs to *aggregate* some generated C code. This means that the resulting executable might contain some cruft even when dead code elimination is turned on. So the final release build should be done with ``--symbolFiles:off``. Due to the aggregation of C code it is also recommended that each project resides in its own directory so that the generated ``nimcache`` directory is not shared between different projects. Cross compilation ================= To cross compile, use for example:: nim c --cpu:i386 --os:linux --compile_only --gen_script myproject.nim Then move the C code and the compile script ``compile_myproject.sh`` to your Linux i386 machine and run the script. Another way is to make Nim invoke a cross compiler toolchain:: nim c --cpu:arm --os:linux myproject.nim For cross compilation, the compiler invokes a C compiler named like ``$cpu.$os.$cc`` (for example arm.linux.gcc) and the configuration system is used to provide meaningful defaults. For example for ``ARM`` your configuration file should contain something like:: arm.linux.gcc.path = "/usr/bin" arm.linux.gcc.exe = "arm-linux-gcc" arm.linux.gcc.linkerexe = "arm-linux-gcc" DLL generation ============== Nim supports the generation of DLLs. However, there must be only one instance of the GC per process/address space. This instance is contained in ``nimrtl.dll``. This means that every generated Nim DLL depends on ``nimrtl.dll``. To generate the "nimrtl.dll" file, use the command:: nim c -d:release lib/nimrtl.nim To link against ``nimrtl.dll`` use the command:: nim c -d:useNimRtl myprog.nim **Note**: Currently the creation of ``nimrtl.dll`` with thread support has never been tested and is unlikely to work! Additional compilation switches =============================== The standard library supports a growing number of ``useX`` conditional defines affecting how some features are implemented. This section tries to give a complete list. ================== ========================================================= Define Effect ================== ========================================================= ``release`` Turns off runtime checks and turns on the optimizer. ``useWinAnsi`` Modules like ``os`` and ``osproc`` use the Ansi versions of the Windows API. The default build uses the Unicode version. ``useFork`` Makes ``osproc`` use ``fork`` instead of ``posix_spawn``. ``useNimRtl`` Compile and link against ``nimrtl.dll``. ``useMalloc`` Makes Nim use C's `malloc`:idx: instead of Nim's own memory manager. This only works with ``gc:none``. ``useRealtimeGC`` Enables support of Nim's GC for *soft* realtime systems. See the documentation of the `gc `_ for further information. ``nodejs`` The JS target is actually ``node.js``. ``ssl`` Enables OpenSSL support for the sockets module. ``memProfiler`` Enables memory profiling for the native GC. ================== ========================================================= Additional Features =================== This section describes Nim's additional features that are not listed in the Nim manual. Some of the features here only make sense for the C code generator and are subject to change. NoDecl pragma ------------- The ``noDecl`` pragma can be applied to almost any symbol (variable, proc, type, etc.) and is sometimes useful for interoperability with C: It tells Nim that it should not generate a declaration for the symbol in the C code. For example: .. code-block:: Nim var EACCES {.importc, noDecl.}: cint # pretend EACCES was a variable, as # Nim does not know its value However, the ``header`` pragma is often the better alternative. **Note**: This will not work for the LLVM backend. Header pragma ------------- The ``header`` pragma is very similar to the ``noDecl`` pragma: It can be applied to almost any symbol and specifies that it should not be declared and instead the generated code should contain an ``#include``: .. code-block:: Nim type PFile {.importc: "FILE*", header: "".} = distinct pointer # import C's FILE* type; Nim will treat it as a new pointer type The ``header`` pragma always expects a string constant. The string contant contains the header file: As usual for C, a system header file is enclosed in angle brackets: ``<>``. If no angle brackets are given, Nim encloses the header file in ``""`` in the generated C code. **Note**: This will not work for the LLVM backend. IncompleteStruct pragma ----------------------- The ``incompleteStruct`` pragma tells the compiler to not use the underlying C ``struct`` in a ``sizeof`` expression: .. code-block:: Nim type TDIR* {.importc: "DIR", header: "", final, pure, incompleteStruct.} = object Compile pragma -------------- The ``compile`` pragma can be used to compile and link a C/C++ source file with the project: .. code-block:: Nim {.compile: "myfile.cpp".} **Note**: Nim computes a CRC checksum and only recompiles the file if it has changed. You can use the ``-f`` command line option to force recompilation of the file. Link pragma ----------- The ``link`` pragma can be used to link an additional file with the project: .. code-block:: Nim {.link: "myfile.o".} PassC pragma ------------ The ``passC`` pragma can be used to pass additional parameters to the C compiler like you would using the commandline switch ``--passC``: .. code-block:: Nim {.passC: "-Wall -Werror".} Note that you can use ``gorge`` from the `system module `_ to embed parameters from an external command at compile time: .. code-block:: Nim {.passC: gorge("pkg-config --cflags sdl").} PassL pragma ------------ The ``passL`` pragma can be used to pass additional parameters to the linker like you would using the commandline switch ``--passL``: .. code-block:: Nim {.passL: "-lSDLmain -lSDL".} Note that you can use ``gorge`` from the `system module `_ to embed parameters from an external command at compile time: .. code-block:: Nim {.passL: gorge("pkg-config --libs sdl").} Emit pragma ----------- The ``emit`` pragma can be used to directly affect the output of the compiler's code generator. So it makes your code unportable to other code generators/backends. Its usage is highly discouraged! However, it can be extremely useful for interfacing with `C++`:idx: or `Objective C`:idx: code. Example: .. code-block:: Nim {.emit: """ static int cvariable = 420; """.} {.push stackTrace:off.} proc embedsC() = var nimVar = 89 # use backticks to access Nim symbols within an emit section: {.emit: """fprintf(stdout, "%d\n", cvariable + (int)`nimVar`);""".} {.pop.} embedsC() ImportCpp pragma ---------------- Similar to the `importc pragma for C `_, the ``importcpp`` pragma can be used to import `C++`:idx: methods. The generated code then uses the C++ method calling syntax: ``obj->method(arg)``. In addition with the ``header`` and ``emit`` pragmas this allows *sloppy* interfacing with libraries written in C++: .. code-block:: Nim # Horrible example of how to interface with a C++ engine ... ;-) {.link: "/usr/lib/libIrrlicht.so".} {.emit: """ using namespace irr; using namespace core; using namespace scene; using namespace video; using namespace io; using namespace gui; """.} const irr = "" type TIrrlichtDevice {.final, header: irr, importc: "IrrlichtDevice".} = object PIrrlichtDevice = ptr TIrrlichtDevice proc createDevice(): PIrrlichtDevice {. header: irr, importc: "createDevice".} proc run(device: PIrrlichtDevice): bool {. header: irr, importcpp: "run".} The compiler needs to be told to generate C++ (command ``cpp``) for this to work. The conditional symbol ``cpp`` is defined when the compiler emits C++ code. ImportObjC pragma ----------------- Similar to the `importc pragma for C `_, the ``importobjc`` pragma can be used to import `Objective C`:idx: methods. The generated code then uses the Objective C method calling syntax: ``[obj method param1: arg]``. In addition with the ``header`` and ``emit`` pragmas this allows *sloppy* interfacing with libraries written in Objective C: .. code-block:: Nim # horrible example of how to interface with GNUStep ... {.passL: "-lobjc".} {.emit: """ #include @interface Greeter:Object { } - (void)greet:(long)x y:(long)dummy; @end #include @implementation Greeter - (void)greet:(long)x y:(long)dummy { printf("Hello, World!\n"); } @end #include """.} type TId {.importc: "id", header: "", final.} = distinct int proc newGreeter: TId {.importobjc: "Greeter new", nodecl.} proc greet(self: TId, x, y: int) {.importobjc: "greet", nodecl.} proc free(self: TId) {.importobjc: "free", nodecl.} var g = newGreeter() g.greet(12, 34) g.free() The compiler needs to be told to generate Objective C (command ``objc``) for this to work. The conditional symbol ``objc`` is defined when the compiler emits Objective C code. CodegenDecl pragma ------------------ The ``codegenDecl`` pragma can be used to directly influence Nim's code generator. It receives a format string that determines how the variable or proc is declared in the generated code: .. code-block:: nim var a {.codegenDecl: "$# progmem $#".}: int proc myinterrupt() {.codegenDecl: "__interrupt $# $#$#".} = echo "realistic interrupt handler" InjectStmt pragma ----------------- The ``injectStmt`` pragma can be used to inject a statement before every other statement in the current module. It is only supposed to be used for debugging: .. code-block:: nim {.injectStmt: gcInvariants().} # ... complex code here that produces crashes ... LineDir option -------------- The ``lineDir`` option can be turned on or off. If turned on the generated C code contains ``#line`` directives. This may be helpful for debugging with GDB. StackTrace option ----------------- If the ``stackTrace`` option is turned on, the generated C contains code to ensure that proper stack traces are given if the program crashes or an uncaught exception is raised. LineTrace option ---------------- The ``lineTrace`` option implies the ``stackTrace`` option. If turned on, the generated C contains code to ensure that proper stack traces with line number information are given if the program crashes or an uncaught exception is raised. Debugger option --------------- The ``debugger`` option enables or disables the *Embedded Nim Debugger*. See the documentation of endb_ for further information. Breakpoint pragma ----------------- The *breakpoint* pragma was specially added for the sake of debugging with ENDB. See the documentation of `endb `_ for further information. Volatile pragma --------------- The ``volatile`` pragma is for variables only. It declares the variable as ``volatile``, whatever that means in C/C++ (its semantics are not well defined in C/C++). **Note**: This pragma will not exist for the LLVM backend. Source code style ================= Nim allows you to `mix freely case and underscores as identifier separators `_, so variables named ``MyPrecioussInt`` and ``my_preciouss_int`` are equivalent: .. code-block:: Nim var MyPrecioussInt = 3 # Following line compiles fine! echo my_preciouss_int Since this can lead to many variants of the same source code (you can use `nimgrep `_ instead of your typical ``grep`` to ignore style problems) the compiler provides the command ``pretty`` to help unifying the style of source code. Running ``nim pretty ugly_test.nim`` with this example will generate a secondary file named ``ugly_test.pretty.nim`` with the following content: .. code-block:: Nim var MyPrecioussInt = 3 # Following line compiles fine! echo MyPrecioussInt During execution the ``pretty`` command will also run on Nim's standard library, since it doesn't differentiate the standard library as something special, and hence will warn of many *errors* which are out of your hand to fix, creating respective ``.pretty.nim`` files all the way. You can ignore these errors if they don't belong to your source and simply compare your original version to the new pretty one. In fact, running ``pretty`` on our test file will show the following:: Hint: ugly_test [Processing] ugly_test.nim(1, 4) Error: name should be: myPrecioussInt ugly_test.nim(1, 4) Error: name should be: myPrecioussInt At the moment ``pretty`` will homogenize the style of symbols but will leave important changes for you to review. In this case the command is warning that a variable name should not start with a capital letter, which is usually reserved to `object types `_. To learn about the accepted `camel case style `_ read `Coding Guidelines in the Internals of Nim Compiler `_ or `Coding Guidelines `_ and `NEP 1 : Style Guide for Nim Code `_ from the Nim `GitHub wiki`_. This command is safe to run because it will never attempt to overwrite your existing sources, but the respective ``.pretty.nim`` files **will** be overwritten without notice. DynlibOverride ============== By default Nim's ``dynlib`` pragma causes the compiler to generate ``GetProcAddress`` (or their Unix counterparts) calls to bind to a DLL. With the ``dynlibOverride`` command line switch this can be prevented and then via ``--passL`` the static library can be linked against. For instance, to link statically against Lua this command might work on Linux:: nim c --dynlibOverride:lua --passL:liblua.lib program.nim Backend language options ======================== The typical compiler usage involves using the ``compile`` or ``c`` command to transform a ``.nim`` file into one or more ``.c`` files which are then compiled with the platform's C compiler into a static binary. However there are other commands to compile to C++, Objective-C or Javascript. More details can be read in the `Nim Backend Integration document `_. Nim documentation tools ======================= Nim provides the `doc`:idx: and `doc2`:idx: commands to generate HTML documentation from ``.nim`` source files. Only exported symbols will appear in the output. For more details `see the docgen documentation `_. Nim idetools integration ======================== Nim provides language integration with external IDEs through the idetools command. See the documentation of `idetools `_ for further information. Nim interactive mode ==================== The Nim compiler supports an interactive mode. This is also known as a `REPL`:idx: (*read eval print loop*). If Nim has been built with the ``-d:useGnuReadline`` switch, it uses the GNU readline library for terminal input management. To start Nim in interactive mode use the command ``nim i``. To quit use the ``quit()`` command. To determine whether an input line is an incomplete statement to be continued these rules are used: 1. The line ends with ``[-+*/\\<>!\?\|%&$@~,;:=#^]\s*$`` (operator symbol followed by optional whitespace). 2. The line starts with a space (indentation). 3. The line is within a triple quoted string literal. However, the detection does not work if the line contains more than one ``"""``. Nim for embedded systems ======================== The standard library can be avoided to a point where C code generation for 16bit micro controllers is feasible. Use the `standalone`:idx: target (``--os:standalone``) for a bare bones standard library that lacks any OS features. To make the compiler output code for a 16bit target use the ``--cpu:avr`` target. For example, to generate code for an `AVR`:idx: processor use this command:: nim c --cpu:avr --os:standalone --deadCodeElim:on --genScript x.nim For the ``standalone`` target you need to provide a file ``panicoverride.nim``. See ``tests/manyloc/standalone/panicoverride.nim`` for an example implementation. Nim for realtime systems ======================== See the documentation of Nim's soft realtime `GC `_ for further information. Debugging with Nim ================== Nim comes with its own *Embedded Nim Debugger*. See the documentation of endb_ for further information. Optimizing for Nim ================== Nim has no separate optimizer, but the C code that is produced is very efficient. Most C compilers have excellent optimizers, so usually it is not needed to optimize one's code. Nim has been designed to encourage efficient code: The most readable code in Nim is often the most efficient too. However, sometimes one has to optimize. Do it in the following order: 1. switch off the embedded debugger (it is **slow**!) 2. turn on the optimizer and turn off runtime checks 3. profile your code to find where the bottlenecks are 4. try to find a better algorithm 5. do low-level optimizations This section can only help you with the last item. Optimizing string handling -------------------------- String assignments are sometimes expensive in Nim: They are required to copy the whole string. However, the compiler is often smart enough to not copy strings. Due to the argument passing semantics, strings are never copied when passed to subroutines. The compiler does not copy strings that are a result from a procedure call, because the callee returns a new string anyway. Thus it is efficient to do: .. code-block:: Nim var s = procA() # assignment will not copy the string; procA allocates a new # string already However it is not efficient to do: .. code-block:: Nim var s = varA # assignment has to copy the whole string into a new buffer! For ``let`` symbols a copy is not always necessary: .. code-block:: Nim let s = varA # may only copy a pointer if it safe to do so If you know what you're doing, you can also mark single string (or sequence) objects as `shallow`:idx:\: .. code-block:: Nim var s = "abc" shallow(s) # mark 's' as shallow string var x = s # now might not copy the string! Usage of ``shallow`` is always safe once you know the string won't be modified anymore, similar to Ruby's `freeze`:idx:. The compiler optimizes string case statements: A hashing scheme is used for them if several different string constants are used. So code like this is reasonably efficient: .. code-block:: Nim case normalize(k.key) of "name": c.name = v of "displayname": c.displayName = v of "version": c.version = v of "os": c.oses = split(v, {';'}) of "cpu": c.cpus = split(v, {';'}) of "authors": c.authors = split(v, {';'}) of "description": c.description = v of "app": case normalize(v) of "console": c.app = appConsole of "gui": c.app = appGUI else: quit(errorStr(p, "expected: console or gui")) of "license": c.license = UnixToNativePath(k.value) else: quit(errorStr(p, "unknown variable: " & k.key))