------------------------------------------------------------------------
--- This module contains an implementation of set functions.
--- The general idea of set functions is described in:
---
--- > S. Antoy, M. Hanus: Set Functions for Functional Logic Programming
--- > Proc. 11th International Conference on Principles and Practice
--- > of Declarative Programming (PPDP'09), pp. 73-82, ACM Press, 2009
--- 
--- The general concept of set functions is as follows.
--- If `f` is an n-ary function, then `(setn f)` is a set-valued
--- function that collects all non-determinism caused by f (but not
--- the non-determinism caused by evaluating arguments!) in a set.
--- Thus, `(setn f a1 ... an)` returns the set of all
--- values of `(f b1 ... bn)` where `b1`,...,`bn` are values
--- of the arguments `a1`,...,`an` (i.e., the arguments are
--- evaluated "outside" this capsule so that the non-determinism
--- caused by evaluating these arguments is not captured in this capsule
--- but yields several results for `(setn...)`.
--- Similarly, logical variables occuring in `a1`,...,`an` are not bound
--- inside this capsule (in PAKCS they cause a suspension until
--- they are bound).
--- 
--- *Remark:*
--- Since there is no special syntax for set functions,
--- one has to write `(setn f)` for the set function of the
--- _n-ary top-level function_ `f`.
--- The correct usage of set functions is currently not checked by
--- the compiler, i.e., one can also write unintended uses
--- like `set0 ((+1) (1 ? 2))`.
--- In order to check the correct use of set functions,
--- it is recommended to apply the tool
--- [CurryCheck](https://cpm.curry-lang.org/pkgs/currycheck.html)
--- on Curry programs which reports illegal uses of set functions
--- (among other properties).
--- 
--- The set of values returned by a set function is represented
--- by an abstract type 'Values' on which several operations are
--- defined in this module. Actually, it is a multiset of values,
--- i.e., duplicates are not removed.
---
--- The handling of failures and nested occurrences of set functions
--- is not specified in the previous paper. Thus, a detailed description
--- of the semantics of set functions as implemented in this library
--- can be found in the paper
---
--- > J. Christiansen, M. Hanus, F. Reck, D. Seidel:
--- > A Semantics for Weakly Encapsulated Search in Functional Logic Programs
--- > Proc. 15th International Conference on Principles and Practice
--- > of Declarative Programming (PPDP'13), pp. 49-60, ACM Press, 2013
--- 
--- Note that the implementation of this library uses multisets
--- instead of sets. Thus, the result of a set function might
--- contain multiple values. From a declarative point of view,
--- this is not relevant. It has the advantage that equality
--- is not required on values, i.e., encapsulated values can also
--- be functional.
--- 
--- The PAKCS implementation of set functions has several restrictions,
--- in particular:
--- 
--- 1. The multiset of values is completely evaluated when demanded.
---    Thus, if it is infinite, its evaluation will not terminate
---    even if only some elements (e.g., for a containment test)
---    are demanded. However, for the emptiness test, at most one
---    value will be computed
--- 2. The arguments of a set function are strictly evaluated before
---    the set functions itself will be evaluated.
--- 3. If the multiset of values contains unbound variables,
---    the evaluation suspends.
---
--- @author Michael Hanus, Fabian Reck
--- @version November 2022
------------------------------------------------------------------------
{-# LANGUAGE CPP #-}
{-# OPTIONS_FRONTEND -Wno-incomplete-patterns #-}

module Control.Search.SetFunctions
  (set0, set1, set2, set3, set4, set5, set6, set7




  , Values, isEmpty, notEmpty, valueOf
  , chooseValue, choose, selectValue, select, getSomeValue, getSome
  , mapValues, foldValues, filterValues
  , minValue, minValueBy, maxValue, maxValueBy
  , values2list, printValues, sortValues, sortValuesBy
  ) where

import Data.List ( delete, minimum, minimumBy, maximum, maximumBy, sortBy )



import Control.Search.Unsafe ( allValues, oneValue )


------------------------------------------------------------------------
--- Combinator to transform a 0-ary function into a corresponding set function.
set0 :: b -> Values b








set0 f = Values (oneValue f) (allValues f)


--- Combinator to transform a unary function into a corresponding set function.
set1 :: (a1 -> b) -> a1 -> Values b








set1 f x | isVal x = Values (oneValue (f x)) (allValues (f x))


--- Combinator to transform a binary function into a corresponding set function.
set2 :: (a1 -> a2 -> b) -> a1 -> a2 -> Values b








set2 f x1 x2
  | isVal x1 & isVal x2
  = Values (oneValue (f x1 x2)) (allValues (f x1 x2))


--- Combinator to transform a function of arity 3
--- into a corresponding set function.
set3 :: (a1 -> a2 -> a3 -> b) -> a1 -> a2 -> a3 -> Values b









set3 f x1 x2 x3
  | isVal x1 & isVal x2 & isVal x3
  = Values (oneValue (f x1 x2 x3)) (allValues (f x1 x2 x3))


--- Combinator to transform a function of arity 4
--- into a corresponding set function.
set4 :: (a1 -> a2 -> a3 -> a4 -> b) -> a1 -> a2 -> a3 -> a4 -> Values b










set4 f x1 x2 x3 x4
  | isVal x1 & isVal x2 & isVal x3 & isVal x4
  = Values (oneValue (f x1 x2 x3 x4)) (allValues (f x1 x2 x3 x4))


--- Combinator to transform a function of arity 5
--- into a corresponding set function.
set5 :: (a1 -> a2 -> a3 -> a4 -> a5 -> b)
      -> a1 -> a2 -> a3 -> a4 -> a5 -> Values b










set5 f x1 x2 x3 x4 x5
  | isVal x1 & isVal x2 & isVal x3 & isVal x4 & isVal x5
  = Values (oneValue (f x1 x2 x3 x4 x5)) (allValues (f x1 x2 x3 x4 x5))


--- Combinator to transform a function of arity 6
--- into a corresponding set function.
set6 :: (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> b)
      -> a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> Values b











set6 f x1 x2 x3 x4 x5 x6
  | isVal x1 & isVal x2 & isVal x3 & isVal x4 & isVal x5 & isVal x6
  = Values (oneValue (f x1 x2 x3 x4 x5 x6))
           (allValues (f x1 x2 x3 x4 x5 x6))


--- Combinator to transform a function of arity 7
--- into a corresponding set function.
set7 :: (a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> b)
      -> a1 -> a2 -> a3 -> a4 -> a5 -> a6 -> a7 -> Values b











set7 f x1 x2 x3 x4 x5 x6 x7
  | isVal x1 & isVal x2 & isVal x3 & isVal x4 & isVal x5 & isVal x6 & isVal x7
  = Values (oneValue (f x1 x2 x3 x4 x5 x6 x7))
           (allValues (f x1 x2 x3 x4 x5 x6 x7))


------------------------------------------------------------------------
-- Auxiliaries:















-- Returns `True` after evaluating the argument to a ground value.
isVal :: a -> Bool
isVal x = (id $## x) `seq` True



------------------------------------------------------------------------
--- Abstract type representing multisets of values.





-- In PAKCS, values are represented as lists but the first argument
-- is used if values are tested for emptiness of a single value
-- is selected. This has the advantage that one can deal with infinite
-- search spaces as long as one is only interested in an emptiness
-- test or a single value.
data Values a = Values (Maybe a) [a]


--- Internal operation to extract all elements of a multiset of values.
valuesOf :: Values a -> [a]



valuesOf (Values _ s) = s


----------------------------------------------------------------------

--- Is a multiset of values empty?
isEmpty :: Values a -> Bool

isEmpty (Values firstval _) = case firstval of Nothing -> True
                                               Just _  -> False




--- Is a multiset of values not empty?
notEmpty :: Values a -> Bool
notEmpty vs = not (isEmpty vs)

--- Is some value an element of a multiset of values?
valueOf :: Eq a => a -> Values a -> Bool
valueOf e s = e `elem` valuesOf s

--- Chooses (non-deterministically) some value in a multiset of values
--- and returns the chosen value. For instance, the expression
---
---     chooseValue (set1 anyOf [1,2,3])
---
--- non-deterministically evaluates to the values `1`, `2`, and `3`.

--- Thus, `(set1 chooseValue)` is the identity on value sets, i.e.,
--- `(set1 chooseValue s)` contains the same elements as the
--- value set `s`.
chooseValue :: Eq a => Values a -> a
chooseValue s = fst (choose s)

--- Chooses (non-deterministically) some value in a multiset of values
--- and returns the chosen value and the remaining multiset of values.
--- Thus, if we consider the operation `chooseValue` defined by
---
---     chooseValue x = fst (choose x)
---
--- then `(set1 chooseValue)` is the identity on value sets, i.e.,
--- `(set1 chooseValue s)` contains the same elements as the
--- value set `s`.
choose :: Eq a => Values a -> (a, Values a)



choose (Values _ vs) =
  (x, Values (if null xs then Nothing else Just (head xs)) xs)

 where x  = foldr1 (?) vs
       xs = delete x vs

--- Selects (indeterministically) some value in a multiset of values
--- and returns the selected value.
--- Thus, `selectValue` has always at most one value, i.e., it is
--- a deterministic operation.
--- It fails if the value set is empty.
---
--- **NOTE:**
--- The usage of this operation is only safe (i.e., does not destroy
--- completeness) if all values in the argument set are identical.
selectValue :: Values a -> a

selectValue (Values (Just val) _) = val




--- Selects (indeterministically) some value in a multiset of values
--- and returns the selected value and the remaining multiset of values.
--- Thus, `select` has always at most one value, i.e., it is
--- a deterministic operation.
--- It fails if the value set is empty.
---
--- **NOTE:**
--- The usage of this operation is only safe (i.e., does not destroy
--- completeness) if all values in the argument set are identical.
select :: Values a -> (a, Values a)



select (Values _ (x:xs)) =
  (x, Values (if null xs then Nothing else Just (head xs)) xs)


--- Returns (indeterministically) some value in a multiset of values.
--- If the value set is empty, `Nothing` is returned.
getSomeValue :: Values a -> IO (Maybe a)




getSomeValue (Values mbval _) = return mbval


--- Selects (indeterministically) some value in a multiset of values
--- and returns the selected value and the remaining multiset of values.
--- Thus, `select` has always at most one value.
--- If the value set is empty, `Nothing` is returned.
getSome :: Values a -> IO (Maybe (a, Values a))




getSome (Values _ [])     = return Nothing
getSome (Values _ (x:xs)) =
  return (Just (x, Values (if null xs then Nothing else Just (head xs)) xs))


--- Maps a function to all elements of a multiset of values.
mapValues :: (a -> b) -> Values a -> Values b



mapValues f (Values mbval s) = Values (maybe Nothing (Just . f) mbval) (map f s)


--- Accumulates all elements of a multiset of values by applying a binary
--- operation. This is similarly to fold on lists, but the binary operation
--- must be **commutative** so that the result is independent of the order
--- of applying this operation to all elements in the multiset.
foldValues :: (a -> a -> a) -> a -> Values a -> a
foldValues f z s = foldr f z (valuesOf s)

--- Keeps all elements of a multiset of values that satisfy a predicate.
filterValues :: (a -> Bool) -> Values a -> Values a



filterValues p (Values _ s) = Values val xs
 where
  xs = filter p s
  val = if null xs then Nothing else Just (head xs)


--- Returns the minimum of a non-empty multiset of values
--- according to the given comparison function on the elements.
minValue :: Ord a => Values a -> a
minValue s = minimum (valuesOf s)

--- Returns the minimum of a non-empty multiset of values
--- according to the given comparison function on the elements.
minValueBy :: (a -> a -> Ordering) -> Values a -> a
minValueBy cmp s = minimumBy cmp (valuesOf s)

--- Returns the maximum of a non-empty multiset of values
--- according to the given comparison function on the elements.
maxValue :: Ord a => Values a -> a
maxValue s = maximum (valuesOf s)

--- Returns the maximum of a non-empty multiset of values
--- according to the given comparison function on the elements.
maxValueBy :: (a -> a -> Ordering) -> Values a -> a
maxValueBy cmp s = maximumBy cmp (valuesOf s)

--- Puts all elements of a multiset of values in a list.
--- Since the order of the elements in the list might depend on
--- the time of the computation, this operation is an I/O action.
values2list :: Values a -> IO [a]
values2list s = return (valuesOf s)

--- Prints all elements of a multiset of values.
printValues :: Show a => Values a -> IO ()
printValues s = values2list s >>= mapM_ print

--- Transforms a multiset of values into a list sorted by
--- the standard term ordering. As a consequence, the multiset of values
--- is completely evaluated.
sortValues :: Ord a => Values a -> [a]
sortValues = sortValuesBy (<=)

--- Transforms a multiset of values into a list sorted by a given ordering
--- on the values. As a consequence, the multiset of values
--- is completely evaluated.
--- In order to ensure that the result of this operation is independent of the
--- evaluation order, the given ordering must be a total order.
sortValuesBy :: (a -> a -> Bool) -> Values a -> [a]
sortValuesBy leq s = sortBy leq (valuesOf s)

------------------------------------------------------------------------