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Man Pages

Manual Reference Pages  -  MLTON (1)


mlton - whole-program compiler for the Standard ML (SML) programming language


Compile-time options
Runtime system options


mlton [option ...] file.{c|mlb|o|sml} [file.{c|o|s|S} ...]


MLton is run from the command line with a collection of options followed by a file name and a list of files with which to compile, assemble, and link with. The simplest case is to run mlton foo.sml, where foo.sml contains a valid SML program, in which case MLton compiles the program to produce an executable foo. Since MLton does not support separate compilation, the program must be the entire program you wish to compile. However, the program may refer to signatures and structures defined in the Basis Library.

Larger programs, spanning many files, may be compiled with the ML Basis system. In this case, mlton will compile the complete SML program described by the basis, which may specify both SML files and additional bases. See the MLton Guide for details.

MLton’s compile-time options control the name of the output file, the verbosity of compile-time messages, and whether or not certain optimizations are performed. They also can specify which intermediate files are saved and can stop the compilation process early, at some intermediate pass, in which case compilation can be resumed by passing the generated files to MLton. MLton uses the input file suffix to determine the type of input program. The possibilities are .c, .mlb, .o, .s, and .sml.

With no arguments, MLton prints the version number and exits. For a usage message, run MLton with an invalid switch, e.g. mlton -z. In the explanation below and in the usage message, for flags that take a number of choices (e.g. {true|false}), the first value listed is the default.

Compile-time options

-align {4|8}
  Aligns object sizes and doubles in memory by the specified alignment. The default varies depending on architecture.

-as-opt option
  Pass option to gcc when assembling.

-cc-opt option
  Pass option to gcc when compiling C code.

-codegen {native|x86|amd64|c}
  Generate native code or C code. With -codegen native (-codegen x86 or -codegen amd64), MLton typically compiles more quickly and generates better code.

-const ’name value
  Set the value of a compile-time constant. Here is a list of available constants, their default values, and what they control.

Exn.keepHistory {false|true} Enable MLton.Exn.history. There is a performance cost to setting this to true, both in memory usage of exceptions and in run time, because of additional work that must be performed at each exception construction, raise, and handle.

-default-ann ann
  Specify default ML Basis annotations. For example, -default-ann ’warnUnused true’ causes unused variable warnings to be enabled by default. Defaults may be overridden by an annotation in an ML Basis file.

-default-type type
  Specify the default binding for a primitive type. For example, -default-type word64 causes the top-level type word and the top-level structure Word in the Basis Library to be equal to Word64.word and Word64:WORD, respectively. Similarly, -default-type intinf causes the top-level type int and the top-level structure Int in the Basis Library to be equal to and IntInf:INTEGER, respectively.

-disable-ann ann
  Ignore the specified ML Basis annotation in every ML Basis File. For example, to see all match and unused warnings, compile with -default-ann ’warnUnused true’, -disable-ann forceUsed, -disable-ann nonexhaustiveMatch, -disable-ann redundantMatch, and -disable-ann warnUnused.

-export-header file
  Write C prototypes to file for all of the functions in the program exported from SML to C.

-ieee-fp {false|true}
  Cause the x86 native code generator to be pedantic about following the IEEE floating point standard. By default, it is not, because of the performance cost. This only has an effect with -codegen x86.

-inline n
  Set the inlining threshold used in the optimizer. The threshold is an approximate measure of code size of a procedure. The default is 320.

-keep {g|o}
  Save intermediate files. If no -keep argument is given, then only the output file is saved. g generated .c and .s files passed to gcc and the assembler
o object (.o) files

-link-opt option
  Pass option to gcc when linking. You can use this to specify library search paths, e.g. -link-opt -Lpath, and libraries to link with, e.g. -link-opt -lfoo, or even both at the same time, e.g. -link-opt ’-Lpath -lfoo’. If you wish to pass an option to the linker, you must use gcc’s -Wl, syntax, e.g., -link-opt ’-Wl,--export-dynamic’.

-mlb-path-map file
  Use file as an ML Basis path map to define additional MLB path variables. Multiple uses of -mlb-path-map and -mlb-path-var are allowed, with variable definitions in later path maps taking precedence over earlier ones.

-mlb-path-var ’name value
  Define an additional MLB path variable. Multiple uses of -mlb-path-map and -mlb-path-var are allowed, with variable definitions in later path maps taking precedence over earlier ones.

-output file
  Specify the name of the final output file. The default name is the input file name with its suffix removed and an appropriate, possibly empty, suffix added.

-profile {no|alloc|count|time}
  Produce an executable that gathers profiling data. When such an executable is run, it will produce an mlmon.out file. The man page on mlprof describes how to extract information from this file.

-profile-branch {false|true}
  If true, the profiler will separately gather profiling data for each branch of a function definition, case expression, and if expression.

-profile-stack {false|true}
  If true, the profiler will gather profiling data for all functions on the stack, not just the currently executing function.

-profile-val {false|true}
  If true, the profiler will separately gather profiling data for each (expansive) val declaration.

-runtime arg
  Pass argument to the runtime system via @MLton. The argument will be processed before other @MLton command line switches. Multiple uses of -runtime are allowed, and will pass all the arguments in order. If the same runtime switch occurs more than once, then the last setting will take effect. There is no need to supply the leading @MLton or the trailing --; these will be supplied automatically.

An argument to -runtime may contain spaces, which will cause the argument to be treated as a sequence of words by the runtime. For example, the command line: mlton -runtime ’ram-slop 0.4’ foo.sml will cause foo to run as if it had been called like foo @MLton ram-slop 0.4 --

An executable created with -runtime stop doesn’t process any @MLton arguments. This is useful to create an executable, e.g. echo, that must treat @MLton like any other command-line argument. % mlton -runtime stop echo.sml % echo @MLton -- @MLton --

-show-basis file
  Pretty print to file the basis defined by the input program.

-show-def-use file
  Output def-use information to file. Each identifier that is defined appears on a line, followed on subsequent lines by the position of each use.

-stop {f|g|o|tc}
  Specify when to stop. f list of files on stdout (only makes sense when input is
g generated .c and .s files
o object (.o) files
tc after type checking If you compile -stop g or -stop o, you can resume compilation by running MLton on the generated .c and .s or .o files.

-target {self|...}
  Generate an executable that runs on the specified platform. The default is self, which means to compile for the machine that MLton is running on. To use any other target, you must first install a cross compiler. See the MLton Guide for details.

-target-as-opt target option
  Like -as-opt, this passes option to gcc when assembling, except it only passes option when the target architecture or operating system is target. Valid values for target are: alpha, amd64, arm,, hppa, ia64, m68k, mips, powerpc, powerpc64, s390, sparc, x86, aix, cygwin, darwin, freebsd, hurd, hpux, linux, mingw, netbsd, openbsd, solaris.

-target-cc-opt target option
  Like -cc-opt, this passes option to gcc when compiling C code, except it only passes option when the target architecture or operating system is target. Valid values for target are as for -target-as-opt.

-target-link-opt target option
  Like -link-opt, this passes option to gcc when linking, except it only passes option when the target architecture or operating system is target. Valid values for target are as for -target-as-opt.

-verbose {0|1|2|3}
  How verbose to be about what passes are running. The default is 0. 0 silent
1 calls to compiler, assembler, and linker
2 1, plus intermediate compiler passes
3 2, plus some data structure sizes

Runtime system options

Executables produced by MLton take command line arguments that control the runtime system. These arguments are optional, and occur before the executable’s usual arguments. To use these options, the first argument to the executable must be @MLton. The optional arguments then follow, must be terminated by --, and are followed by any arguments to the program. The optional arguments are not made available to the SML program via CommandLine.arguments. For example, a valid call to hello-world is: hello-world @MLton gc-summary fixed-heap 10k -- a b c In the above example, CommandLine.arguments () = ["a", "b", "c"].

It is allowed to have a sequence of @MLton arguments, as in: hello-world @MLton gc-summary -- @MLton fixed-heap 10k -- a b c

Run-time options can also control MLton, as in mlton @MLton fixed-heap 0.5g -- foo.sml

fixed-heap x{k|K|m|M|g|G}
  Use a fixed size heap of size x, where x is a real number and the trailing letter indicates its units. k or K 1024
m or M 1,048,576
g or G 1,073,741,824 A value of 0 means to use almost all the RAM present on the machine.

The heap size used by fixed-heap includes all memory allocated by SML code, including memory for the stack (or stacks, if there are multiple threads). It does not, however, include any memory used for code itself or memory used by C globals, the C stack, or malloc.

  Print a message at the start and end of every garbage collection.

  Print a summary of garbage collection statistics upon program termination.

load-world world
  Restart the computation with the file specified by world, which must have been created by a call to by the same executable. See the MLton Guide for details.

max-heap x{k|K|m|M|g|G}
  Run the computation with an automatically resized heap that is never larger than x, where x is a real number and the trailing letter indicates the units as with fixed-heap. The heap size for max-heap is accounted for as with fixed-heap.

may-page-heap {false|true}
  Enable paging the heap to disk when unable to grow the heap to a desired size.

  Disable load-world. This can be used as an argument to the compiler via -runtime no-load-world to create executables that will not load a world. This may be useful to ensure that set-uid executables do not load some strange world.

ram-slop x
  Multiply x by the amount of RAM on the machine to obtain what the runtime views as the amount of RAM it can use. Typically x is less than 1, and is used to account for space used by other programs running on the same machine.

stop Causes the runtime to stop processing @MLton arguments once the next -- is reached. This can be used as an argument to the compiler via -runtime stop to create executables that don’t process any @MLton arguments.


MLton’s type error messages are not in a form suitable for processing by Emacs. For details on how to fix this, see


mlprof(1) and the MLton Guide.
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--> MLTON (1) June 8, 2010

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