record-wrangler

This package uses TemplateHaskell to alter records in ways that may be convenient for you.

Inspiration

Let’s say you’ve got a set of API types and a set of domain types. The API types define your HTTP contract, and you generically derive ToJSON and FromJSON instances from their field labels. They look like this:

data User = User
    { id :: Int
    , name :: String
    , age :: Int
    }

data Dog = Dog
    { id :: Int
    , name :: String
    , toy :: FavoriteToy
    , ownerId :: Int
    }

The domain types for your business logic look a little different, though. The record fields are prefixed with an _ underscore character, to allow for lens derivation. We can pretend they have other differences that are relevant to internal details and irrelevant for showing off record-wrangler if that makes you feel better.

data Dog = Dog
    { _id :: Int
    , _name :: String
    , _toy :: FavoriteToy
    , _ownerId :: Int
    }

makeLenses ''Dog

data User = User
    { _id :: Int
    , _name :: String
    , _age :: Int
    }

makeLenses ''User

If the record fields were the same, then we could just write the conversion function using RecordWildCards and it’d be fine:

convertUser Api.User{..} = Domain.User{..}

However, because the field labels are different, we have to do it manually:

convertUser Api.User{..} = Domain.User
    { _id = id
    , _name = name
    , _age = age 
    }

This is annoying and boilerplatey. I don’t know about you, but I hate writing code, for every line of code I write is a chance to mess up. Let’s use record-wrangler to make these conversions easier.

wrangle ''Api.User defWrangleOpts 
    { fieldLabelModifier = \fieldStr -> '_' : fieldStr 
    }

This is going to take the Api.User type and modify it slightly. By default, we append a ' to the constructor name, type name, and field labels. Here, we’ve decided to prepend a _ character to the field label. It also generates a function wrangleUserToUser' which converts the old record to the modified one.

With the power of view patterns, our conversion function is now quite concise:

convertUser :: Api.User -> Domain.User
convertUser (wrangleUserToUser' -> User'{..}) = Domain.User{..}