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------------------------------------------------------------------------------
--- Library with operations to encapsulate search, i.e., non-deterministic
--- computations. Note that these operations are not fully declarative,
--- i.e., the results depend on the order of evaluation and program rules.
--- This is due to the fact that the search operators work on a copy
--- of the current expression to be encapsulated.
--- The potential problems of this method are discussed in this paper:
---
--- > B. Brassel, M. Hanus, F. Huch:
--- > Encapsulating Non-Determinism in Functional Logic Computations
--- > Journal of Functional and Logic Programming, No. 6, EAPLS, 2004
---
--- There are newer and better approaches the encapsulate search,
--- in particular, set functions (see module `Control.Search.SetFunctions`
--- which should be used.
---
--- @author Michael Hanus
--- @version October 2023
------------------------------------------------------------------------------
{-# LANGUAGE CPP #-}

module Control.Search.Unsafe
  ( allValues, someValue, oneValue, isFail

  , rewriteAll, rewriteSome

  ) where





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

--- Returns all values of an expression.
--- Conceptually, the value is computed on a copy of the expression,
--- i.e., the evaluation of the expression does not share any results.
--- In PAKCS, the evaluation suspends as long as the expression
--- contains unbound variables or the computed
--- value contains unbound variables.
---
--- Note that this operation is not purely declarative since the ordering
--- of the computed values depends on the ordering of the program rules.
allValues :: a -> [a]



allValues external


--- Returns just one value for an expression.
--- If the expression has no value, `Nothing` is returned.
--- Conceptually, the value is computed on a copy of the expression,
--- i.e., the evaluation of the expression does not share any results.
--- In PAKCS, the evaluation suspends as long as the expression
--- contains unbound variables or the computed
--- value contains unbound variables.
---
--- Note that this operation is not purely declarative since
--- the computed value depends on the ordering of the program rules.
--- Thus, this operation should be used only if the expression
--- has a single value.
oneValue :: a -> Maybe a





oneValue external


--- Returns some value for an expression.
--- If the expression has no value, the computation fails.
--- Conceptually, the value is computed on a copy of the expression,
--- i.e., the evaluation of the expression does not share any results.
--- In PAKCS, the evaluation suspends as long as the expression
--- contains unbound variables or the computed
--- value contains unbound variables.
---
--- Note that this operation is not purely declarative since
--- the computed value depends on the ordering of the program rules.
--- Thus, this operation should be used only if the expression
--- has a single value.
someValue :: a -> a
someValue x = case oneValue x of Just v  -> v
                                 Nothing -> failed

--- Does the computation of the argument to a value fail?
--- Conceptually, the argument is evaluated on a copy, i.e.,
--- even if the computation does not fail, it has not been evaluated.
isFail :: a -> Bool
isFail x = case oneValue x of Nothing -> True
                              Just _  -> False


------------------------------------------------------------------------------
--- Gets all values computable by term rewriting.
--- In contrast to `allValues`, this operation does not wait
--- until all "outside" variables are bound to values,
--- but it returns all values computable by term rewriting
--- and ignores all computations that requires bindings for outside variables.
rewriteAll :: a -> [a]
rewriteAll external

--- Similarly to 'rewriteAll' but returns only some value computable
--- by term rewriting. Returns `Nothing` if there is no such value.
rewriteSome :: a -> Maybe a
rewriteSome external


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