WebGear.Trait

Traits are optional attributes that a value might posess. For example, a list containing totally ordered values might have a Maximum trait where the associated attribute is the maximum value. The trait exists only if the list is non-empty.

Traits help to access these attributes in a type-safe manner.

Traits are somewhat similar to refinement types, but allow arbitrary attributes to be associated with a value instead of only a predicate.

Core Types

class Monad m => Trait t a m where

A Trait is an optional attribute t associated with a value a.

The check function validates the presence of the trait for a given value. Checking the presence of the trait can optionally modify the value as well.

Associated Types

type Val t a

Type of the associated attribute

type Fail t a

Type of check failures

Methods

check :: a -> m (CheckResult t a)

Checks the presence of the associated attribute.

Instances

Monad m => Trait ('[] :: [k]) a m

A trivial trait that is always present and whose attribute does not carry any meaningful information.

Defined in WebGear.Trait

Associated Types

type Val '[] a

type Fail '[] a

Methods

check :: a -> m (CheckResult '[] a)

(Monad m, IsStdMethod t) => Trait (Method t :: Type) Request m

Defined in WebGear.Trait.Method

Associated Types

type Val (Method t) Request

type Fail (Method t) Request

Methods

check :: Request -> m (CheckResult (Method t) Request)

(KnownSymbol s, Monad m) => Trait (Path s :: Type) Request m

Defined in WebGear.Trait.Path

Associated Types

type Val (Path s) Request

type Fail (Path s) Request

Methods

check :: Request -> m (CheckResult (Path s) Request)

(FromJSON t, MonadIO m) => Trait (JSONRequestBody t :: Type) Request m

Defined in WebGear.Trait.Body

Associated Types

type Val (JSONRequestBody t) Request

type Fail (JSONRequestBody t) Request

Methods

check :: Request -> m (CheckResult (JSONRequestBody t) Request)

(KnownSymbol s, KnownSymbol t, Monad m) => Trait (HeaderMatch s t :: Type) Request m

Defined in WebGear.Trait.Header

Associated Types

type Val (HeaderMatch s t) Request

type Fail (HeaderMatch s t) Request

Methods

check :: Request -> m (CheckResult (HeaderMatch s t) Request)

(KnownSymbol s, FromHttpApiData t, Monad m) => Trait (Header s t :: Type) Request m

Defined in WebGear.Trait.Header

Associated Types

type Val (Header s t) Request

type Fail (Header s t) Request

Methods

check :: Request -> m (CheckResult (Header s t) Request)

(Trait t a2 m, Trait ts a2 m) => Trait (t ': ts :: [a1]) a2 m

Combination of many traits all of which are present for a value.

Defined in WebGear.Trait

Associated Types

type Val (t ': ts) a2

type Fail (t ': ts) a2

Methods

check :: a2 -> m (CheckResult (t ': ts) a2)

(FromHttpApiData val, Monad m) => Trait (PathVar tag val :: Type) Request m

Defined in WebGear.Trait.Path

Associated Types

type Val (PathVar tag val) Request

type Fail (PathVar tag val) Request

Methods

check :: Request -> m (CheckResult (PathVar tag val) Request)

data CheckResult t a

Result of a check operation

Constructors

CheckSuccess a (Val t a)

CheckFail (Fail t a)

Instances

(Eq a, Eq (Val t a), Eq (Fail t a)) => Eq (CheckResult t a)

Defined in WebGear.Trait

Methods

(==) :: CheckResult t a -> CheckResult t a -> Bool

(/=) :: CheckResult t a -> CheckResult t a -> Bool

(Read a, Read (Val t a), Read (Fail t a)) => Read (CheckResult t a)

Defined in WebGear.Trait

Methods

readsPrec :: Int -> ReadS (CheckResult t a)

readList :: ReadS [CheckResult t a]

readPrec :: ReadPrec (CheckResult t a)

readListPrec :: ReadPrec [CheckResult t a]

(Show a, Show (Val t a), Show (Fail t a)) => Show (CheckResult t a)

Defined in WebGear.Trait

Methods

showsPrec :: Int -> CheckResult t a -> ShowS

show :: CheckResult t a -> String

showList :: [CheckResult t a] -> ShowS

data Linked (ts :: [Type]) a

A value linked with a type-level list of traits.

type family Traits ts a m :: Constraint where ...

Constraint for functions that use multiple traits

Equations

Traits '[] a m = ()

Traits (t ': ts) a m = (Trait t a m, Traits ts a m)

Linking values with traits

linkzero :: a -> Linked '[] a

Link a value with the trivial trait

linkplus :: Trait t a m => Linked ts a -> m (Either (Fail t a) (Linked (t ': ts) a))

Attempt to link an additional trait with an already linked value

linkminus :: Linked (t ': ts) a -> Linked ts a

Remove the leading trait from the linked value

unlink :: Linked ts a -> a

Retrive the value from a linked value

Retrive trait attributes from linked values

class Has t ts where

Constraint that proves that the trait t is present somewhere in the list of traits ts.

Methods

traitValue :: Linked ts a -> Tagged t (Val t a)

Instances

Has (t :: Type) (t ': ts)

Defined in WebGear.Trait

Methods

traitValue :: Linked (t ': ts) a -> Tagged t (Val t a)

Has t ts => Has (t :: k) (t' ': ts)

Defined in WebGear.Trait

Methods

traitValue :: Linked (t' ': ts) a -> Tagged t (Val t a)

type family Have ts qs :: Constraint where ...

Constraint that proves that all the traits in the list ts are present in the list qs.

Equations

Have '[] qs = ()

Have (t ': ts) qs = (Has t qs, Have ts qs)