Difference between revisions of "Internationalization of Haskell programs using Haskell data types"
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(Created page from a haskell-cafe post by Felipe Lessa) |
(Attribute this page to Felipe Lessa) |
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You can use Haskell data types for internationalization. |
You can use Haskell data types for internationalization. |
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+ | The following example is adapted from a |
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+ | [http://www.haskell.org/pipermail/haskell-cafe/2011-September/095715.html haskell-cafe mailing list post] |
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+ | by Felipe Lessa. |
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+ | |||
The idea is to have a data type |
The idea is to have a data type |
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with all your messages, like |
with all your messages, like |
Revision as of 12:47, 3 October 2011
You can use Haskell data types for internationalization. The following example is adapted from a haskell-cafe mailing list post by Felipe Lessa.
The idea is to have a data type with all your messages, like
data Message =
Hello |
WhatsYourName |
MyNameIs String |
Ihave_apples Int
GoodBye
For each of your supported languages, you provide a rendering function (they may be in separate source files):
render_en_US :: Message -> String
render_en_US Hello = "Hello!"
render_en_US WhatsYourName = "What's your name?"
render_en_US (MyNameIs name) = "My name is " ++ name ++ "."
render_en_US (Ihave_apples 0) = "I don't have any apples."
render_en_US (Ihave_apples 1) = "I have one apple."
render_en_US (Ihave_apples n) = "I have " ++ n ++ " apples."
render_en_US GoodBye = "Good bye!"
render_pt_BR :: Message -> String
render_pt_BR Hello = "Olá!"
render_pt_BR WhatsYourName = "Como você se chama?"
render_pt_BR (MyNameIs name) = "Eu me chamo " ++ name ++ "."
render_pt_BR (Ihave_apples 0) = "Não tenho nenhuma maçã."
render_pt_BR (Ihave_apples 1) = "Tenho uma maçã."
render_pt_BR (Ihave_apples 2) = "Tenho uma maçã."
render_pt_BR (Ihave_apples n) = "Tenho " ++ show n ++ " maçãs."
render_pt_BR GoodBye = "Tchau!"
Given those functions, you can construct something like
type Lang = String
render :: [Lang] -> Message -> String
render ("pt" :_) = render_pt_BR
render ("pt_BR":_) = render_pt_BR
render ("en" :_) = render_en_US
render ("en_US":_) = render_en_US
render (_:xs) = render xs
render _ = render_en_US
So r = render ["fr", "pt"]
will do the right thing.
You just need to pass this r
around in your code.
Using is easy and clear:
putStrLn $ r Hello
putStrLn $ r WhatsYourName
name <- getLine
putStrLn $ r MyNameIs "Alice"
putStrLn $ r (Ihave_apples $ length name `mod` 4)
putStrLn $ r GoodBye
This approach is used for internationalization in the Yesod web framework, except that instead of one big data type, some type classes are used.