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add filename: learntcl.tcl
This commit is contained in:
Poor Yorick
parent
aac20efb70
commit
79b730dcaa
@ -1,446 +1,447 @@
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---
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language: Tcl
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contributors:
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- ["Poor Yorick", "http://pooryorick.com/"]
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---
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Tcl was created by [John Ousterhout](http://wiki.tcl.tk/John Ousterout) as a
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reusable scripting language for chip design tools he was creating. In 1997 he
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was awarded the [ACM Software System
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Award](http://en.wikipedia.org/wiki/ACM_Software_System_Award) for Tcl. Tcl
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can be used both as an embeddable scripting language and as a general
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programming language. It can also be used as a portable C library, even in
|
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cases where no scripting capability is needed, as it provides data structures
|
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such as dynamic strings, lists, and hash tables. The C library also provides
|
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portable functionality for loading dynamic libraries, string formatting and
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code conversion, filesystem operations, network operations, and more.
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Various features of Tcl stand out:
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* Convenient cross-platform networking API
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* Fully virtualized filesystem
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* Stackable I/O channels
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* Asynchronous to the core
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* Full coroutines
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* A threading model recognized as robust and easy to use
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If Lisp is a list processor, then Tcl is a string processor. All values are
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strings. A list is a string format. A procedure definition is a string
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format. To achieve performance, Tcl internally caches structured
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representations of these values. The list commands, for example, operate on
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the internal cached representation, and Tcl takes care of updating the string
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representation if it is ever actually needed in the script. The copy-on-write
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design of Tcl allows script authors can pass around large data values without
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actually incurring additional memory overhead. Procedures are automatically
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byte-compiled unless they use the more dynamic commands such as "uplevel",
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"upvar", and "trace".
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Tcl is a pleasure to program in. It will appeal to hacker types who find Lisp,
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Forth, or Smalltalk interesting, as well as to engineers and scientists who
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just want to get down to business with a tool that bends to their will. Its
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discipline of exposing all programmatic functionality as commands, including
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things like loops and mathematical operations that are usually baked into the
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syntax of other languages, allows it to fade into the background of whatever
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domain-specific functionality a project needs. It's syntax, which is even
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lighter that that of Lisp, just gets out of the way.
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```tcl
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#! /bin/env tclsh
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################################################################################
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## 1. Guidelines
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################################################################################
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# Tcl is not Bash or C! This needs to be said because standard shell quoting
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# habits almost work in Tcl and it is common for people to pick up Tcl and try
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# to get by with syntax they know from another language. It works at first,
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# but soon leads to frustration with more complex scripts.
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# Braces are just a quoting mechanism, not a code block constructor or a list
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# constructor. Tcl doesn't have either of those things. Braces are used,
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# though, to escape special characters in procedure bodies and in strings that
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# are formatted as lists.
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################################################################################
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## 2. Syntax
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################################################################################
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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, 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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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$greeting3
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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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# 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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# A word enclosed in braces is not subject to any special interpretation or
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# substitutions, except that a backslash before a brace is not counted when look#ing for the closing brace
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set somevar {
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This is a literal $ sign, and this \} escaped
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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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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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set greeting "Hello $people::person::name"
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################################################################################
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## 3. A Few Notes
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################################################################################
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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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# 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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}
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}
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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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################################################################################
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## 4. Commands
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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 + $b}]
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# Since "expr" performs variable substitution on its own, brace the expression
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# to prevent Tcl from performing variable substitution first. See
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# "http://wiki.tcl.tk/Brace%20your%20#%20expr-essions" for details.
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# The "expr" command understands variable and command substitution
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set c [expr {$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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# rewritten to not use braces at all:
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proc greet greeting\ 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 $arg]
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}
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}
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fold ::tcl::mathop::* 5 3 3 ;# -> 45
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# Conditional execution is implemented as a command
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if {3 > 4} {
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puts {This will never happen}
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} elseif {4 > 4} {
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puts {This will also never happen}
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} else {
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puts {This will always happen}
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}
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# Loops are implemented as commands. The first, second, and third
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# arguments of the "for" command are treated as mathematical expressions
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for {set i 0} {$i < 10} {incr i} {
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set res [expr {$res + $i}]
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}
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# The first argument of the "while" command is also treated as a mathematical
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# expression
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set i 0
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while {$i < 10} {
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incr i 2
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}
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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. 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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{three four}
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five\{six
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}
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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 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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# 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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}
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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 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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to "set" in {set name Archibald Sorbisol}
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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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eval $command ;# The same error as before: to many arguments to "set" in \
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{set name Archibald Sorbisol}
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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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eval $command
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# It is extremely common to see the "list" command being used to properly
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# format values that are substituted into Tcl script templates. There are
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# several examples of this, below.
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# The "apply" command evaluates a string as a command.
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set cmd {{greeting name} {
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return "$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
|
||||
|
||||
[Official Tcl Documentation](http://www.tcl.tk/man/tcl/)
|
||||
|
||||
[Tcl Wiki](http://wiki.tcl.tk)
|
||||
|
||||
[Tcl Subreddit](http://www.reddit.com/r/Tcl)
|
||||
---
|
||||
language: Tcl
|
||||
contributors:
|
||||
- ["Poor Yorick", "http://pooryorick.com/"]
|
||||
filename: learntcl.tcl
|
||||
---
|
||||
|
||||
Tcl was created by [John Ousterhout](http://wiki.tcl.tk/John Ousterout) as a
|
||||
reusable scripting language for chip design tools he was creating. In 1997 he
|
||||
was awarded the [ACM Software System
|
||||
Award](http://en.wikipedia.org/wiki/ACM_Software_System_Award) for Tcl. Tcl
|
||||
can be used both as an embeddable scripting language and as a general
|
||||
programming language. It can also be used as a portable C library, even in
|
||||
cases where no scripting capability is needed, as it provides data structures
|
||||
such as dynamic strings, lists, and hash tables. The C library also provides
|
||||
portable functionality for loading dynamic libraries, string formatting and
|
||||
code conversion, filesystem operations, network operations, and more.
|
||||
Various features of Tcl stand out:
|
||||
|
||||
* Convenient cross-platform networking API
|
||||
|
||||
* Fully virtualized filesystem
|
||||
|
||||
* Stackable I/O channels
|
||||
|
||||
* Asynchronous to the core
|
||||
|
||||
* Full coroutines
|
||||
|
||||
* A threading model recognized as robust and easy to use
|
||||
|
||||
|
||||
If Lisp is a list processor, then Tcl is a string processor. All values are
|
||||
strings. A list is a string format. A procedure definition is a string
|
||||
format. To achieve performance, Tcl internally caches structured
|
||||
representations of these values. The list commands, for example, operate on
|
||||
the internal cached representation, and Tcl takes care of updating the string
|
||||
representation if it is ever actually needed in the script. The copy-on-write
|
||||
design of Tcl allows script authors can pass around large data values without
|
||||
actually incurring additional memory overhead. Procedures are automatically
|
||||
byte-compiled unless they use the more dynamic commands such as "uplevel",
|
||||
"upvar", and "trace".
|
||||
|
||||
Tcl is a pleasure to program in. It will appeal to hacker types who find Lisp,
|
||||
Forth, or Smalltalk interesting, as well as to engineers and scientists who
|
||||
just want to get down to business with a tool that bends to their will. Its
|
||||
discipline of exposing all programmatic functionality as commands, including
|
||||
things like loops and mathematical operations that are usually baked into the
|
||||
syntax of other languages, allows it to fade into the background of whatever
|
||||
domain-specific functionality a project needs. It's syntax, which is even
|
||||
lighter that that of Lisp, just gets out of the way.
|
||||
|
||||
|
||||
|
||||
|
||||
|
||||
```tcl
|
||||
#! /bin/env tclsh
|
||||
|
||||
################################################################################
|
||||
## 1. Guidelines
|
||||
################################################################################
|
||||
|
||||
# Tcl is not Bash or C! This needs to be said because standard shell quoting
|
||||
# habits almost work in Tcl and it is common for people to pick up Tcl and try
|
||||
# to get by with syntax they know from another language. It works at first,
|
||||
# but soon leads to frustration with more complex scripts.
|
||||
|
||||
# Braces are just a quoting mechanism, not a code block constructor or a list
|
||||
# constructor. Tcl doesn't have either of those things. Braces are used,
|
||||
# though, to escape special characters in procedure bodies and in strings that
|
||||
# are formatted as lists.
|
||||
|
||||
|
||||
################################################################################
|
||||
## 2. Syntax
|
||||
################################################################################
|
||||
|
||||
# Every line is a command. The first word is the name of the command, and
|
||||
# subsequent words are arguments to the command. Words are delimited by
|
||||
# whitespace. Since every word is a string, in the simple case no special
|
||||
# markup such as quotes, braces, or backslash, is necessary. Even when quotes
|
||||
# are used, they are not a string constructor, but just another escaping
|
||||
# character.
|
||||
|
||||
set greeting1 Sal
|
||||
set greeting2 ut
|
||||
set greeting3 ations
|
||||
|
||||
|
||||
#semicolon also delimits commands
|
||||
set greeting1 Sal; set greeting2 ut; set greeting3 ations
|
||||
|
||||
|
||||
# Dollar sign introduces variable substitution
|
||||
set greeting $greeting1$greeting2$greeting3
|
||||
|
||||
|
||||
# Bracket introduces command substitution. The result of the command is
|
||||
# substituted in place of the bracketed script. When the "set" command is
|
||||
# given only the name of a variable, it returns the value of that variable.
|
||||
set greeting $greeting1$greeting2[set greeting3]
|
||||
|
||||
|
||||
# Command substitution should really be called script substitution, because an
|
||||
# entire script, not just a command, can be placed between the brackets. The
|
||||
# "incr" command increments the value of a variable and returns its value.
|
||||
set greeting $greeting[
|
||||
incr i
|
||||
incr i
|
||||
incr i
|
||||
]
|
||||
|
||||
|
||||
# backslash suppresses the special meaning of characters
|
||||
set amount \$16.42
|
||||
|
||||
|
||||
# backslash adds special meaning to certain characters
|
||||
puts lots\nof\n\n\n\n\n\nnewlines
|
||||
|
||||
|
||||
# A word enclosed in braces is not subject to any special interpretation or
|
||||
# substitutions, except that a backslash before a brace is not counted when look#ing for the closing brace
|
||||
set somevar {
|
||||
This is a literal $ sign, and this \} escaped
|
||||
brace remains uninterpreted
|
||||
}
|
||||
|
||||
|
||||
# In a word enclosed in double quotes, whitespace characters lose their special
|
||||
# meaning
|
||||
set name Neo
|
||||
set greeting "Hello, $name"
|
||||
|
||||
|
||||
#variable names can be any string
|
||||
set {first name} New
|
||||
|
||||
|
||||
# The brace form of variable substitution handles more complex variable names
|
||||
set greeting "Hello, ${first name}"
|
||||
|
||||
|
||||
# The "set" command can always be used instead of variable substitution
|
||||
set greeting "Hello, [set {first name}]"
|
||||
|
||||
|
||||
# To promote the words within a word to individual words of the current
|
||||
# command, use the expansion operator, "{*}".
|
||||
set {*}{name Neo}
|
||||
|
||||
# is equivalent to
|
||||
set name Neo
|
||||
|
||||
|
||||
# An array is a special variable that is a container for other variables.
|
||||
set person(name) Neo
|
||||
set person(gender) male
|
||||
set greeting "Hello, $person(name)"
|
||||
|
||||
|
||||
# A namespace holds commands and variables
|
||||
namespace eval people {
|
||||
namespace eval person1 {
|
||||
set name Neo
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
#The full name of a variable includes its enclosing namespace(s), delimited by two colons:
|
||||
set greeting "Hello $people::person::name"
|
||||
|
||||
|
||||
|
||||
################################################################################
|
||||
## 3. A Few Notes
|
||||
################################################################################
|
||||
|
||||
# All other functionality is implemented via commands. From this point on,
|
||||
# there is no new syntax. Everything else there is to learn about Tcl is about
|
||||
# the behaviour of individual commands, and what meaning they assign to their
|
||||
# arguments.
|
||||
|
||||
|
||||
# To end up with an interpreter that can do nothing, delete the global
|
||||
# namespace. It's not very useful to do such a thing, but it illustrates the
|
||||
# nature of Tcl.
|
||||
namespace delete ::
|
||||
|
||||
|
||||
# Because of name resolution behaviour, its safer to use the "variable" command to declare or to assign a value to a namespace.
|
||||
namespace eval people {
|
||||
namespace eval person1 {
|
||||
variable name Neo
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
# The full name of a variable can always be used, if desired.
|
||||
set people::person1::name Neo
|
||||
|
||||
|
||||
|
||||
################################################################################
|
||||
## 4. Commands
|
||||
################################################################################
|
||||
|
||||
# Math can be done with the "expr" command.
|
||||
set a 3
|
||||
set b 4
|
||||
set c [expr {$a + $b}]
|
||||
|
||||
# Since "expr" performs variable substitution on its own, brace the expression
|
||||
# to prevent Tcl from performing variable substitution first. See
|
||||
# "http://wiki.tcl.tk/Brace%20your%20#%20expr-essions" for details.
|
||||
|
||||
|
||||
# The "expr" command understands variable and command substitution
|
||||
set c [expr {$a + [set b]}]
|
||||
|
||||
|
||||
# The "expr" command provides a set of mathematical functions
|
||||
set c [expr {pow($a,$b)}]
|
||||
|
||||
|
||||
# Mathematical operators are available as commands in the ::tcl::mathop
|
||||
# namespace
|
||||
::tcl::mathop::+ 5 3
|
||||
|
||||
# Commands can be imported from other namespaces
|
||||
namespace import ::tcl::mathop::+
|
||||
set result [+ 5 3]
|
||||
|
||||
|
||||
# New commands can be created via the "proc" command.
|
||||
proc greet name {
|
||||
return "Hello, $name!"
|
||||
}
|
||||
|
||||
#multiple parameters can be specified
|
||||
proc greet {greeting name} {
|
||||
return "$greeting, $name!"
|
||||
}
|
||||
|
||||
|
||||
# As noted earlier, braces do not construct a code block. Every value, even
|
||||
# the third argument of the "proc" command, is a string. The previous command
|
||||
# rewritten to not use braces at all:
|
||||
proc greet greeting\ name return\ \"Hello,\ \$name!
|
||||
|
||||
|
||||
|
||||
# When the last parameter is the literal value, "args", it collects all extra
|
||||
# arguments when the command is invoked
|
||||
proc fold {cmd args} {
|
||||
set res 0
|
||||
foreach arg $args {
|
||||
set res [cmd $res $arg]
|
||||
}
|
||||
}
|
||||
fold ::tcl::mathop::* 5 3 3 ;# -> 45
|
||||
|
||||
|
||||
# Conditional execution is implemented as a command
|
||||
if {3 > 4} {
|
||||
puts {This will never happen}
|
||||
} elseif {4 > 4} {
|
||||
puts {This will also never happen}
|
||||
} else {
|
||||
puts {This will always happen}
|
||||
}
|
||||
|
||||
|
||||
# Loops are implemented as commands. The first, second, and third
|
||||
# arguments of the "for" command are treated as mathematical expressions
|
||||
for {set i 0} {$i < 10} {incr i} {
|
||||
set res [expr {$res + $i}]
|
||||
}
|
||||
|
||||
|
||||
# The first argument of the "while" command is also treated as a mathematical
|
||||
# expression
|
||||
set i 0
|
||||
while {$i < 10} {
|
||||
incr i 2
|
||||
}
|
||||
|
||||
|
||||
# A list is a specially-formatted string. In the simple case, whitespace is sufficient to delimit values
|
||||
set amounts 10\ 33\ 18
|
||||
set amount [lindex $amounts 1]
|
||||
|
||||
|
||||
# Braces and backslash can be used to format more complex values in a list. A
|
||||
# list looks exactly like a script, except that the newline character and the
|
||||
# semicolon character lose their special meanings. This feature makes Tcl
|
||||
# homoiconic. There are three items in the following list.
|
||||
set values {
|
||||
|
||||
one\ two
|
||||
|
||||
{three four}
|
||||
|
||||
five\{six
|
||||
|
||||
}
|
||||
|
||||
|
||||
# Since a list is a string, string operations could be performed on it, at the
|
||||
# risk of corrupting the formatting of the list.
|
||||
set values {one two three four}
|
||||
set values [string map {two \{} $values] ;# $values is no-longer a \
|
||||
properly-formatted listwell-formed list
|
||||
|
||||
|
||||
# The sure-fire way to get a properly-formmated list is to use "list" commands
|
||||
set values [list one \{ three four]
|
||||
lappend values { } ;# add a single space as an item in the list
|
||||
|
||||
|
||||
# Use "eval" to evaluate a value as a script
|
||||
eval {
|
||||
set name Neo
|
||||
set greeting "Hello, $name"
|
||||
}
|
||||
|
||||
|
||||
# A list can always be passed to "eval" as a script composed of a single
|
||||
# command.
|
||||
eval {set name Neo}
|
||||
eval [list set greeting "Hello, $name"]
|
||||
|
||||
|
||||
# Therefore, when using "eval", use [list] to build up a desired command
|
||||
set command {set name}
|
||||
lappend command {Archibald Sorbisol}
|
||||
eval $command
|
||||
|
||||
|
||||
# A common mistake is not to use list functions when building up a command
|
||||
set command {set name}
|
||||
append command { Archibald Sorbisol}
|
||||
eval $command ;# There is an error here, because there are too many arguments \
|
||||
to "set" in {set name Archibald Sorbisol}
|
||||
|
||||
|
||||
# This mistake can easily occur with the "subst" command.
|
||||
set replacement {Archibald Sorbisol}
|
||||
set command {set name $replacement}
|
||||
set command [subst $command]
|
||||
eval $command ;# The same error as before: to many arguments to "set" in \
|
||||
{set name Archibald Sorbisol}
|
||||
|
||||
|
||||
# The proper way is to format the substituted value using use the "list"
|
||||
# command.
|
||||
set replacement [list {Archibald Sorbisol}]
|
||||
set command {set name $replacement}
|
||||
set command [subst $command]
|
||||
eval $command
|
||||
|
||||
|
||||
# It is extremely common to see the "list" command being used to properly
|
||||
# format values that are substituted into Tcl script templates. There are
|
||||
# several examples of this, below.
|
||||
|
||||
|
||||
# The "apply" command evaluates a string as a command.
|
||||
set cmd {{greeting name} {
|
||||
return "$greeting, $name!"
|
||||
}}
|
||||
apply $cmd Whaddup Neo
|
||||
|
||||
|
||||
# The "uplevel" command evaluates a script in some enclosing scope.
|
||||
proc greet {} {
|
||||
uplevel {puts "$greeting, $name"}
|
||||
}
|
||||
|
||||
proc set_double {varname value} {
|
||||
if {[string is double $value]} {
|
||||
uplevel [list variable $varname $value]
|
||||
} else {
|
||||
error [list {not a double} $value]
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
# The "upvar" command links a variable in the current scope to a variable in
|
||||
# some enclosing scope
|
||||
proc set_double {varname value} {
|
||||
if {[string is double $value]} {
|
||||
upvar 1 $varname var
|
||||
set var $value
|
||||
} else {
|
||||
error [list {not a double} $value]
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
#get rid of the built-in "while" command.
|
||||
rename ::while {}
|
||||
|
||||
|
||||
# Define a new while command with the "proc" command. More sophisticated error
|
||||
# handling is left as an exercise.
|
||||
proc while {condition script} {
|
||||
if {[uplevel 1 [list expr $condition]]} {
|
||||
uplevel 1 $script
|
||||
tailcall [namespace which while] $condition $script
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
# The "coroutine" command creates a separate call stack, along with a command
|
||||
# to enter that call stack. The "yield" command suspends execution in that
|
||||
# stack.
|
||||
proc countdown {} {
|
||||
#send something back to the initial "coroutine" command
|
||||
yield
|
||||
|
||||
set count 3
|
||||
while {$count > 1} {
|
||||
yield [incr count -1]
|
||||
}
|
||||
return 0
|
||||
}
|
||||
coroutine countdown1 countdown
|
||||
coroutine countdown2 countdown
|
||||
puts [countdown 1] ;# -> 2
|
||||
puts [countdown 2] ;# -> 2
|
||||
puts [countdown 1] ;# -> 1
|
||||
puts [countdown 1] ;# -> 0
|
||||
puts [coundown 1] ;# -> invalid command name "countdown1"
|
||||
puts [countdown 2] ;# -> 1
|
||||
|
||||
|
||||
```
|
||||
|
||||
## Reference
|
||||
|
||||
[Official Tcl Documentation](http://www.tcl.tk/man/tcl/)
|
||||
|
||||
[Tcl Wiki](http://wiki.tcl.tk)
|
||||
|
||||
[Tcl Subreddit](http://www.reddit.com/r/Tcl)
|
||||
|
||||
Loading…
Reference in New Issue
Block a user