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Adding custom datatype in Yesod

I'm using the scaffolded yesod template and want to add a new type for monetary values. Ideally this should be done in one file called Money.hs for example. But this will form cyclic imports:

  • Foundation imports Model.

  • The new Money type will be used in the model to store monetary values. Therefore Model will have to import Money

  • Money will have to import foundation for a moneyField :: Field Handler Money function

TL DR: Is there a "correct" way to add new types to a scaffolded yesod application including sql db type, shakespeare field widget and ToHtml instance?

Solution

  • For your third bullet: why do you need to reference Handler? You can structure your field code the same way as the built in fields in Yesod.Form.Fields, e.g.:

    textField :: Monad m => RenderMessage (HandlerSite m) FormMessage => Field m Text

    With that: I'd say that a Money.hs (or even Data/Money.hs) makes the most sense.