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better verbiage, add more commands.
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Poor Yorick
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@ -49,8 +49,10 @@ programming in the best way. Even Lisp is more syntactically heavy than Tcl.
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# Every line is a command. The first word is the name of the command, and
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# subsequent words are arguments to the command. Words are delimited by
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# whitespace. Since every word is a string, no escaping is necessary in the
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# simple case.
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# whitespace. Since every word is a string, in the simple case no special
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# markup such as quotes, braces, or backslash, is necessary. Even when quotes
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# are used, they are not a string constructor, but just another escaping
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# character.
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set greeting1 Sal
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set greeting2 ut
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@ -58,27 +60,34 @@ set greeting3 ations
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#semicolon also delimits commands
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set greeting1 Sal; set greeting2 ut; set greeting3 ations
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# Dollar sign introduces variable substitution
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set greeting $greeting1$greeting2
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set greeting $greeting1$greeting2$greeting3
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# Bracket introduces command substitution
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# Bracket introduces command substitution. The result of the command is
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# substituted in place of the bracketed script. When the "set" command is
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# given only the name of a variable, it returns the value of that variable.
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set greeting $greeting1$greeting2[set greeting3]
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set greeting $greeting[set greeting3]
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# Command substitution should really be called script substitution, because an
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# entire script, not just a command, can be placed between the brackets. The
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# "incr" command increments the value of a variable and returns its value.
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set greeting $greeting[
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incr i
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incr i
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incr i
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]
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# backslash suppresses the special meaning of characters
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set amount \$16.42
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# backslash adds special meaning to certain characters
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puts lots\nof\n\n\n\n\n\nnewlines
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@ -89,55 +98,48 @@ set somevar {
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brace remains uninterpreted
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}
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# In a word enclosed in double quotes, whitespace characters lose their special
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# meaning
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set name Neo
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set greeting "Hello, $name"
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#variable names can be any string
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set {first name} New
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# The brace form of variable substitution handles more complex variable names
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set greeting "Hello, ${first name}"
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# The "set" command can always be used instead of variable substitution
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set greeting "Hello, [set {first name}]"
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# To promote the words within a word to individual words of the current
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# command, use the expansion operator, "{*}".
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set {*}{name Neo}
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# is equivalent to
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set name Neo
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# An array is a special variable that is a container for other variables.
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set person(name) Neo
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set person(gender) male
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set greeting "Hello, $person(name)"
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# A namespace holds commands and variables
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# A namespace holds commands and variables
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namespace eval people {
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namespace eval person1 {
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set name Neo
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}
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}
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#The full name of a variable includes its enclosing namespace(s), delimited by two colons:
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#The full name of a variable includes its enclosing namespace(s), delimited by two colons:
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set greeting "Hello $people::person::name"
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@ -146,20 +148,19 @@ set greeting "Hello $people::person::name"
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## 3. A Few Notes
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################################################################################
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# From this point on, there is no new syntax. Everything else there is to
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# learn about Tcl is about the behaviour of individual commands, and what
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# meaning they assign to their arguments.
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# All other functionality is implemented via commands. From this point on,
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# there is no new syntax. Everything else there is to learn about Tcl is about
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# the behaviour of individual commands, and what meaning they assign to their
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# arguments.
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# All other functionality is implemented via commands. To end up with an
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# interpreter that can do nothing, delete the global namespace. It's not very
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# useful to do such a thing, but it illustrates the nature of Tcl.
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# To end up with an interpreter that can do nothing, delete the global
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# namespace. It's not very useful to do such a thing, but it illustrates the
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# nature of Tcl.
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namespace delete ::
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# Because of name resolution behaviour, its safer to use the "variable" command to declare or to assign a value to a namespace.
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namespace eval people {
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namespace eval person1 {
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variable name Neo
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@ -168,7 +169,6 @@ namespace eval people {
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# The full name of a variable can always be used, if desired.
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set people::person1::name Neo
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@ -178,7 +178,6 @@ set people::person1::name Neo
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################################################################################
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# Math can be done with the "expr" command.
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set a 3
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set b 4
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set c [expr {$a + $b}]
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@ -189,56 +188,52 @@ set c [expr {$a + $b}]
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# The "expr" command understands variable and command substitution
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set c [expr {$a + [set b]}]
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# The "expr" command provides a set of mathematical functions
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set c [expr {pow($a,$b)}]
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# Mathematical operators are available as commands in the ::tcl::mathop
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# namespace
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::tcl::mathop::+ 5 3
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# Commands can be imported from other namespaces
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namespace import ::tcl::mathop::+
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set result [+ 5 3]
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# New commands can be created via the "proc" command.
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proc greet name {
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return "Hello, $name!"
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}
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#multiple parameters can be specified
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proc greet {greeting name} {
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return "$greeting, $name!"
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}
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# As noted earlier, braces do not construct a code block. Every value, even
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# the third argument of the "proc" command, is a string. The previous command
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# could be defined without using braces at all:
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# rewritten to not use braces at all:
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proc greet greeting\ name return\ \"Hello,\ \$name!
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proc greet name return\ \"Hello,\ \$name!
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# When the last parameter is the literal value, "args", it collects all extra
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# arguments when the command is invoked
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proc fold {cmd args} {
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set res 0
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foreach arg $args {
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set res [cmd $res $arg]
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}
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}
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fold ::tcl::mathop::* 5 3 3 ;# -> 45
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# Conditional execution is implemented as a command
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if {3 > 4} {
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puts {This will never happen}
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} elseif {4 > 4} {
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@ -250,7 +245,6 @@ if {3 > 4} {
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# Loops are implemented as commands. The first, second, and third
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# arguments of the "for" command are treated as mathematical expressions
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for {set i 0} {$i < 10} {incr i} {
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set res [expr {$res + $i}]
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}
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@ -258,7 +252,6 @@ for {set i 0} {$i < 10} {incr i} {
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# The first argument of the "while" command is also treated as a mathematical
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# expression
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set i 0
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while {$i < 10} {
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incr i 2
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@ -266,14 +259,14 @@ while {$i < 10} {
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# A list is a specially-formatted string. In the simple case, whitespace is sufficient to delimit values
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set amounts 10\ 33\ 18
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set amount [lindex $amounts 1]
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# Braces and backslash can be used to format more complex values in a list.
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# There are three items in the following
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# Braces and backslash can be used to format more complex values in a list. A
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# list looks exactly like a script, except that the newline character and the
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# semicolon character lose their special meanings. This feature makes Tcl
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# homoiconic. There are three items in the following list.
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set values {
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one\ two
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@ -286,8 +279,7 @@ set values {
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# Since a list is a string, string operations could be performed on it, at the
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# risk of corrupting the list.
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# risk of corrupting the formatting of the list.
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set values {one two three four}
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set values [string map {two \{} $values] ;# $values is no-longer a \
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properly-formatted listwell-formed list
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@ -295,12 +287,10 @@ set values [string map {two \{} $values] ;# $values is no-longer a \
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# The sure-fire way to get a properly-formmated list is to use "list" commands
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set values [list one \{ three four]
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lappend values { } ;# add a single space as an item in the list
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# Use "eval" to evaluate a value as a script
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eval {
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set name Neo
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set greeting "Hello, $name"
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@ -309,20 +299,17 @@ eval {
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# A list can always be passed to "eval" as a script composed of a single
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# command.
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eval {set name Neo}
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eval [list set greeting "Hello, $name"]
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# Therefore, when using "eval", use [list] to build up a desired command
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set command {set name}
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lappend command {Archibald Sorbisol}
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eval $command
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# A common mistake is not to use list functions
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# A common mistake is not to use list functions when building up a command
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set command {set name}
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append command { Archibald Sorbisol}
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eval $command ;# There is an error here, because there are too many arguments \
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@ -330,7 +317,6 @@ eval $command ;# There is an error here, because there are too many arguments \
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# This mistake can easily occur with the "subst" command.
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set replacement {Archibald Sorbisol}
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set command {set name $replacement}
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set command [subst $command]
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@ -340,7 +326,6 @@ eval $command ;# The same error as before: to many arguments to "set" in \
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# The proper way is to format the substituted value using use the "list"
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# command.
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set replacement [list {Archibald Sorbisol}]
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set command {set name $replacement}
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set command [subst $command]
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@ -348,24 +333,80 @@ eval $command
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# It is extremely common to see the "list" command being used to properly
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# format values that are substituted into Tcl script templates. There is an
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# example of this in the following replacement "while" implementation.
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# format values that are substituted into Tcl script templates. There are
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# several examples of this, below.
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# The "apply" command evaluates a string as a command.
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set cmd {{greeting name} {
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return "$greeting, $name!"
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}}
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apply $cmd Whaddup Neo
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# The "uplevel" command evaluates a script in some enclosing scope.
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proc greet {} {
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uplevel {puts "$greeting, $name"}
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}
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proc set_double {varname value} {
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if {[string is double $value]} {
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uplevel [list variable $varname $value]
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} else {
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error [list {not a double} $value]
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}
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}
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# The "upvar" command links a variable in the current scope to a variable in
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# some enclosing scope
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proc set_double {varname value} {
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if {[string is double $value]} {
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upvar 1 $varname var
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set var $value
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} else {
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error [list {not a double} $value]
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}
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}
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#get rid of the built-in "while" command.
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rename ::while {}
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# Define a new while command with the "proc" command. More sophisticated error
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# handling is left as an exercise.
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proc while {condition script} {
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if {[uplevel 1 [list expr $condition]]} {
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uplevel 1 $script
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tailcall [namespace which while] $condition $script
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}
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}
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# The "coroutine" command creates a separate call stack, along with a command
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# to enter that call stack. The "yield" command suspends execution in that
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# stack.
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proc countdown {} {
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#send something back to the initial "coroutine" command
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yield
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set count 3
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while {$count > 1} {
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yield [incr count -1]
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}
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return 0
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}
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coroutine countdown1 countdown
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coroutine countdown2 countdown
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puts [countdown 1] ;# -> 2
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puts [countdown 2] ;# -> 2
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puts [countdown 1] ;# -> 1
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puts [countdown 1] ;# -> 0
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puts [coundown 1] ;# -> invalid command name "countdown1"
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puts [countdown 2] ;# -> 1
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```
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## Reference
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