Is there a mechanism whereby I can create constants or even better, enumerated types within Tcl?
KBK: In Tcl, everything is a string, so you can simply use your names as constants. If you want to assign numeric values, you can do something like:
set fruits { apple blueberry cherry date elderberry }
set i 0
foreach fruit $fruits {
set fruit_number($fruit) $i
incr i
}
proc fruit_to_number { fruit } {
variable fruit_number
if { [catch { set fruit_number($fruit) } number] } {
return -code error "$fruit: no such fruit"
} else {
return $number
}
}
proc number_to_fruit { number } {
variable fruits
if { [catch {
if { $number < 0 || $number >= [llength $fruits] } {
error {out of range}
lindex $fruits $number
} fruit] } {
return -code error "no fruit with number $number"
} else {
return $fruit
}
}DKF adds: I reckon it is easier to just load up a pair of arrays to map in each direction, and then use them directly, which we can dress up like this...
proc makeEnum {type identifiers} {
upvar #0 ${type}_number a1 number_${type} a2
set n 0
foreach id $identifiers {
incr n
set a1($id) $n
set a2($n) $id
}
proc ${type}_to_number $type "
upvar #0 ${type}_number ary
if {\[catch {set ary(\$$type)} num\]} {
return -code error \"unknown $type \\\"\$$type\\\"\"
}
return \$num
"
proc number_to_${type} {number} "
upvar #0 number_${type} ary
if {\[catch {set ary(\$number)} $type\]} {
return -code error \"no $type for \\\"\$number\\\"\"
}
return \$$type
"
}
makeEnum fruit {apple blueberry cherry date elderberry}KBK: I like the 'makeEnum' syntax, but [lindex] is faster than an array search, at least in Tcl 8.3.2. Also, was there a reason you switched from zero-based to one-based indexing? (DKF: no, it just came out that way. :^)
Consider the alternative implementation:
proc makeEnumKBK {type identifiers} {
upvar #0 ${type}_number a1
set n 0
foreach id $identifiers {
set a1($id) $n
lappend list $id
incr n
}
proc ${type}_to_number $type "
upvar #0 ${type}_number ary
if {\[catch {set ary(\$$type)} num\]} {
return -code error \"unknown $type \\\"\$$type\\\"\"
}
return \$num
"
proc number_to_${type} {number} "
if { \[catch {
if { \$number < 0 || \$number >= [llength $list] } {
error {out of range}
}
lindex [list $list] \$number
} $type\] } {
return -code error \"no $type for \\\"\$number\\\"\"
}
return \$$type
"
}
makeEnumKBK froot {apple blueberry cherry date elderberry}Running comparative timings on my machine shows that the array+list implementation is significantly faster, mostly because it avoids the cost of the [upvar]:
Time (us; 550 MHz PIII)
Action
Two arrays Array + list
------------------------------------------------------------
Convert enum to number 23 23
Try to convert nonexistent
enum to number 80 81
Convert number to enum 23 13
Try to convert out-of-range
number to enum 79 60
Try to convert non-number to enum 80 65
------------------------------------------------------------