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⎕SM is a system variable that defines a character-based user interface (as opposed to a graphical user interface).  In versions of Dyalog APL that support asynchronous terminals, ⎕SM defines a form that is displayed on the USER SCREEN.  The implementation of ⎕SM in "window" environments is compatible with these versions.  In Dyalog APL/X, ⎕SM occupies its own separate window on the display, but is otherwise equivalent.  In versions of Dyalog APL with GUI support, ⎕SM either occupies its own separate window (as in Dyalog APL/X) or, if it exists, uses the window assigned to the SM object.  This allows ⎕SM to be used in a GUI application in conjunction with other GUI components.

In general ⎕SM is a nested matrix containing between 3 and 13 columns.  Each row of ⎕SM represents a field; each column a field attribute.

The columns have the following meanings:

With the exception of columns 1 and 8, all elements in ⎕SM are integer scalar values.

Elements in column 1 (Field Contents) may be:

• A numeric scalar
• A numeric vector
• A 1-column numeric matrix
• A character scalar
• A character vector
• A character matrix (rank 2)
• A nested matrix defining a sub-form whose structure and contents must conform to that defined for ⎕SM as a whole.  This definition is recursive.  Note however that a sub-form must be a matrix - a vector is not allowed.

Elements in column 8 (Video Attributes) may be:

• An integer scalar that specifies the appearance of the entire field.
• An integer array of the same shape as the field contents.  Each element specifies the appearance of the corresponding element in the field contents.

Screen Management (Async Terminals)

Dyalog APL for UNIX systems on tty devices (async terminals or on terminal emulators) manages two screens; the SESSION screen and the USER screen.  If the SESSION screen is current, an assignment to ⎕SM causes the display to switch to the USER screen and show the form defined by ⎕SM.

Note that the RIDE does not directly support ⎕SM, although it is possible to display ⎕SM in the tty session to which a RIDE client is connected.

If the USER screen is current, any change in the value of ⎕SM is immediately reflected by a corresponding change in the appearance of the display.  However, an assignment to ⎕SM that leaves its value unchanged has no effect.

Dyalog APL automatically switches to the SESSION screen for default output, if it enters immediate input mode (6-space prompt), or through use of ⎕ or ⍞.  This means that typing

      ⎕SM ← expression

in the APL session will cause the screen to switch first to the USER screen, display the form defined by ⎕SM, and then switch back to the SESSION screen to issue the 6-space prompt.  This normally happens so quickly that all the user sees is a flash on the screen.To retain the USER screen in view it is necessary to issue a call to ⎕SR or for APL to continue processing. e.g.

      ⎕SM ← expression  ⋄  ⎕SR 1

or

      ⎕SM ← expression  ⋄  ⎕DL 5

Screen Management (Window Versions)

In Dyalog APL/X, and optionally in Dyalog APL/W, ⎕SM is displayed in a separate USER WINDOW on the screen.  In an end-user application this may be the only Dyalog APL window.  However, during development, there will be a SESSION window, and perhaps EDIT and TRACE windows too.

The USER Window will only accept input during execution of ⎕SR.  It is otherwise "output-only".  Furthermore, during the execution of ⎕SR it is the only active window, and the SESSION, EDIT and TRACE Windows will not respond to user input.

Screen Management (GUI Versions)

In versions of Dyalog APL that provide GUI support, there is a special SM object that defines the position and size of the window to be associated with ⎕SM.  This allows character-mode applications developed for previous versions of Dyalog APL to be migrated to and integrated with GUI environments without the need for a total re-write.

Effect of Localisation

Like all system variables (with the exception of ⎕TRAP) ⎕SM is subject to "pass-through localisation".  This means that a localised ⎕SM assumes its value from the calling environment.  The localisation of ⎕SM does not, of itself therefore, affect the appearance of the display.  However, reassignment of a localised ⎕SM causes the new form to overlay rather than replace whatever forms are defined further down the stack.  The localisation of ⎕SM thus provides a simple method of defining pop-up forms, help messages, etc.

The user may edit the form defined by ⎕SM using the system function ⎕SR.  Under the control of ⎕SR the user may change the following elements in ⎕SM which may afterwards be referenced to obtain the new values.