Standard ML of New Jersey Interactive System
This section explains some of the basic elements of using the Standard
ML of New Jersey compiler as an interactive system into which you
enter declarations and expressions or load code from source files.
This is the most accessible mode of using the compiler,
but for any programs of nontrivial size we recommend that
the
Compilation Manager be used to organize the compilation process.
We assume below that you are using the
compiler under Unix. The behavior will be somewhat different under
MS Windows or the Macintosh OS.
Running Standard ML
Type ``sml'' from the shell. This puts you into the
interactive system. The top level prompt is ``-'', and the
secondary prompt (printed when input is incomplete) is ``=''.
If you get the secondary prompt when you don't expect it,
typing ``; return'' will often complete your input, or
typing your interrupt character (e.g., control-C)
will cancel your input and return to the ML top level.
If ``sml'' doesn't work, ask where sml has
been installed on your machine and use the appropriate path name or
redefine your PATH environment variable.
Interactive input
Input to the top level interpreter (i.e., declarations and
expressions) must be terminated by a semicolon (and carriage
return) before the system will evaluate it. The system then prints
out a response indicating the effect of the evaluation.
Expressions are treated as implicit declarations of a standard
variable it. For example,
| - 3; | user input after prompt
|
val it = 3 : int| system response
| |
This means that the value of the last top level expression evaluated
can be referred to using the variable ``it.''
Interrupting compilation or execution
Typing the interrupt character (typically control-C)
should interrupt the compiler and return you
to top level.
Exiting the interactive system.
Typing control-D (EOF) at top level will cause an exit to the shell
(or the parent process from which sml was run). One can also terminate
by calling OS.Process.exit(OS.Process.success).
Loading ML source text from a file
The function use: string -> unit interprets its argument as a
file name relative to sml's current directory and loads the text
from that file as though it had been typed in. This should normally
be executed at top level, but the loaded files can also contain calls
of use to recursively load other files. It is a bad idea to call
use within an expression or declaration, because the effects are
not well-defined.
For industrial-strength multi-module software systems, the
the
Compilation Manager is more appropriate than use.
Error messages
The compiler attempts to recover from syntactic errors so that
it can also produce semantic (type-checking) errors during
the same compilation. Syntactic error recovery is more accurate
for source files loaded by use or CM than it is
from declarations typed directly into
the interactive system -- this is because lookahead is not
possible when text is entered one line at a time.
When compiling files, the error messages include line numbers and
character positions within the line. For example:
- if true
= then 5 true
= else 6;
std_in:7.6-7.11 Error: operator is not a function
operator: int
in expression:
5 true
-
Here the location information std_in:7.6-7.11 indicates that the
erroneous expression ``5 true'' occupies characters 6 through 11
of the 7th line of input from std_in. For simple syntactic
errors this position information is usually accurate or perhaps off by
just one line. For some classes of errors the line numbers may not be very
useful, because they delineate a potentially large declaration
containing the error. If the error occurs in a file being loaded by
use, the line numbers will refer to lines in the file being
loaded.
There are a number of different forms of type error message, and it
may require some practice before you become adept at interpreting
them. The most common form indicates a mismatch between the type of a
function (or operator) and its argument (or operand).
A representation of the offending expression is usually included, but
this is an image of the internal abstract syntax for the expression
and may differ significantly from the original source code. For
instance, an expression ``if e1 then e2 else e3''
is represented internally as a case expression over a boolean value:
``$case e1 of true => e2 | false => e3.''
Compiler interface
Most users will interact with the compiler via the {\it use} command
or the Separate Compilation system. However, direct control of
individual compiler phases is possible through the {\it Compiler}
structure, which is
documented in Section CONTROL(\comp{}).
Printing
The structure Compiler.Control.Print contains several useful flags and
functions with which you can control or redirect compiler error,
diagnostic, and result messages. See PRINTCONTROL(\comp).
You can also control the depth of printing of large structured values.
Interacting with the operating system
The structures IO, OS, and others
contain functions
for such tasks as executing
other programs from within sml and changing
the current directory. These are documented in
The Standard ML Basis Library manual.
Emacs support
The directory contrib/emacs contains packages supporting editing ML
source code and interacting with sml under GNU emacs.