How to prompt a user for input to set a variable

#!/usr/bin/tclsh

# EDIT here to set ::the_pin_name as required:
set ::the_pin_name   motion.analog-in-00
set ::default_value  0

# Note: This file must:
#    have execute permissions (chmod +x this_file_name)
#    be located according to rules for user M codes
#    exist before starting LinuxCNC

#------------------------------------------------------------------
set ::prog [file tail $::argv0]
set ::value $::default_value
package require Hal
package require Tk

proc go {} {
  if [catch {
    puts "$::prog:$::the_pin_name before: [hal getp $::the_pin_name]"
    hal setp $::the_pin_name $::value
    puts "$::prog:$::the_pin_name  after: [hal getp $::the_pin_name]"
  } msg ] {
    set ::value "FIXME"
    popup "ERROR\n\n$msg"
  } else {
    exit 0
  }
}
proc abort {} {
  popup "Aborting Gcode program"
  exit 1
}
proc err_exit {msg} {
  popup $msg
  exit 1
}
proc popup {msg} {
  puts stderr "$::prog: $msg"
  tk_messageBox \
    -title "$::prog:$::the_pin_name" \
    -type ok \
    -message "$msg"
}

wm title . "$::prog pin: $::the_pin_name"
wm protocol . WM_DELETE_WINDOW  {puts "$::prog: window close disallowed"}
pack [label .l -text "Enter Value:"] -side left
pack [entry .e -textvar ::value] -side left
bind .e <Return> go
pack [button .b -text Go -command go] -side left
pack [button .a -text Abort -command abort] -side left

%
; demonstrate user prompt using m131
g0 x0
f100 g1 x1
m66 e0 l0            ; read motion.analog-in-00 pin
(debug, before=#5399)
m131                 ; prompt user for input to motion.analog-in-00 pin
m66 e0 l0           ; read motion.analog-in-00 pin
(debug, after=#5399)
g1 x0                ; continue program
%