Command Lookup Tables in Various Languages

Sometimes it's fun to see how an idea translates when coded in several languages. In this article, the central idea is having a table that maps a command name to a function. A runloop prompts the user for a command, looks up the command in the table, and, if it exists, executes the associated function. The python cmd module is a take on this idea. Here we code up something much more rudimentary in python, ruby, and lua.

cmd.py

---

!/usr/bin/env python

import sys 

def connect(host):
    print('connecting to %s...' % (host))

def download(path):
    print('downloading %s...' % (path))

def upload(path):
    print('uploading %s...' % (path))

def disconnect():
    print ('disconnecting...')
    sys.exit(0)

commands = { 
    'connect': connect,
    'download': download,
    'upload': upload,
    'disconnect' : disconnect
}

while True:
    line = raw_input('% ')
    tokens = line.split()
    if not tokens: continue
    cmd = tokens[0]
    args = tokens[1:]
    if cmd in commands:
        try: commands[cmd](*args)
        except TypeError as e:
            print(e)
    else:
        print("cmd '%s' not found" % (cmd))

cmd.rb

---

!/usr/bin/env ruby

def connect(host) 
    puts "connecting to #{host}..."
end

def download(path) 
    puts "downloading #{path}..."
end

def upload(path) 
    puts "uploading #{path}..."
end

def disconnect() 
    puts "disconnecting..."
    exit
end

commands = { 
    'connect' => :connect,
    'download' => :download,
    'upload' => :upload,
    'disconnect' => :disconnect,
}

while true do
    print '% '
    tokens = gets.split
    cmd = tokens[0]
    args = tokens[1..-1]
    if cmd and commands[cmd] then
        begin
            send(commands[cmd], *args)
        rescue ArgumentError => e
            puts e.message
        end 
    else
        puts "cmd '#{cmd}' not found"
    end 
end

cmd.lua

---

#!/usr/bin/env lua

---------------------------------------
-- utility functions
---------------------------------------
local function split(s) 
    local tokens = {}
    for t in string.gmatch(s, "%S+") do
        --print('token: ' .. t)
        tokens[#tokens + 1] = t 
    end 
    return tokens
end

local function printf(fmt, ...)
    if fmt then
        return io.write(string.format(fmt, ...))
    else
        return print()
    end 
end

local function perror(msg)
    -- strip file and line number where error occurs
    local i, j = string.find(msg, ':%d+:%s+')
    if j then
        print(string.sub(msg, j+1, -1))
    else
        print(msg)
    end 
end

---------------------------------------
-- commands
---------------------------------------
local function connect(host)
    if not host then
        error('must specify host')
    end 
    printf('connecting to %s...\n', host)
end

local function download(path)
    if not path then
        error('must specify path')
    end 
    printf('downloading %s...\n', path)
end

local function upload(path)
    if not path then
        error('must specify path')
    end
    printf('uploading %s..\n', path)
end

local function disconnect()
    printf('disconnecting...\n')
    os.exit(0)
end

---------------------------------------
-- command table
---------------------------------------
local commands = {
    connect = connect,
    upload = upload,
    download = download,
    disconnect = disconnect
}

---------------------------------------
-- runloop
---------------------------------------
while true do
    io.write('% ')
    io.flush()
    local line = io.read('*line')
    local tokens = split(line)
    if #tokens > 0 then
        local cmd = tokens[1]
        table.remove(tokens, 1)
        local args = tokens
        if commands[cmd] then
            local status, err = pcall(commands[cmd], unpack(args))
            if not status then
                perror(err)
            end
        else
            printf("cmd '%s' not found\n", cmd)
        end
    end
end