Add a oneOf function to improve the ergonomics of NonDet.

ChangeLog.md

@@ -1,5 +1,7 @@
 - Adds an `Empty` effect, modelling nondeterminism without choice ([#196](https://github.com/fused-effects/fused-effects/pull/196)).
 
+- Adds a `oneOf` function to `Control.Effect.NonDet` to provide an idiom for the common case of nondeterministically selecting from a container. ([#201](https://github.com/fused-effects/fused-effects/pull/201))
+
 ## Backwards-incompatible changes
 
 - Improves the performance of `runInterpret` using reflection, changing its signature slightly ([#193](https://github.com/fused-effects/fused-effects/pull/193), h/t [@ocharles](https://github.com/ocharles)).

src/Control/Effect/NonDet.hs

@@ -5,6 +5,7 @@ module Control.Effect.NonDet
   -- * NonDet carrier
 , runNonDet
 , NonDetC(..)
+, oneOf
   -- * Re-exports
 , Alternative(..)
 , Carrier
@@ -19,6 +20,7 @@ import qualified Control.Monad.Fail as Fail
 import Control.Monad.Fix
 import Control.Monad.IO.Class
 import Control.Monad.Trans.Class
+import Data.Monoid
 import GHC.Generics (Generic1)
 
 data NonDet m k
@@ -37,6 +39,20 @@ data NonDet m k
 runNonDet :: (Alternative f, Applicative m) => NonDetC m a -> m (f a)
 runNonDet (NonDetC m) = m (fmap . (<|>) . pure) (pure empty)
 
+-- | Nondeterministically choose an element from a 'Foldable' collection.
+-- This can be used to emulate the style of nondeterminism associated with
+-- programming in the list monad:
+-- @
+--   pythagoreanTriples = do
+--     a <- oneOf [1..10]
+--     b <- oneOf [1..10]
+--     c <- oneOf [1..10]
+--     guard (a^2 + b^2 == c^2)
+--     pure (a, b, c)
+-- @
+oneOf :: (Foldable t, Alternative m) => t a -> m a
+oneOf = getAlt . foldMap (Alt . pure)
+
 -- | A carrier for 'NonDet' effects based on Ralf Hinze’s design described in [Deriving Backtracking Monad Transformers](https://www.cs.ox.ac.uk/ralf.hinze/publications/#P12).
 newtype NonDetC m a = NonDetC
   { -- | A higher-order function receiving two parameters: a function to combine each solution with the rest of the solutions, and an action to run when no results are produced.