Safe Haskell	None
Language	GHC2021

Strongweak.Generic

Contents

Generic derivation compatibility
Generic derivers
Generic wrapper

Description

Generic strengthen and weaken.

Synopsis

weakenGeneric :: (Generic s, Generic w, GWeaken (Rep s) (Rep w)) => s -> w
strengthenGeneric :: (Generic w, Generic s, GStrengthenD (Rep w) (Rep s)) => w -> Either StrengthenFailure' s
newtype GenericallySW s w = GenericallySW {
- unGenericallySW :: s
}

Generic derivation compatibility

The Strengthen and Weaken generic derivers allow you to derive instances between any compatible pair of types. Compatibility is defined as follows:

Both types' generic representation (the SOP tree structure) match exactly.
For each leaf pair of types, either the types are identical, or the appropriate instance exists to transform from source to target.

If they aren't compatible, the derivation will fail with a type error. I'm fairly certain that if it succeeds, your instance is guaranteed correct (assuming the instances it uses internally are all OK!).

I don't think GHC strongly guarantees the SOP property, so if you receive surprising derivation errors, the types might have differing generic representation structure, even if their flattened representations are identical. If you experience this let me know, since in my experience GHC's stock Generic derivation is highly deterministic.

Also, generic strengthening requires that all metadata is present for both types: for the datatype, constructors and selectors. GHC will always add this metadata for you, but manually-derived Generic instances (which are usually a bad idea) do not require it.

Generic derivers

weakenGeneric :: (Generic s, Generic w, GWeaken (Rep s) (Rep w)) => s -> w Source #

Weaken a value generically.

The weak and strong types must be compatible. See Generic for the definition of compatibility in this context.

strengthenGeneric :: (Generic w, Generic s, GStrengthenD (Rep w) (Rep s)) => w -> Either StrengthenFailure' s Source #

Strengthen a value generically.

The weak and strong types must be compatible. See Generic for the definition of compatibility in this context.

Generic wrapper

newtype GenericallySW s w Source #

DerivingVia wrapper for strongweak instances.

We can't use Generically conveniently because we need to talk about two data types, not one -- we would have to do something like Generically (Tagged w s), which is ugly. So we instead define our own adorable little "via type" here!

Use like so:

data XYZ (s :: Strength) = XYZ
  { xyz1 :: SW s Word8
  , xyz2 :: Word8
  , xyz3 :: ()
  } deriving stock Generic
deriving via (GenericallySW (XYZ 'Strong) (XYZ 'Weak)) instance Weaken (XYZ 'Strong)
deriving via (GenericallySW (XYZ 'Strong) (XYZ 'Weak)) instance Strengthen (XYZ 'Strong)

TODO can't figure out a way around multiple standalone deriving declarations :(

TODO maybe GenericallySW1? but even so instances differ between weaken and strengthen (weaken needs nothing) so it's kinda better this way. :)

Constructors