SML mode - The Emacs SML editing mode

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SML Mode

Interaction Mode


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1 Introduction

SML mode is a major mode for Emacs for editing Standard ML. It has some novel bugs, and some nice features:

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1.1 Contributors to the SML mode

Contributions to the package are welcome. I have limited time to work on this project, but I will gladly add any code that you contribute to me to this package.

Although the history of sml-mode is obscure, it seems that the following persons have made contributions to sml-mode:

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1.2 Getting started

With luck your system administrator will have installed SML mode somewhere convenient, so it will just magically all work—you can skip the rest of this getting started section. Otherwise you will need to tell Emacs where to find all the SML mode .el files, and when to use them. The where is addressed by locating the Lisp code on your Emacs Lisp load path—you may have to create a directory for this, say /home/mjm/elisp, and then insert the following lines in your /home/mjm/.emacs file:

     (add-to-list 'load-path "/home/mjm/elisp")
     (autoload 'sml-mode "sml-mode" "Major mode for editing SML." t)
     (autoload 'run-sml "sml-proc" "Run an inferior SML process." t)

The first line adjusts Emacs' internal search path so it can locate the Lisp source you have copied to that directory; the second and third lines tell Emacs to load the code automatically when it is needed. You can then switch any Emacs buffer into SML mode by entering the command

     M-x sml-mode

It is usually more convenient to have Emacs automatically place the buffer in SML mode whenever you visit a file containing ML programs. The simplest way of achieving this is to put something like

     (add-to-list 'auto-mode-alist '("\\.\\(sml\\|sig\\)\\'" . sml-mode))

also in your .emacs file. Subsequently (after a restart), any files with these extensions will be placed in SML mode buffers when you visit them.

You may want to pre-compile the sml-*.el files (M-x byte-compile-file) for greater speed—byte compiled code loads and runs somewhat faster.

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1.3 Help!

You're reading it. Apart from the on-line info tree (C-h i is the Emacs key to enter the info system—you should follow the brief tutorial if this is unfamiliar), there are further details on specific commands in their documentation strings. Only the most useful SML mode commands are documented in the info tree: to find out more use Emacs' help facilities.

Briefly, to get help on a specific function use C-h f and enter the command name. All (almost all, then) SML mode commands begin with sml-, so if you type this and press <TAB> (for completion) you will get a list of all commands. Another way is to use C-h a and enter the string sml. This is command apropos; it will list all commands with that sub-string in their names, and any key binding they may have in the current buffer. Command apropos gives a one-line synopsis of what each command does.

Some commands are also variables—such things are allowed in Lisp, if not in ML! See Command Index, for a list of (info) documented functions. See Variable Index, for a list of user settable variables to control the behaviour of SML mode.

Before accessing this information on-line from within Emacs you may have to set the variable sml-mode-info. Put in your .emacs file something like:

     (setq sml-mode-info "/home/mjm/info/")

When different from the default this variable should be a string giving the absolute name of the .info file. Then C-c C-i in SML mode (i.e., the command M-x sml-mode-info) will bring up the manual. This help is also accessible from the menu. (Resetting this variable will not be necessary if your site administrator has been kind enough to install SML mode and its attendant documentation in the Emacs hierarchy.)

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2 Editing with SML Mode

Now SML mode provides just a few additional editing commands. Most of the work has gone into implementing the indentation algorithm which, if you think about it, has to be complicated for a language like ML. See Indentation Defaults, for details on how to control some of the behaviour of the indentation algorithm. Principal goodies are the `electric pipe' feature, and the ability to insert common SML forms (macros or templates).

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2.1 On entering SML mode

— Command: sml-mode

This switches a buffer into SML mode. This is a major mode in Emacs. To get out of SML mode the buffer's major mode must be set to something else, like text-mode. See Getting Started, for details on how to set this up automatically when visiting an SML file.

Emacs is all hooks of course. A hook is a variable: if the variable is non-nil it binds a list of Emacs Lisp functions to be run in some order (usually left to right). You can customise SML mode with these hooks:

— Hook: sml-mode-hook

Default: nil

This is run every time a new SML mode buffer is created (or if you type M-x sml-mode). This is one place to put your preferred key bindings. See Configuration, for some examples.

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2.2 Automatic indentation

ML is a complicated language to parse, let alone compile. The indentation algorithm is a little wooden (for some tastes), and the best advice is not to fight it! There are several variables that can be adjusted to control the indentation algorithm (see Customising SML Mode, below).

— Command: indent-for-tab-command

Key: <TAB> This command indents the current line. If you set the indentation of the previous line by hand, indent-for-tab-command will indent relative to this setting.

— Command: indent-region

Key: C-M-\ Indent the current region. Be patient if the region is large (like the whole buffer).

— Command: sml-back-to-outer-indent

Key: M-<TAB> Unindents the line to the next outer level of indentation.

Further indentation commands that Emacs provides (generically, for all modes) that you may like to recall:

As with other language modes, M-; gives you a comment at the end of the current line. The column where the comment starts is determined by the variable comment-column—default is 40, but it can be changed with set-comment-column (on C-x ; by default).

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2.3 Electric features

Electric keys are generally pretty irritating, so those provided by SML mode are fairly muted. The only truly electric key is ;, and this has to be enabled to take effect.

— Command: sml-electric-pipe

Key: M-| When the point is in a `case' statement this opens a new line, indents and inserts | => leaving point just before the double arrow; if the enclosing construct is a `fun' declaration, the newline is indented and the function name copied at the appropriate column. Generally, try it whenever a | is wanted—you'll like it!

— Command: sml-electric-space

Key: M-SPC When the point is after a keyword like `let', this inserts the corresponding predefined skeleton if one exists. Else it just inserts a space. Another way to insert those skeletons is to use sml-insert-form, described below.

— Command: sml-electric-semi

Key: ; Just inserts a semi-colon, usually. The behaviour of this command is governed by the variable sml-electric-semi-mode.

— Variable: sml-electric-semi-mode

Default: nil

If this variable is nil, sml-electric-semi just inserts a semi-colon, otherwise it inserts a semi-colon and a newline, and indents the newline for SML.

— Command: sml-insert-form

Key: C-c <RET> Interactive short-cut to insert common ML forms (a.k.a. macros, or templates). Recognised forms are `let', `local', `case', `abstype', `datatype', `signature', `structure', and `functor'. Except for `let' and `local', these will prompt for appropriate parameters like functor name and signature, etc.. This command prompts in the mini-buffer, with completion.

By default C-c <RET> will insert at point, with the indentation of the current column; if you give a prefix argument (i.e., C-u C-c <RET>) the command will insert a newline first, indent, and then insert the template.

sml-insert-form is also extensible: see Configuration for further details.

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2.4 Indentation defaults

Several variables try to control the indentation algorithm and other features of SML mode. Most of them are still in flux so they are not described here yet. If the default values are not acceptable you can set these variables permanently in your .emacs file. See Configuration, for details and examples.

— Variable: sml-indent-level

Default: 4

This variable controls the block indentation level.

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3 Running ML under Emacs

The most useful feature of SML mode is that it provides a convenient interface to the compiler. How serious users of ML put up with a teletype interface to the compiler is beyond me... but perhaps there are other interfaces to compilers that require one to part with serious money. Such remarks can quickly become dated—in this case, let's hope so!

Anyway, SML mode provides an interaction mode, inferior-sml-mode, where the compiler runs in a separate buffer in a window or frame of its own. You can use this buffer just like a terminal, but it's usually more convenient to mark some text in the SML mode buffer and have Emacs communicate with the sub-process. The features discussed below are syntax-independent, so they should work with a wide range of ML-like tools and compilers. See Process Defaults, for some hints.

inferior-sml-mode is a specialisation of the comint package that comes with Emacs and XEmacs.

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3.1 Starting the compiler

Start your favourite ML compiler with the command

     M-x run-sml

This creates a process interaction buffer that inherits some key bindings from SML mode and from comint (see Shell Mode). Starting the ML compiler adds some functions to SML mode buffers so that program text can be communicated between editor and compiler (see ML Interaction).

The name of the ML compiler is the first thing you should know how to specify:

— Variable: sml-program-name

Default: "sml"

The program to run as ML. You might need to specify the full path name of the program.

— Variable: sml-default-arg

Default: ""

Useful for Poly/ML users who may supply a database file, or others who have wrappers for setting various options around the command to run the compiler. Moscow ML people might set this to "-P full", etc..

The variable sml-program-name is a string holding the name of the program as you would type it at the shell. You can always choose a program different to the default by invoking

     C-u M-x run-sml

With the prefix argument Emacs will prompt for the command name and any command line arguments to pass to the compiler. Thereafter Emacs will use this new name as the default, but for a permanent change you should set this in your .emacs with, e.g.:

     (setq sml-program-name "nj-sml")
— Command: run-sml

Launches ML as an inferior process in another buffer; if an ML process already exists, just switch to the process buffer. A prefix argument allows you to edit the command line to specify the program, and any command line options.

— Hook: inferior-sml-mode-hook

Default: nil

M-x run-sml runs comint-mode-hook and inferior-sml-mode-hook hooks in that order, but after the compiler is started. Use inferior-sml-mode-hook to set any comint buffer-local configurations for SML mode you like.

— Command: switch-to-sml

Key: C-c C-s Switch from the SML buffer to the interaction buffer. By default point will be placed at the end of the process buffer, but a prefix argument will leave point wherever it was before. If you try C-c C-s before an ML process has been started, you'll just get an error message to the effect that there's no current process buffer.

— Command: sml-cd

When started, the ML compiler's default working directory is the current buffer's default directory. This command allows the working directory to be changed, if the compiler can do this. The variable sml-cd-command specifies the compiler command to invoke (see Process Defaults).

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3.2 Speaking to the compiler

Several commands are defined for sending program fragments to the running compiler. Each of the following commands takes a prefix argument that will switch the input focus to the process buffer afterwards (leaving point at the end of the buffer):

— Command: sml-load-file

Key: C-c C-l Send a `use file' command to the current ML process. The variable sml-use-command is used to define the correct template for the command to invoke (see Process Defaults). The default file is the file associated with the current buffer, or the last file loaded if you are in the interaction buffer.

— Command: sml-send-region

Key: C-c C-r Send the current region of text in the SML buffer. sml-send-region-and-go is a similar command for you to bind in SML mode if you wish: it'll send the region and then switch-to-sml.

— Command: sml-send-buffer

Key: C-c C-b Send the contents of the current buffer to ML.

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3.3 Finding errors

SML mode provides one customisable function for locating the source position of errors reported by the compiler. This should work whether you type use "puzzle.sml"; into the interaction buffer, or use one of the mechanisms provided for sending programs directly to the compiler—see ML Interaction.

— Command: next-error

Key: C-x` Jump to the source location of the next error reported by the compiler. All the usual error-navigation commands are available, see see Compilation Mode.

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3.4 Process defaults

The process interaction code is independent of the compiler used, deliberately, so SML mode will work with a variety of ML compilers and ML-based tools. There are therefore a number of variables that may need to be set correctly before SML mode can speak to the compiler. Things are by default set up for Standard ML of New Jersey, but switching to a new system is quite easy.

— Variable: sml-use-command

Default: "use \"%s\""

Use file command template. Emacs will replace the %s with a file name. Note that Emacs requires double quote characters inside strings to be quoted with a backslash.

— Variable: sml-cd-command

Default: "OS.FileSys.chDir \"%s\""

Compiler command to change the working directory. Not all ML systems support this feature (well, Edinburgh (core) ML didn't), but they should.

— Variable: sml-prompt-regexp

Default: "^[-=>#] *"

Matches the ML compiler's prompt: comint uses this for various purposes.

To customise error reportage for different ML compilers you need to set two further variables before next-error can be useful:

— Variable: sml-error-regexp-alist

Alist that specifies how to match errors in compiler output. Each elt has the form (REGEXP FILE-IDX LINE-IDX [COLUMN-IDX FILE-FORMAT...]) If REGEXP matches, the FILE-IDX'th subexpression gives the file name, and the LINE-IDX'th subexpression gives the line number. If COLUMN-IDX is given, the COLUMN-IDX'th subexpression gives the column number on that line. If any FILE-FORMAT is given, each is a format string to produce a file name to try; %s in the string is replaced by the text matching the FILE-IDX'th subexpression.

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4 Configuration Summary

This (sort of pedagogic) section gives more information on how to configure SML mode: menus, key bindings, hooks and highlighting are discussed, along with a few other random topics.

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4.1 Hooks

One way to set SML mode variables (see Indentation Defaults), and other defaults, is through the sml-mode-hook in your .emacs. A simple example:

     (defun my-sml-mode-hook () "Local defaults for SML mode"
       (setq sml-indent-level 2)        ; conserve on horizontal space
       (setq words-include-escape t)    ; \ loses word break status
       (setq indent-tabs-mode nil))     ; never ever indent with tabs
     (add-hook 'sml-mode-hook 'my-sml-mode-hook)

The body of my-sml-mode-hook is a sequence of assignments. In this case it is not really necessary to set sml-indent-level in a hook because this variable is global (most SML mode variables are). With similar effect:

     (setq sml-indent-level 2)

anywhere in your .emacs file. The variable indent-tabs-mode is automatically made local to the current buffer whenever it is set explicitly, so it must be set in a hook if you always want SML mode to behave like this.

Another hook is inferior-sml-mode-hook. This can be used to control the behaviour of the interaction buffer through various variables meaningful to comint-based packages:

     (defun my-inf-sml-mode-hook () "Local defaults for inferior SML mode"
       (add-hook 'comint-output-filter-functions 'comint-truncate-buffer)
       (setq      comint-scroll-show-maximum-output t)
       (setq      comint-input-autoexpand nil))
     (add-hook 'inferior-sml-mode-hook 'my-inf-sml-mode-hook)

Again, the body is a sequence of assignments. Unless you run several ML compilers simultaneously under one Emacs, this hook will normally only get run once. You might want to look up the documentation (C-h v and C-h f) for these buffer-local comint things.

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4.2 Key bindings

Customisation (in Emacs) usually entails putting favourite commands on easily remembered keys. Two `keymaps' are defined in SML mode: one is effective in program text buffers (sml-mode-map) and the other is effective in interaction buffers (inferior-sml-mode-map). The initial design ensures that (many of) the default key bindings from the former keymap will also be available in the latter (e.g., C-c`).

Type C-h m in an SML mode buffer to find the default key bindings (and similarly in an ML interaction buffer), and use the hooks provided to install your preferred key bindings. Given that the keymaps are global (variables):

     (defun my-sml-mode-hook () "Global defaults for SML mode"
       (define-key sml-mode-map "\C-cd" 'sml-cd))
     (add-hook 'sml-mode-hook 'my-sml-mode-hook)

This has the effect of binding sml-cd to the key C-c d. If you want the same behaviour from C-c d in the ML buffer:

     (defun my-inf-sml-mode-hook () "Global defaults for inferior SML mode"
       (define-key inferior-sml-mode-map "\C-cd" 'sml-cd)
       ;; NB. for SML/NJ '96
       (setq sml-cd-command "OS.FileSys.chDir \"%s\""))
     (add-hook 'inferior-sml-mode-hook 'my-inf-sml-mode-hook)

There is nothing to stop you rebuilding the entire keymap for SML mode and the ML interaction buffer in your .emacs of course: SML mode won't define sml-mode-map or inferior-sml-mode-map if you have already done so.

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4.3 Syntax colouring

Highlighting is very handy for picking out keywords in the program text, spotting misspelled kewyords, and, if you have Emacs' ps-print package installed (you usually do these days), obtaining pretty, even colourful code listings—quite properly for your colourful ML programs.

The indentation scheme (strangely enough) also relies on the highlighting code to properly handle nested comments, which is yet another reason to turn on highlighting. To turn on highlighting, use either of:

     M-x font-lock-mode
     (add-hook 'sml-mode-hook 'turn-on-font-lock)
     (global-font-lock-mode 1)

The first will turn it on in the current buffer. The second will turn it on in all sml-mode buffers. The last will turn it on everywhere. This is valid for Emacs but maybe not for XEmacs. Check font-lock documentation if you encounter problems.

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4.4 Advanced Topics

These forms are bloody useless; can't we have better ones?

You can indeed. sml-insert-form is extensible so all you need to do is create the macros yourself. Define a keybord macro (C-x ( <something> C-x )) and give it a suitable name: sml-addto-forms-alist prompts for a name, say NAME, and binds the macro sml-form-NAME. Thereafter C-c <RET> NAME will insert the macro at point, and C-u C-c <RET> NAME will insert the macro after a newline-and-indent. If you want to keep your macros from one editing session to the next, go to your .emacs file and call insert-kbd-macro; you'll need to add NAME to sml-forms-alist permanently yourself:
     (defun my-sml-mode-hook () "Global defaults for SML mode"
       ;; whatever else you do
       (add-to-list 'sml-forms-alist '("NAME" . FUNCTION)))

If you want to create templates like `case' that prompt for parameters you'll have to do some Lisp programming. The skeleton package is a good stating point. Better yet, you can reuse the wrappers used by sml-mode itself in your sml-mode-hook:

     (add-hook 'sml-mode-hook
       (lambda ()
         (sml-def-skeleton "case" "Case expr: "
           str " of" \n _ " => ")))

This will redefine `case' in order to leave the `of' on the first line. See the documentation of skeleton-insert to get a better understanding of how this works.

I hate that indentation algorithm; can't I tweak it?

Ah, yes, of course, but this manual will not tell you how.

Can SML mode handle more than one compiler running at once?

Sure, just rename the `*sml*' buffer and then use run-sml as usual.

What needs to be done to support other ML compilers?

Not much really. Just add the right regular expressions to sml-error-regexp-alist and that should be all.

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Command Index

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Variable Index

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Key Index

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