Internationalization of Haskell programs using gettext

Most common in the GNU world approach to internationalization (i18n) of software is to use GNU gettext utilities. In this tutorial we will create simple "Hello world" program, with multilingual support.

Prepare program to internationalization

Consider we want to make the following program multilingual (file Main.hs):

module Main where
    
import IO 

main = do
  putStrLn "Please enter your name:"
  name <- getLine
  putStrLn $ "Hello, " ++ name ++ ", how are you?"

First of all, wrap all strings you want to translate in function __:

module Main where
    
import IO 

__ = id

main = do
  putStrLn (__ "Please enter your name:")
  name <- getLine
  putStrLn $ (__ "Hello, ") ++ name ++ (__ ", how are you?")

We will return to the definition of __ a bit later, for now leave this function as a no-op (id).

Translate

The next step is to generate a POT file (a template which contain all strings needing translation). For Python, C, C++ and Scheme languages there is the xgettext utility, but it doesn't support Haskell. On Hackage you can download hgettext library and utilities, which process haskell source files in the same way as xgettext does for C/C++ files:

cabal install --global hgettext

Now run this from the directory where your project is:

hgettext -k __ -o messages.pot Main.hs

This gathers all strings marked by the function __ from the file Main.hs and writes everything to messages.pot.

Now look at the resulting pot file:

# Translation file
 
msgid ""
msgstr ""
 
"Project-Id-Version: PACKAGE VERSION\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2009-01-13 06:05-0800\n"
"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
"Last-Translator: FULL NAME \n"
"Language-Team: LANGUAGE \n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"

#: Main.hs:0
msgid "Please enter your name:"
msgstr ""

#: Main.hs:0
msgid "Hello, "
msgstr ""

#: Main.hs:0
msgid ", how are you?"
msgstr ""

We are interested in the bottom part of this file (starting from '#: Main.hs:...'). Here we can see pairs of lines: msgid and msgstr: msgid is the original text from the code, and msgstr is the translated string. Each language should have its own translation file. I will create two translations, German and English.

To create a PO file for specific locale we should use the msginit utility:

To generate the German translation file template run:

msginit --input=messages.pot --locale=de.UTF-8

And for the English translation run:

msginit --input=messages.pot --locale=en.UTF-8

If we look at the generated files (en.po and de.po), we will see that the English translation is completely filled, we have only to edit the German PO file. So fill it with the following strings:

#: Main.hs:0
msgid "Please enter your name:"
msgstr "Wie heißen Sie?"

#: Main.hs:0
msgid "Hello, "
msgstr "Hallo, "

#: Main.hs:0
msgid ", how are you?"
msgstr ", wie geht es Ihnen?"

Install translation files

Now we have to create the directories where these translations should be placed. By default all translation files are placed on /usr/share/locale/ folder, but we are free to select different places. Run:

mkdir -p {de,en}/LC_MESSAGES

It will create two directories in the current directory, de and en, that contain LC_MESSAGES. Now use msgfmt tool to encode our po files to mo files (binary translation files):

msgfmt --output-file=en/LC_MESSAGES/hello.mo en.po
msgfmt --output-file=de/LC_MESSAGES/hello.mo de.po

Enable internationalization in the code

As the final step we have to modify the code to support the internationalization:

module Main where
    
import IO 
import Text.I18N.GetText
import System.Locale.SetLocale
import System.IO.Unsafe

__ :: String -> String
__ = unsafePerformIO . getText

main = do
  setLocale LC_ALL (Just "") 
  bindTextDomain "hello" "." 
  textDomain "hello" 

  putStrLn (__ "Please enter your name:")
  name <- getLine
  putStrLn $ (__ "Hello, ") ++ name ++ (__ ", how are you?")

Here we added three initialization commands:

setLocale LC_ALL (Just "") 
bindTextDomain "hello" "." 
textDomain "hello"

The first one (you'll have to download setlocale package to enable this function) sets the current locale to a default value. The next two functions tell gettext to take the "hello.mo" message file from the locale directory (I set it to ".", but, in the general case, this directory should be passed from the package configuration).

The final step — define function __. It simply calls getText from the module Text.I18N.GetText, but its type is String -> IO String so here we used unsafePerformIO to make it simpler to call.

Run the program

Now you can build and try this program in different locales:

user> ghc --make Main.hs
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...

user> LOCALE=en_US.UTF-8 ./Main
Please enter your name:
Bond
Hello, Bond, how are you?

user> LOCALE=de_DE.UTF-8 ./Main
Wie heißen Sie?
Bond
Hallo, Bond, wie geht es Ihnen?

user>

Distribute internationalized cabal package

TBD