Add scoped type variable example

Author: garyb

README.md

@@ -30,7 +30,27 @@ class AjaxResponse a where
 
 We can now call `responseType (Proxy :: Proxy SomeContentType)` to produce
 a `ResponseType` for `SomeContentType` without having to construct some
-empty version of `SomeContentType` first.
+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:
+
+``` purescript
+_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:
+
+``` purescript
+makeRequest :: URL -> ResponseType -> Aff _ Foreign
+makeRequest = ...
+
+fetchData :: forall a. (AjaxResponse a) => URL -> Aff _ a
+fetchData url = fromResponse <$> makeRequest url (responseType (Proxy :: Proxy a))
+```
 
 #### `Proxy`
 

src/Type/Proxy.purs

@@ -25,7 +25,27 @@
 -- |
 -- | We can now call `responseType (Proxy :: Proxy SomeContentType)` to produce
 -- | a `ResponseType` for `SomeContentType` without having to construct some
--- | empty version of `SomeContentType` first.
+-- | 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:
+-- |
+-- | ``` purescript
+-- | _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:
+-- |
+-- | ``` purescript
+-- | makeRequest :: URL -> ResponseType -> Aff _ Foreign
+-- | makeRequest = ...
+-- |
+-- | fetchData :: forall a. (AjaxResponse a) => URL -> Aff _ a
+-- | fetchData url = fromResponse <$> makeRequest url (responseType (Proxy :: Proxy a))
+-- | ```
 module Type.Proxy where
 
 -- | Value proxy for kind `*` types.