derivingvia-extras-0.1.0.0: README.md

# derivingvia-extras

Miscellaneous via types, for use with `DerivingVia`.

## `On`

The type `On T "field"` derives non-structural instances for `T` by
comparing an evaluting it only at the record field `"field"`.

```haskell
{-# Language DataKinds     #-}
{-# Language DerivingVia   #-}
{-# Language TypeOperators #-}

import Deriving.On
import Data.Hashable

data User = User
  { name   :: String
  , age    :: Int
  , userID :: Integer
  }
  deriving (Eq, Ord, Hashable)
  via User `On` "userID"
```

such that all other fields are ignored

```haskell
>> alice = User "Alice" 50 0xDEADBEAF
>> bob   = User "Bob"   20 0xDEADBEAF
>>
>> alice == bob
True
>> alice <= bob
True
>> hash alice == hash bob
True
```