asProxyTypeOf is a type-restricted version of
const. It
is usually used as an infix operator, and its typing forces its first
argument (which is usually overloaded) to have the same type as the
tag of the second.
>>> import Data.Word
>>> :type asProxyTypeOf 123 (Proxy :: Proxy Word8)
asProxyTypeOf 123 (Proxy :: Proxy Word8) :: Word8
Note the lower-case
proxy in the definition. This allows any
type constructor with just one argument to be passed to the function,
for example we could also write
>>> import Data.Word
>>> :type asProxyTypeOf 123 (Just (undefined :: Word8))
asProxyTypeOf 123 (Just (undefined :: Word8)) :: Word8