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Adding keyword parameters to Tcl procs

Published on 2025-04-21 Tags: tcl, programming

编程语言中，有两件事让我特别恼火：一是缺少关键字（可选、始终命名且与顺序无关）参数，二是语言内置功能对用户不可用。

Tcl 在这方面表现得很糟糕，因为它的许多标准命令都有类似 UNIX 的选项，但这些在 proc () 或 apply () 中都无法使用。

但由于 proc 可以“轻松地”被包装（稍后详细说明），我花了几个小时实现了这个：

source .../util.tcl
namespace path {::util ::tcl::mathop ::tcl::mathfunc}
proc* p {-flag {-opt foo} x args} {
  puts [lmap v {flag opt x args} {string cat "$v=[set $v]"}]
}
p 1 a;        # => flag=0 opt=foo x=1 args=a
p -opt bar -flag 1 a; # => flag=1 opt=bar x=1 args=a
p -flag;       # => flag=0 opt=foo x=-flag args=

# Relatively constrained overhead
proc p1 {x} {}
proc* p2 {-flag {-opt foo} x} {}
timerate {p1 1}; # => 0.127049 µs/# 7870986 # 7870986 #/sec 1000.000 net-ms
timerate {p2 1}; # => 0.794317 µs/# 1258943 # 1258943 #/sec 1000.000 net-ms

还不错吧？正如你在第三个例子中看到的，我甚至实现了 Tcl 的行为，即强制性的位置参数数量用于消除它们与选项的歧义（这就是你如何执行 puts $string 而不必使用 -- 标记保护它的方式）。

实现方式 §

关于前面提到的“轻松地”，我想补充说明一下它的实现过程；绝对 不适合胆小的人。

proc proc* {name args body} {
  if {[llength $args] == 0 || ![string match "-*" [lindex $args 0]]} {
    proc $name $args $body
    return
  }
  while {[llength $args] && [string match "-*" [lindex $args 0]]} {
    set args [lassign $args[set args {}] arg]
    switch [llength $arg] {
      1 { # Flag
        lappend optargs [set var [string range $arg 1 end]]
        lappend optinit 0
        lappend optswitch $arg [list set $var 1]
      }
      2 { # Option
        lassign $arg opt init
        lappend optargs [set var [string range $opt 1 end]]
        lappend optinit $init
        lappend optswitch $opt "set [list $var] \[lindex \$args \[incr i]]"
      }
      default {error "$arg: unknown option format"}
    }
  }
  lappend optswitch default {error "$arg: unknown option"}
  # Count mandatory positional args to disambiguate with options
  set mcount 0
  foreach arg $args {
    if {[llength $arg] != 1 || $arg eq "args"} break
    incr mcount
  }
  tailcall proc $name args [quasiquote {
    lassign `$optinit `@$optargs
    for {set i 0; set end [expr {[llength $args] - `$mcount}]} {$i < $end} {incr i} {
      set arg [lindex $args $i]
      if {![string match "-*" $arg] || $arg eq "--"} break
      switch -- $arg `$optswitch
    }
    apply `[list [list {*}$optargs {*}$args] $body]` \
      `@[join [lmap o $optargs {string cat \$$o}]]` {*}[lrange $args $i end]
  }]
}

你可能会说，还不错。我同意，这里只是一些非常基本的预处理和代码生成。不，问题在于用于进行最终生成的那个不起眼的 quasiquote 命令。

你看，Tcl 的元编程能力很差，因为它缺乏选择性地在字符串中 subst () 的方法，所以每个人都在做什么呢？自己实现基于字符串的模板系统！事实上，我认为一个 hacker 如果不制作类似的东西，就无法使用 Tcl 一段时间（参见 wiki)。

在我的例子中，这是令人厌恶但有效的尝试（忽略 ? 和 move 辅助函数）：

# CL-like quasiquoting, with `$ and `[...]` for unquoting and `@$ and `@[...]` for splicing
# NB: ` chosen for its lack of standard meaning in T