Whenever the user changes the value of a slider, its callback procedure is invoked. A callback procedure is provided as an initialization argument when each slider is created.

If & occurs in label, it is specially parsed as for button%.

The min-value and max-value arguments specify the range of the slider, inclusive. The init-value argument optionally specifies the slider’s initial value. The sequence [min-value, init-value, max-value] must be non-decreasing. Otherwise, an exn:fail:contract exception is raised.

The callback procedure is called (with the event type 'slider) when the user changes the slider’s value.

The style argument must include either 'horizontal for a horizontal slider going left-to-right, 'upward for a vertical slider going up, or 'vertical for a vertical slider going down (but beware that 'vertical might render with misleading colors on Mac OS, where the system toolkit supports only upward sliders). If style includes 'plain, the slider does not display numbers for its range and current value to the user. If style includes 'vertical-label, then the slider is created with a label above the control; if style does not include 'vertical-label (and optionally includes 'horizontal-label), then the label is created to the left of the slider. If style includes 'deleted, then the slider is created as hidden, and it does not affect its parent’s geometry; the slider can be made active later by calling parent’s add-child method.

The font argument determines the font for the control. For information about the enabled argument, see window<%>. For information about the horiz-margin and vert-margin arguments, see subarea<%>. For information about the min-width, min-height, stretchable-width, and stretchable-height arguments, see area<%>.

Changed in version 1.73 of package gui-lib: Added 'upward as a possible style element.