Type.Proxy
The Proxy
type and values are for situations where type information is
required for an input to determine the type of an output, but where it is
not possible or convenient to provide a value for the input.
A hypothetical example: if you have a class that is used to handle the result of an AJAX request, you may want to use this information to set the expected content type of the request, so you might have a class something like this:
class AjaxResponse a where
responseType :: a -> ResponseType
fromResponse :: Foreign -> a
The problem here is responseType
requires a value of type a
, but we
won't have a value of that type until the request has been completed. The
solution is to use a Proxy
type instead:
class AjaxResponse a where
responseType :: Proxy a -> ResponseType
fromResponse :: Foreign -> a
We can now call responseType (Proxy :: Proxy SomeContentType)
to produce
a ResponseType
for SomeContentType
without having to construct some
empty version of SomeContentType
first. In situations like this where
the Proxy
type can be statically determined, it is recommended to pull
out the definition to the top level and make a declaration like:
_SomeContentType :: Proxy SomeContentType
_SomeContentType = Proxy
That way the proxy value can be used as responseType _SomeContentType
for improved readability. However, this is not always possible, sometimes
the type required will be determined by a type variable. As PureScript has
scoped type variables, we can do things like this:
makeRequest :: URL -> ResponseType -> Aff _ Foreign
makeRequest = ...
fetchData :: forall a. (AjaxResponse a) => URL -> Aff _ a
fetchData url = fromResponse <$> makeRequest url (responseType (Proxy :: Proxy a))
#Proxy2
Packages
assert
catenable-lists
const
convertable-options
datetime-parsing
debug
distributive
erl-atom
erl-gun
erl-lager
erl-logger
erl-maps
erl-modules
erl-nativerefs
erl-opentelemetry
- OpenTelemetry.Metrics
- OpenTelemetry.Metrics.Counter
- OpenTelemetry.Metrics.Meter
- OpenTelemetry.Metrics.SumObserver
- OpenTelemetry.Metrics.UpDownCounter
- OpenTelemetry.Metrics.UpDownSumObserver
- OpenTelemetry.Metrics.ValueObserver
- OpenTelemetry.Metrics.ValueRecorder
- OpenTelemetry.Tracing
- OpenTelemetry.Tracing.Baggage
- OpenTelemetry.Tracing.Ctx
- OpenTelemetry.Tracing.Propagator.TextMap
- OpenTelemetry.Tracing.Span
- OpenTelemetry.Tracing.Tracer
- Tracing.Attributes
erl-process
erl-queue
erl-ranch
erl-simplebus
erl-ssl
erl-test-eunit
erl-test-eunit-discovery
erl-tuples
erl-untagged-union
exceptions
exists
expect-inferred
functions
graphs
heterogeneous
identity
integers
invariant
js-uri
lazy
lcg
math
media-types
newtype
nonempty
nullable
ordered-collections
partial
prelude
- Control.Applicative
- Control.Apply
- Control.Bind
- Control.Category
- Control.Monad
- Control.Semigroupoid
- Data.Boolean
- Data.BooleanAlgebra
- Data.Bounded
- Data.Bounded.Generic
- Data.CommutativeRing
- Data.DivisionRing
- Data.Eq
- Data.Eq.Generic
- Data.EuclideanRing
- Data.Field
- Data.Function
- Data.Functor
- Data.Generic.Rep
- Data.HeytingAlgebra
- Data.HeytingAlgebra.Generic
- Data.Monoid
- Data.Monoid.Additive
- Data.Monoid.Conj
- Data.Monoid.Disj
- Data.Monoid.Dual
- Data.Monoid.Endo
- Data.Monoid.Generic
- Data.Monoid.Multiplicative
- Data.NaturalTransformation
- Data.Ord
- Data.Ord.Generic
- Data.Ordering
- Data.Ring
- Data.Ring.Generic
- Data.Semigroup
- Data.Semigroup.First
- Data.Semigroup.Generic
- Data.Semigroup.Last
- Data.Semiring
- Data.Semiring.Generic
- Data.Show
- Data.Show.Generic
- Data.Symbol
- Data.Unit
- Data.Void
- Record.Unsafe
- Type.Data.Row
- Type.Data.RowList
- Type.Proxy
profunctor-lenses
- Data.Lens.AffineTraversal
- Data.Lens.At
- Data.Lens.Common
- Data.Lens.Fold
- Data.Lens.Fold.Partial
- Data.Lens.Getter
- Data.Lens.Grate
- Data.Lens.Index
- Data.Lens.Indexed
- Data.Lens.Internal.Bazaar
- Data.Lens.Internal.Exchange
- Data.Lens.Internal.Focusing
- Data.Lens.Internal.Forget
- Data.Lens.Internal.Grating
- Data.Lens.Internal.Indexed
- Data.Lens.Internal.Market
- Data.Lens.Internal.Re
- Data.Lens.Internal.Shop
- Data.Lens.Internal.Stall
- Data.Lens.Internal.Tagged
- Data.Lens.Internal.Wander
- Data.Lens.Internal.Zipping
- Data.Lens.Iso
- Data.Lens.Iso.Newtype
- Data.Lens.Lens
- Data.Lens.Lens.Product
- Data.Lens.Lens.Tuple
- Data.Lens.Lens.Unit
- Data.Lens.Lens.Void
- Data.Lens.Prism
- Data.Lens.Prism.Coproduct
- Data.Lens.Prism.Either
- Data.Lens.Prism.Maybe
- Data.Lens.Record
- Data.Lens.Setter
- Data.Lens.Traversal
- Data.Lens.Types
- Data.Lens.Zoom
psci-support
quickcheck-laws
- Test.QuickCheck.Laws
- Test.QuickCheck.Laws.Control.Alt
- Test.QuickCheck.Laws.Control.Alternative
- Test.QuickCheck.Laws.Control.Applicative
- Test.QuickCheck.Laws.Control.Apply
- Test.QuickCheck.Laws.Control.Bind
- Test.QuickCheck.Laws.Control.Category
- Test.QuickCheck.Laws.Control.Comonad
- Test.QuickCheck.Laws.Control.Extend
- Test.QuickCheck.Laws.Control.Monad
- Test.QuickCheck.Laws.Control.MonadPlus
- Test.QuickCheck.Laws.Control.MonadZero
- Test.QuickCheck.Laws.Control.Plus
- Test.QuickCheck.Laws.Control.Semigroupoid
- Test.QuickCheck.Laws.Data.BooleanAlgebra
- Test.QuickCheck.Laws.Data.Bounded
- Test.QuickCheck.Laws.Data.BoundedEnum
- Test.QuickCheck.Laws.Data.CommutativeRing
- Test.QuickCheck.Laws.Data.DivisionRing
- Test.QuickCheck.Laws.Data.Eq
- Test.QuickCheck.Laws.Data.EuclideanRing
- Test.QuickCheck.Laws.Data.Field
- Test.QuickCheck.Laws.Data.Foldable
- Test.QuickCheck.Laws.Data.Functor
- Test.QuickCheck.Laws.Data.FunctorWithIndex
- Test.QuickCheck.Laws.Data.HeytingAlgebra
- Test.QuickCheck.Laws.Data.Monoid
- Test.QuickCheck.Laws.Data.Ord
- Test.QuickCheck.Laws.Data.Ring
- Test.QuickCheck.Laws.Data.Semigroup
- Test.QuickCheck.Laws.Data.Semiring
random
rationals
record-prefix
refs
safe-coerce
semirings
simple-json
strings
- Data.Char
- Data.Char.Gen
- Data.String.CaseInsensitive
- Data.String.CodePoints
- Data.String.CodeUnits
- Data.String.Common
- Data.String.Gen
- Data.String.NonEmpty.CaseInsensitive
- Data.String.NonEmpty.CodePoints
- Data.String.NonEmpty.CodeUnits
- Data.String.NonEmpty.Internal
- Data.String.Pattern
- Data.String.Regex
- Data.String.Regex.Flags
- Data.String.Regex.Unsafe
- Data.String.Unsafe
tailrec
transformers
- Control.Comonad.Env
- Control.Comonad.Env.Class
- Control.Comonad.Env.Trans
- Control.Comonad.Store
- Control.Comonad.Store.Class
- Control.Comonad.Store.Trans
- Control.Comonad.Traced
- Control.Comonad.Traced.Class
- Control.Comonad.Traced.Trans
- Control.Comonad.Trans.Class
- Control.Monad.Cont
- Control.Monad.Cont.Class
- Control.Monad.Cont.Trans
- Control.Monad.Error.Class
- Control.Monad.Except
- Control.Monad.Except.Trans
- Control.Monad.Identity.Trans
- Control.Monad.List.Trans
- Control.Monad.Maybe.Trans
- Control.Monad.RWS
- Control.Monad.RWS.Trans
- Control.Monad.Reader
- Control.Monad.Reader.Class
- Control.Monad.Reader.Trans
- Control.Monad.State
- Control.Monad.State.Class
- Control.Monad.State.Trans
- Control.Monad.Trans.Class
- Control.Monad.Writer
- Control.Monad.Writer.Class
- Control.Monad.Writer.Trans
type-equality
undefinable
unfoldable
unsafe-coerce
unsafe-reference
uri
- URI.AbsoluteURI
- URI.Authority
- URI.Common
- URI.Extra.MultiHostPortPair
- URI.Extra.QueryPairs
- URI.Extra.UserPassInfo
- URI.Fragment
- URI.HierarchicalPart
- URI.Host
- URI.Host.Gen
- URI.Host.IPv4Address
- URI.Host.IPv6Address
- URI.Host.RegName
- URI.HostPortPair
- URI.HostPortPair.Gen
- URI.Path
- URI.Path.Absolute
- URI.Path.NoScheme
- URI.Path.Rootless
- URI.Path.Segment
- URI.Port
- URI.Port.Gen
- URI.Query
- URI.RelativePart
- URI.RelativeRef
- URI.Scheme
- URI.Scheme.Common
- URI.URI
- URI.URIRef
- URI.UserInfo