-- Hoogle documentation, generated by Haddock
-- See Hoogle, http://www.haskell.org/hoogle/
-- | Named and unnamed implicit parameters with defaults.
--
-- Named and unnamed implicit parameters with defaults using type classes
-- and constraint hacks. For examples, see Data.Implicit.
@package implicit-params
@version 0.2
-- | Data.Implicit provides both named and unnamed implicit
-- parameters that support default values (given by the Default
-- class from the data-default package). It makes no use of the
-- ImplicitParams extension and instead everything is done using
-- type classes.
--
-- Here is an example of unnamed implicit parameters:
--
--
-- {-# LANGUAGE FlexibleContexts #-}
-- import Data.Implicit
--
-- putParam :: Implicit_ String => IO ()
-- putParam = putStrLn $ "Param was: " ++ show (param_ :: String)
--
--
-- We define putParam, which is a simple function which takes an
-- implicit parameter of type String, and prints it to the
-- screen. The param_ function is used to retrieve the unnamed
-- implicit parameter of type String from putParam's
-- context. The type signature is necessary to force param_ to
-- return a String, as this cannot be inferred due to the
-- polymorphism of show.
--
--
-- >>> putParam
-- Param was ""
--
--
-- This is how we call putParam without specifying its implicit
-- parameters. If an implicit parameter is left unspecified, its value is
-- defaulted to def, assuming that its type has a Default
-- instance. If not, then it is a type error not to specify the value of
-- an implicit parameter.
--
--
-- >>> putParam $~ "hello, world"
-- Param was "hello, world"
--
--
-- The operator $~ takes a function f and a value to
-- which to set the homotypic implicit parameter on f. It
-- applies the implicit parameter to f and returns the result.
-- There is also a prefix version of $~ whose arguments are
-- flipped called setParam_.
--
-- Here is an example of named implicit parameters:
--
--
-- {-# LANGUAGE DataKinds, FlexibleContexts, RankNTypes #-}
-- import Data.Implicit
-- import Data.Proxy
--
-- foo :: Proxy "foo"
-- foo = Proxy
--
-- bar :: Proxy "bar"
-- bar = Proxy
--
-- putFooBar :: (Implicit "foo" String, Implicit "bar" String) => IO ()
-- putFooBar = do
-- putStrLn $ "foo was: " ++ show (param foo :: String)
-- putStrLn $ "bar was: " ++ show (param bar :: String)
--
--
-- The Implicit constraint is the named equivalent of
-- Implicit_. It takes an additional argument s to
-- specify the name of the implicit parameter. Implicit parameters can be
-- "named" using any type (of any kind, on compilers that support the
-- PolyKinds extension). (The above code uses type-level strings
-- of the Symbol kind from the GHC.TypeLits module, which
-- is the recommended way to name implicit parameters. However,
-- Symbol requires the DataKinds extension and at least
-- version 7.8 of GHC (7.6 is broken; see GHC bug #7502), so you are free
-- to use other types of other kinds if you want to support older
-- versions of GHC.) param and setParam work like their
-- unnamed counterparts param_ and setParam_, but they also
-- take a proxy argument to specify the name of the implicit parameter.
-- The code above defines foo and bar to hide away the
-- (slightly ugly) proxy stuff.
--
--
-- >>> putFooBar
-- foo was: ""
-- bar was: ""
--
--
-- Once again, the defaults of unspecified implicit parameters are given
-- by the Default class.
--
--
-- >>> setParam foo "hello, world" putFooBar
-- foo was: "hello, world"
-- bar was: ""
--
--
--
-- >>> setParam bar "goodbye" $ setParam foo "hello, world" putFooBar
-- foo was: "hello, world"
-- bar was: "goodbye"
--
--
-- An infix version of setParam is also provided, ~$.
-- Using ~$, the above example would be:
--
--
-- >>> putFooBar ~$ foo ~$ bar $$ "goodbye" $$ "hello, world"
-- foo was: "hello, world"
-- bar was: "goodbye
--
module Data.Implicit
-- | The constraint Implicit "foo" String on a function
-- f indicates that an implicit parameter named "foo"
-- of type String is passed to f.
class Implicit s a
param :: Implicit s a => proxy s -> a
-- | setParam supplies a value for an implicit parameter named
-- s to a function which takes a homotypic and homonymous
-- implicit parameter. The name s is specified by a proxy
-- argument passed to setParam.
setParam :: proxy s -> a -> (Implicit s a => b) -> b
-- | An infix version of setParam with flipped arguments.
(~$) :: (Implicit s a => b) -> proxy s -> a -> b
-- | A left-associated version of $.
($$) :: (a -> b) -> a -> b
-- | The constraint Implicit_ String on a function
-- f indicates that an unnamed implicit parameter of type
-- String is passed to f.
class Implicit_ a
param_ :: Implicit_ a => a
-- | setParam_ supplies a value for an unnamed implicit parameter to
-- a function which takes a homotypic implicit parameter.
setParam_ :: a -> (Implicit_ a => b) -> b
-- | An infix version of setParam_ with flipped arguments.
($~) :: (Implicit_ a => b) -> a -> b
instance Default a => Implicit_ a
instance Default a => Implicit k s a