[Richard Suchenwirth] 2002-11-27 - A visitor in the [Tcl chatroom] asked for a modified [foreach] to cover all combinations of the input lists. In database technical terms, this is called a "nested-loop join", at least one functional programming tutorial tells me so.

Well, and reason enough to do some Tcl'ing (where [metaprogramming] is as trivial as virtual destructors in other languages ;-) - from the input, build up a nested command of foreaches, insert the specified body in the middle, and presto:

 proc nljoin args {
    set body [lindex $args end]
    set cmd ""
    foreach {vars list} [lrange $args 0 end-1] {
       append cmd "[list foreach $vars $list] \{\n"
       append body "\}"
    }
    uplevel 1 $cmd$body
 }
#------------------------- Testing:
 % nljoin x {a b} y {c d} {puts x:$x,y:$y}
 x:a,y:c
 x:a,y:d
 x:b,y:c
 x:b,y:d
No error-checking yet - the length of ''args'' must be odd and >2. Caveat user.

Filtering the lists in advance reduces the runtime needed - for instance, after an example in  [[?]], to get the list of all professors who have published since 1990, using the fancy ''all'' [list comprehension]:
 nljoin pub  [all x from $publications where {$date($x)>=1990}] \
        prof [all x from $staff where {$status($x)=="professor"}] {
            if {$empID($pub)==$empID($prof)} {
                lappend res $prof
            }
        }
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Come to think, what this does is just iterate over the [Cartesian product of a list of lists]...

A join is not the same as a cartesian product.  Joining a table of 3 records with one of 5 does not necessarily produce a table of 15 records. -[jcw]
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[Category Concept] | [Arts and crafts of Tcl-Tk programming]