Safe Haskell | None |
---|---|

Language | Haskell2010 |

This module uses `RecAll`

to extend common typeclass methods to records.
Generally, it is preferable to use the original typeclass methods to these
variants. For example, in most places where `recCompare`

could be used,
you could use `compare`

instead. They are useful in scenarios
that involve working on unknown subsets of a record's fields
because `RecAll`

constraints can easily be weakened. An example of this
is given at the bottom of this page.

- recEq :: RecAll f rs Eq => Rec f rs -> Rec f rs -> Bool
- recCompare :: RecAll f rs Ord => Rec f rs -> Rec f rs -> Ordering
- recMempty :: RecAll f rs Monoid => Rec proxy rs -> Rec f rs
- recMappend :: RecAll f rs Monoid => Rec f rs -> Rec f rs -> Rec f rs
- recMconcat :: RecAll f rs Monoid => Rec proxy rs -> [Rec f rs] -> Rec f rs
- recAdd :: RecAll f rs Num => Rec f rs -> Rec f rs -> Rec f rs
- recSubtract :: RecAll f rs Num => Rec f rs -> Rec f rs -> Rec f rs
- recMultiply :: RecAll f rs Num => Rec f rs -> Rec f rs -> Rec f rs
- recAbs :: RecAll f rs Num => Rec f rs -> Rec f rs
- recSignum :: RecAll f rs Num => Rec f rs -> Rec f rs
- recNegate :: RecAll f rs Num => Rec f rs -> Rec f rs
- recMinBound :: RecAll f rs Bounded => Rec proxy rs -> Rec f rs
- recMaxBound :: RecAll f rs Bounded => Rec proxy rs -> Rec f rs

# Eq Functions

# Ord Functions

# Monoid Functions

recMempty :: RecAll f rs Monoid => Rec proxy rs -> Rec f rs Source

This function differs from the original `mempty`

in that
it takes an argument. In some cases, you will already
have a record of the type you are interested in, and
that can be passed an the argument. In other situations
where this is not the case, you may need the
interpretation function of the argument record to be
`Const ()`

or `Proxy`

so the you can generate the
argument with `rpure`

.

# Num Functions

# Bounded Functions

# Example

This module provides variants of typeclass methods that have
a `RecAll`

constraint instead of the normal typeclass
constraint. For example, a type-specialized `compare`

would
look like this:

compare :: Ord (Rec f rs) => Rec f rs -> Rec f rs -> Ordering

The `recCompare`

function looks like this:

recCompare :: RecAll f rs Ord => Rec f rs -> Rec f rs -> Ordering

The only difference is the constraint. Let's look at a potential use case for these functions.

Let's write a function that projects out a subrecord from two records and
then compares those for equality. We can write this with
the `<:`

operator from `Data.Vinyl.Lens`

and the normal `compare`

function. We don't need `recCompare`

:

-- This needs ScopedTypeVariables projectAndCompare :: forall super sub f. (super <: sub, Ord (Rec f sub)) => Proxy sub -> Rec f super -> Rec f super -> Ordering projectAndCompare _ a b = compare (rcast a :: Rec f sub) (rcast b :: Rec f sub)

That works fine for the majority of use cases, and it is probably how you should write the function if it does everything you need. However, let's consider a somewhat more complicated case.

What if the exact subrecord we were projecting couldn't be
known at compile time? Assume that the end user was allowd to
choose the fields on which he or she wanted to compare records.
The `projectAndCompare`

function cannot handle this because of the
`Ord (Rec f sub)`

constraint. Even if we amend the constraint to
read `Ord (Rec f super)`

instead, we cannot use this information
to recover the `Ord (Rec f sub)`

constraint that we need. Let's
try another approach.

We can use the following GADT to prove subsethood:

data Sublist (super :: [k]) (sub :: [k]) where SublistNil :: Sublist '[] SublistSuper :: Proxy r -> Sublist super sub -> Sublist (r ': super) sub SublistBoth :: Proxy r -> Sublist super sub -> Sublist (r ': super) (r ': sub) projectRec :: Sublist super sub -> Rec f super -> Rec f sub projectRec s r = case s of SublistNil -> RNil SublistBoth n snext -> case r of rhead :& rtail -> rhead :& projectRec snext rtail SublistSuper n snext -> case r of rhead :& rtail -> projectRec snext rtail

It is also possible to write a typeclass to generate `Sublist`

s
implicitly, but that is beyond the scope of this example. Let's
now write a function to use `Sublist`

to weaken a `RecAll`

constraint:

import Data.Vinyl.Core hiding (Dict) import Data.Constraint weakenRecAll :: Proxy f -> Proxy c -> Sublist super sub -> RecAll f super c :- RecAll f sub c weakenRecAll f c s = case s of SublistNil -> Sub Dict SublistSuper _ snext -> Sub $ case weakenRecAll f c snext of Sub Dict -> Dict SublistBoth _ snext -> Sub $ case weakenRecAll f c snext of Sub Dict -> Dict

Now we can write a different version of our original function:

-- This needs ScopedTypeVariables projectAndCompare2 :: forall super sub f. (RecAll f super Ord) => Sublist super sub -> Rec f super -> Rec f super -> Ordering projectAndCompare2 s a b = case weakenRecAll (Proxy :: Proxy f) (Proxy :: Proxy Ord) s of Sub Dict -> recCompare (projectRec s a) (projectRec s b)

Notice that in this case, the `Ord`

constraint applies to the full set of fields
and is then weakened to target a subset of them instead.