Proxies

Using Proxy

The Proxy :: k -> * type, found in Data.Proxy (opens new window), is used when you need to give the compiler some type information - eg, to pick a type class instance - which is nonetheless irrelevant at runtime.

{-# LANGUAGE PolyKinds #-}

data Proxy a = Proxy

Functions which use a Proxy typically use ScopedTypeVariables to pick a type class instance based on the a type.

For example, the classic example of an ambiguous function,

showread :: String -> String
showread = show . read

which results in a type error because the elaborator doesn't know which instance of Show or Read to use, can be resolved using Proxy:

{-# LANGUAGE ScopedTypeVariables #-}

import Data.Proxy

showread :: forall a. (Show a, Read a) => Proxy a -> String -> String
showread _ = (show :: a -> String) . read

When calling a function with Proxy, you need to use a type annotation to declare which a you meant.

ghci> showread (Proxy :: Proxy Int) "3"
"3"
ghci> showread (Proxy :: Proxy Bool) "'m'"  -- attempt to parse a char literal as a Bool
"*** Exception: Prelude.read: no parse

The "polymorphic proxy" idiom

Since Proxy contains no runtime information, there is never a need to pattern-match on the Proxy constructor. So a common idiom is to abstract over the Proxy datatype using a type variable.

showread :: forall proxy a. (Show a, Read a) => proxy a -> String -> String
showread _ = (show :: a -> String) . read

Now, if you happen to have an f a in scope for some f, you don't need to write out Proxy :: Proxy a when calling f.

ghci> let chars = "foo"  -- chars :: [Char]
ghci> showread chars "'a'"
"'a'"

Proxy is like ()

Since Proxy contains no runtime information, you can always write a natural transformation f a -> Proxy a for any f.

proxy :: f a -> Proxy a
proxy _ = Proxy

This is just like how any given value can always be erased to ():

unit :: a -> ()
unit _ = ()

Technically, Proxy is the terminal object in the category of functors, just like () is the terminal object in the category of values.