content: {-# LANGUAGE TemplateHaskell #-}
  {-# LANGUAGE TypeAbstractions #-}
  {-# LANGUAGE TypeFamilies #-}
  {-# LANGUAGE UndecidableInstances #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  Data.Maybe.Singletons
  -- Copyright   :  (C) 2013-2014 Richard Eisenberg, Jan Stolarek
  -- License     :  BSD-style (see LICENSE)
  -- Maintainer  :  Ryan Scott
  -- Stability   :  experimental
  -- Portability :  non-portable
  --
  -- Defines functions and datatypes relating to the singleton for 'Maybe',
  -- including singled versions of all the definitions in @Data.Maybe@.
  --
  -- Because many of these definitions are produced by Template Haskell,
  -- it is not possible to create proper Haddock documentation. Please look
  -- up the corresponding operation in @Data.Maybe@. Also, please excuse
  -- the apparent repeated variable names. This is due to an interaction
  -- between Template Haskell and Haddock.
  --
  ----------------------------------------------------------------------------
  
  
  module Data.Maybe.Singletons (
    -- The 'Maybe' singleton
    Sing, SMaybe(..),
  
    -- * Singletons from @Data.Maybe@
    maybe_, Maybe_, sMaybe_,
    -- | The preceding two definitions are derived from the function 'maybe' in
    -- @Data.Maybe@. The extra underscore is to avoid name clashes with the type
    -- 'Maybe'.
  
    IsJust, sIsJust, IsNothing, sIsNothing,
    FromJust, sFromJust, FromMaybe, sFromMaybe, ListToMaybe, sListToMaybe,
    MaybeToList, sMaybeToList, CatMaybes, sCatMaybes, MapMaybe, sMapMaybe,
  
    -- * Defunctionalization symbols
    NothingSym0, JustSym0, JustSym1,
  
    Maybe_Sym0, Maybe_Sym1, Maybe_Sym2, Maybe_Sym3,
    IsJustSym0, IsJustSym1, IsNothingSym0, IsNothingSym1,
    FromJustSym0, FromJustSym1, FromMaybeSym0, FromMaybeSym1, FromMaybeSym2,
    ListToMaybeSym0, ListToMaybeSym1, MaybeToListSym0, MaybeToListSym1,
    CatMaybesSym0, CatMaybesSym1, MapMaybeSym0, MapMaybeSym1, MapMaybeSym2
    ) where
  
  import Data.Singletons.Base.Instances
  import Data.Singletons.TH
  import GHC.TypeLits.Singletons
  
  $(singletons [d|
    -- Renamed to avoid name clash
    -- -| The 'maybe' function takes a default value, a function, and a 'Maybe'
    -- value.  If the 'Maybe' value is 'Nothing', the function returns the
    -- default value.  Otherwise, it applies the function to the value inside
    -- the 'Just' and returns the result.
    maybe_ :: b -> (a -> b) -> Maybe a -> b
    maybe_ n _ Nothing  = n
    maybe_ _ f (Just x) = f x
   |])
  
  $(singletonsOnly [d|
    -- -| The 'isJust' function returns 'True' iff its argument is of the
    -- form @Just _@.
    isJust         :: Maybe a -> Bool
    isJust Nothing  = False
    isJust (Just _) = True
  
    -- -| The 'isNothing' function returns 'True' iff its argument is 'Nothing'.
    isNothing         :: Maybe a -> Bool
    isNothing Nothing  = True
    isNothing (Just _) = False
  
    -- -| The 'fromJust' function extracts the element out of a 'Just' and
    -- throws an error if its argument is 'Nothing'.
    fromJust          :: Maybe a -> a
    fromJust Nothing  = error "Maybe.fromJust: Nothing" -- yuck
    fromJust (Just x) = x
  
    -- -| The 'fromMaybe' function takes a default value and and 'Maybe'
    -- value.  If the 'Maybe' is 'Nothing', it returns the default values;
    -- otherwise, it returns the value contained in the 'Maybe'.
    fromMaybe     :: a -> Maybe a -> a
    fromMaybe d x = case x of {Nothing -> d;Just v  -> v}
  
    -- -| The 'maybeToList' function returns an empty list when given
    -- 'Nothing' or a singleton list when not given 'Nothing'.
    maybeToList            :: Maybe a -> [a]
    maybeToList  Nothing   = []
    maybeToList  (Just x)  = [x]
  
    -- -| The 'listToMaybe' function returns 'Nothing' on an empty list
    -- or @'Just' a@ where @a@ is the first element of the list.
    listToMaybe           :: [a] -> Maybe a
    listToMaybe []        =  Nothing
    listToMaybe (a:_)     =  Just a
  
    -- Modified to avoid list comprehensions
    -- -| The 'catMaybes' function takes a list of 'Maybe's and returns
    -- a list of all the 'Just' values.
    catMaybes              :: [Maybe a] -> [a]
    catMaybes []             = []
    catMaybes (Just x  : xs) = x : catMaybes xs
    catMaybes (Nothing : xs) = catMaybes xs
  
    -- -| The 'mapMaybe' function is a version of 'map' which can throw
    -- out elements.  In particular, the functional argument returns
    -- something of type @'Maybe' b@.  If this is 'Nothing', no element
    -- is added on to the result list.  If it just @'Just' b@, then @b@ is
    -- included in the result list.
    mapMaybe          :: (a -> Maybe b) -> [a] -> [b]
    mapMaybe _ []     = []
    mapMaybe f (x:xs) =
     let rs = mapMaybe f xs in
     case f x of
      Nothing -> rs
      Just r  -> r:rs
    |])
  