Type.Proxy

Package

purescript-prelude

Repository

purescript/purescript-prelude

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))

data Proxy :: forall k. k -> Typedata Proxy a