record-wrangler: Alter your records with ease

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Dependencies base (>=4.11 && <5), template-haskell [details]
License LicenseRef-Apache
Copyright 2019 Matt Parsons, Lumi
Author Matt Parsons
Category Development
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Source repo head: git clone
Uploaded by parsonsmatt at 2019-04-01T20:14:07Z
Downloads 1032 total (9 in the last 30 days)
Rating 2.0 (votes: 1) [estimated by Bayesian average]
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Status Docs available [build log]
Last success reported on 2019-04-01 [all 1 reports]

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


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{..}