if 0 {[Richard Suchenwirth] 2004-07-24 - [Tk]'s [text] widget has recently
received an -elide feature which I hadn't tried yet. So in this
weekend fun project, I experimented how to implement "folding" parts of a
[text] content. The command ''elidify'' adds buttons in a text widget
wherever an open brace appears at end of line (the typical pattern for
[proc] bodies). If such a button displays "-", clicking it will elide (hide) the
braces and its contents; clicking a "+" button will make the braces and
its contents reappear. This way one can "close" details one is not
interested in, and see more of the grand picture instead.
}
 proc elidify w {
    set from 1.0
    set id 0
    while 1 {
        set here [$w search -regexp {\{$} $from end]
        if {$here eq ""} break
        set n 1
        while {![info complete [$w get $here "$here + $n chars"]]} {
            incr n
        }
        set there [$w index "$here + $n chars"]
        set b $w.[incr id]
        button $b -text - -command "eliToggle $b" -pady 0
        $w window create $here -window $b
        $w tag config $b -elide 0
        $w tag add    $b "$here + 1 char" "$there + 1 char"
        set from [$w index "$there + 2 chars"]
    }
    set id ;# returns the number of elidable sections
 }
# This procedure is called whenever a "folding" button is clicked 
 proc eliToggle w {
    if {[$w cget -text] eq "-"} {
        $w config -text +
        [winfo parent $w] tag config $w -elide 1
    } else {
        $w config -text -
        [winfo parent $w] tag config $w -elide 0
    }
 }
if 0 {Let's now test the whole thing on its own source code:}
 pack [text .t -wrap word] -fill both -expand 1
 set fp [open [info script]]
 .t insert end [read $fp]
 close $fp
 elidify .t

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[Arts and crafts of Tcl-Tk programming]