A filter is a pipeline that produces data which represents an edited version of its input.

See Also

the filter idiom
Tcl's simple syntax for reading and writing to stdin

and stdout makes it fairly easy to write these types of programs

How can I run data through an external filter?
a wrapper to handle the details of pipeline filtering:

Filter in functional programming

RS 2005-03-08: filter is also a classic operator in functional programming, and it makes nice simple code:

proc filter {list script} {
    set res {}
    foreach e $list {if {[uplevel 1 $script $e]} {lappend res $e}}
    set res

Testing, with a simple way to do intersection:

proc in {list e} {expr {[lsearch -exact $list $e]>=0}}

% filter {a b c} {in {b c d}}
b c

lpz 2013-06-09: The elements I use have spaces, so I changed the filter routine to:

proc filter {list script} {
    set res {}
    foreach e $list {if {[uplevel 1 $script [list $e]]} {lappend res $e}}
    set res


% filter { {s pace} x d}  {in {{s pace} a b c d e}}
{s pace} d

smh2 2006-08-26: Here is a routine based on this to find the intersection of many lists:

proc intersection { items } {
    set res [lindex $items 0]             ;    # start with the first sub-list
    foreach item [lrange $items 1 end] {  ;    # loop through the rest
        set res [filter $res {in $item}]   ;    # and filter each new subset against the next
    set res

AMG: Implementation using lcomp:

proc filter {list test_expression} {
    lcomp {$x} for x in $list if $test_expression

test_expression is a boolean if expression that should be a function of $x, the list element being tested.