Strongweak

Main import module for basic use.

For defining Strengthen instances, import Strongweak.Strengthen.

Instance design

A given strong type a has exactly one associated weak type Weaken a. Multiple strong types may weaken to the same weak type.

The following laws must hold:

• weaken a == weaken b |= a == b

• strengthen (weaken a) == pure a

strongweak is largely a programmer convenience library. There is a lot of room to write instances which may seem useful on first glance, but are inconsistent with the overall design. Here is some relevant guidance.

• Weak types should have _simpler invariants to manage_ than strong ones.
• In general, weak types should be easier to use than strong ones.
• Most (all?) instances should handle (relax or assert) a single invariant.
• Most instances should not have a recursive context.

Some types may not have any invariants which may be usefully relaxed e.g. Either a b. For these, you may write a recursive instance that weakens/strengthens "through" the type e.g. (Weaken a, Weaken b) => Weak (Either a b)). Don't combine the two instance types.

An example is NonEmpty a. We could weaken this to [a], but also to [Weaken a]. However, the latter would mean decomposing and removing an invariant simultaneously. It would be two separate strengthens in one instance. And now, your a must be in the strongweak ecosystem, which isn't necessarily what you want - indeed, it appears this sort of design would require a Weaken a = a, weaken = id overlapping instance, which I do not want. On the other hand, [a] does weaken to [Weaken a], because there are no invariants present to remove, so decomposing is all the user could hope to do.

Classes

class Weaken a where

Weaken some a, relaxing certain invariants.

See Strongweak for class design notes and laws.

Associated Types

type Weak a :: Type

The weakened type for some type.

Methods

weaken :: a -> Weak a

Weaken some a to its associated weak type Weaken a.

Instances

Weaken Int16

Defined in Strongweak.Weaken

Associated Types

type Weak Int16

Methods

weaken :: Int16 -> Weak Int16

Weaken Int32

Defined in Strongweak.Weaken

Associated Types

type Weak Int32

Methods

weaken :: Int32 -> Weak Int32

Weaken Int64

Defined in Strongweak.Weaken

Associated Types

type Weak Int64

Methods

weaken :: Int64 -> Weak Int64

Weaken Int8

Defined in Strongweak.Weaken

Associated Types

type Weak Int8

Methods

weaken :: Int8 -> Weak Int8

Weaken Word16

Defined in Strongweak.Weaken

Associated Types

type Weak Word16

Methods

weaken :: Word16 -> Weak Word16

Weaken Word32

Defined in Strongweak.Weaken

Associated Types

type Weak Word32

Methods

weaken :: Word32 -> Weak Word32

Weaken Word64

Defined in Strongweak.Weaken

Associated Types

type Weak Word64

Methods

weaken :: Word64 -> Weak Word64

Weaken Word8

Defined in Strongweak.Weaken

Associated Types

type Weak Word8

Methods

weaken :: Word8 -> Weak Word8

Weaken (Identity a)

Strip wrapper.

Defined in Strongweak.Weaken

Associated Types

type Weak (Identity a)

Methods

weaken :: Identity a -> Weak (Identity a)

Weaken (NonEmpty a)

Weaken non-empty lists into plain lists.

Defined in Strongweak.Weaken

Associated Types

type Weak (NonEmpty a)

Methods

weaken :: NonEmpty a -> Weak (NonEmpty a)

Weaken a => Weaken [a]

Decomposer. Weaken every element in a list.

Defined in Strongweak.Weaken

Associated Types

type Weak [a]

Methods

weaken :: [a] -> Weak [a]

(Weaken a, Weaken b) => Weaken (Either a b)

Decomposer. Weaken either side of an Either.

Defined in Strongweak.Weaken

Associated Types

type Weak (Either a b)

Methods

weaken :: Either a b -> Weak (Either a b)

(Generic s, Generic w, GWeaken (Rep s) (Rep w)) => Weaken (GenericallySW s w)

Defined in Strongweak.Generic.Via

Associated Types

type Weak (GenericallySW s w)

Methods

weaken :: GenericallySW s w -> Weak (GenericallySW s w)

(Weaken a, Weaken b) => Weaken (a, b)

Decomposer. Weaken both elements of a tuple.

Defined in Strongweak.Weaken

Associated Types

type Weak (a, b)

Methods

weaken :: (a, b) -> Weak (a, b)

Weaken (Const a b)

Strip wrapper.

Defined in Strongweak.Weaken

Associated Types

type Weak (Const a b)

Methods

weaken :: Const a b -> Weak (Const a b)

Weaken (Refined p a)

Strip refined type refinement.

Defined in Strongweak.Weaken

Associated Types

type Weak (Refined p a)

Methods

weaken :: Refined p a -> Weak (Refined p a)

Vector v a => Weaken (Vector v n a)

Weaken sized vectors into plain lists.

Defined in Strongweak.Weaken

Associated Types

type Weak (Vector v n a)

Methods

weaken :: Vector v n a -> Weak (Vector v n a)

Weaken (Refined1 p f a)

Strip refined functor type refinement.

Defined in Strongweak.Weaken

Associated Types

type Weak (Refined1 p f a)

Methods

weaken :: Refined1 p f a -> Weak (Refined1 p f a)

class Weaken a => Strengthen a where

Attempt to strengthen some Weak a, asserting certain invariants.

We take Weaken as a superclass in order to maintain strong/weak type pair consistency. We choose this dependency direction because we treat the strong type as the "canonical" one, so Weaken is the more natural (and straightforward) class to define. That does mean the instances for this class are a little confusingly worded. Alas.

See Strongweak for class design notes and laws.

Methods

strengthen :: Weak a -> Result a

Attempt to strengthen some Weak a to its associated strong type a.

Instances

Strengthen Int16

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak Int16 -> Result Int16

Strengthen Int32

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak Int32 -> Result Int32

Strengthen Int64

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak Int64 -> Result Int64

Strengthen Int8

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak Int8 -> Result Int8

Strengthen Word16

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak Word16 -> Result Word16

Strengthen Word32

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak Word32 -> Result Word32

Strengthen Word64

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak Word64 -> Result Word64

Strengthen Word8

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak Word8 -> Result Word8

Strengthen (Identity a)

Add wrapper.

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak (Identity a) -> Result (Identity a)

Typeable a => Strengthen (NonEmpty a)

Strengthen a plain list into a non-empty list by asserting non-emptiness.

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak (NonEmpty a) -> Result (NonEmpty a)

Strengthen a => Strengthen [a]

Decomposer. Strengthen every element in a list.

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak [a] -> Result [a]

(Strengthen a, Strengthen b) => Strengthen (Either a b)

Decomposer. Strengthen either side of an Either.

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak (Either a b) -> Result (Either a b)

(Generic s, Generic w, GStrengthenD (Rep w) (Rep s), Weaken (GenericallySW s w)) => Strengthen (GenericallySW s w)

Defined in Strongweak.Generic.Via

Methods

strengthen :: Weak (GenericallySW s w) -> Result (GenericallySW s w)

(Strengthen a, Strengthen b) => Strengthen (a, b)

Decomposer. Strengthen both elements of a tuple.

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak (a, b) -> Result (a, b)

Strengthen (Const a b)

Add wrapper.

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak (Const a b) -> Result (Const a b)

(Predicate p a, Typeable a) => Strengthen (Refined p a)

Assert a predicate to refine a type.

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak (Refined p a) -> Result (Refined p a)

(Vector v a, KnownNat n, Typeable v, Typeable a) => Strengthen (Vector v n a)

Strengthen a plain list into a sized vector by asserting length.

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak (Vector v n a) -> Result (Vector v n a)

(Predicate1 p f, Typeable f, Typeable a, Typeable ak) => Strengthen (Refined1 p f a)

Assert a functor predicate to refine a type.

Defined in Strongweak.Strengthen

Methods

strengthen :: Weak (Refined1 p f a) -> Result (Refined1 p f a)

Other definitions

liftWeakF :: Weaken a => (Weak a -> b) -> a -> b

Lift a function on a weak type to the associated strong type by weakening first.

Strength switch wrapper

data Strength

Strength enumeration: is it strong, or weak?

Primarily interesting at the type level (using DataKinds).

Constructors

Strong

Weak

type family SW (s :: Strength) a :: Type where ...

Get either the strong or weak representation of a type, depending on the type-level "switch" provided.

This is intended to be used in data types that take a Strength type. Define your type using strong fields wrapped in SW s. You then get the weak representation for free, using the same definition.

data A (s :: Strength) = A
  { a1 :: SW s Word8
  , a2 :: String }

Equations

SW 'Strong a = a

SW 'Weak a = Weak a