[Haskell] Re: I18N, external strings

apfelmus at quantentunnel.de apfelmus at quantentunnel.de
Thu Nov 16 09:51:53 EST 2006

Johannes Waldmann wrote:
> What methods and tools are there for i18n of Haskell programs?
> (I.e. I want output in several languages,
> with the language configurable at runtime,
> and I want to add languages without recompilation.)
> a typical source text (for my application) is here
> it contains (German) text strings all over the place.
> In Java/Eclipse, I would "Source -> Externalize Strings"
> and this replaces each string by something like
> Messages.getString("Foobar.0")  where  Messages  is a global variable
> (constructed at program start from reading a properties file)
> In Haskell, I see at least two problems:
> a) reading the file is in IO
> b) there are no "global variables". implicit parameters perhaps?
> c) when I'm trying to be clever, I use "deriving Show/Read" or similar.
> then i18n should rename the constructors/accessors? I rather not.

Fortunately, a) is unavoidable in general :) But assuming that the
language loading only happens at startup, I'd go for an unsafePerformIO:

    module Main where
    main = do
         language <- getArgs
         initInternationalization language

    module Internationalization where

    initInternationalization lang = do
        writeIORef stringTable ...

    {-# NOINLINE stringTable #-}
    stringTable :: IORef (Data.Map String String)
    stringTable =
        unsafePerformIO $ newIORef $ Map.empty

    {-# NOINLINE getString #-}
    getString :: String -> String
    getString = unsafePerformIO $ do
            map <- readIORef stringTable
            return $ \s -> fromJust' s $ lookup s map
        fromJust' (Just x) _ = x
        fromJust' Nothing  s = "[untranslated message] " ++ s

You can use (getString) just as you would use
(Messages.getString("...")). Note that the language is fixed after first
call to getString and it's not wise to call getString before
initInternationalization. There is no way to change the language after
startup which is probably what you want. If not, then you just have to
float (\s ->) out of unsafePerformIO:

    getString = \s -> unsafePerformIO $ do ... return $ fromJust' ...

Of course, you can also integrate the initialization into (getString).


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