feat(SourceT): add fromConsumableActionStep that creates a StepT from…

changelog.d/generalize-fromActionStep-to-consumable

@@ -0,0 +1,22 @@
+synopsis: Add a `StepT` constructor method that consumes a resource
+prs: #1533
+issues: #1448
+
+description: {
+
+`fromActionStep` always runs the same action, which makes it impossible with such monadic actions to
+"consume" a resource (that is to say to pass the modified resource to the following action), or
+"unfold" an input structure.
+
+`unfoldStep` gives this possibility.
+
+This allows for example to build a `StepT m` directly from a `Streaming.Prelude.Stream`, by passing
+`unfoldStep` the following argument:
+
+```haskell
+import Streaming.Prelude as S
+
+action :: Stream (Of a) m r -> m (Maybe ( a, Stream (Of a) m r ))
+action = S.uncons
+```
+}

servant/src/Servant/Types/SourceT.hs

@@ -2,6 +2,7 @@
 {-# LANGUAGE GADTs               #-}
 {-# LANGUAGE RankNTypes          #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TupleSections       #-}
 module Servant.Types.SourceT where
 
 import           Control.Monad.Except
@@ -312,6 +313,28 @@ fromActionStep stop action = loop where
         | otherwise = Yield x loop
 {-# INLINE fromActionStep #-}
 
+-- | Create a `StepT' from a consumable @c@, that is to say from an input and an action that returns
+-- and is called again on an updated version of that input.
+--
+-- >>> import qualified Streaming.Prelude as S
+-- >>> foreachStep mempty print (unfoldStep S.uncons $ S.each [1..3] :: StepT IO Int)
+-- 1
+-- 2
+-- 3
+-- 
+unfoldStep :: Functor m
+                     => (c -> m (Maybe (a,c)))
+                     -- ^ Action. Return @Nothing@ to stop or @Just (a,c)@ where @a@ is the
+                     -- output element of the action and @c@ the updated input
+                     -> c
+                     -- ^ Input
+                     -> StepT m a
+unfoldStep action = loop where
+  loop c = Effect $ step <$> action c
+  step Nothing = Stop
+  step (Just (x,t)) = Yield x $ loop t
+{-# INLINE unfoldStep #-}
+
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